Why do most fish move fins orthogonally with their axis of symmetry?

Why do most fish move fins orthogonally with their axis of symmetry?

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So recently I read the question why are most fishes vertical (in the sense of distribution of their body mass) and it got me thinking what is the reason behind the direction they move their fins. My explanation is that they need to catch the image they several times to truly percieve it, so the eyes have to be orthogonal on the direction they move their fins. Am I correct, therefore they swim like this rather than this because there is more to see on the horizontal dimension in shallow waters?

First, we should define symmetry. Symmetry is the arrangement of body parts so they can be divided equally along an imaginary line or axis. In marine life, the two main types of symmetry are bilateral symmetry and radial symmetry, although there are some organisms that exhibit biradial symmetry (e.g., ctenophores) or asymmetry (e.g., sponges).

When an organism is radially symmetrical, you could cut from one side of the organism through the center to the other side, anywhere on the organism, and this cut would produce two equal halves. Think of a pie: no matter which way you slice it, if you slice from one side to the other through the center, you'll end up with equal halves. You can continue slicing the pie to end up with any number of equal-sized pieces. Thus, the pieces of this pie radiate out from the central point.

You can apply the same slicing demonstration to a sea anemone. If you draw an imaginary line across the top of a sea anemone starting at any one point, that would divide it into roughly equal halves.


The skull of fishes is formed from a series of loosely connected bones. Lampreys and sharks only possess a cartilaginous endocranium, with both the upper and lower jaws being separate elements. Bony fishes have additional dermal bone, forming a more or less coherent skull roof in lungfish and holost fish.

The simpler structure is found in jawless fish, in which the cranium is represented by a trough-like basket of cartilaginous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril. Distinctively, these fish have no jaws. [9]

Cartilaginous fish, such as sharks, also have simple skulls. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle. The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra. There are, in addition, at various points throughout the cranium, smaller foramina for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper. [9]

In ray-finned fishes, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced there is little cheek region behind the enlarged orbits, and little, if any bone in between them. The upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium. [9]

Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from the pterygoids and vomers alone, all of which bear teeth. Much of the skull is formed from cartilage, and its overall structure is reduced. [9]

Lower Edit

In vertebrates, the lower jaw (mandible or jawbone) [10] is a bone forming the skull with the cranium. In lobe-finned fishes and the early fossil tetrapods, the bone homologous to the mandible of mammals is merely the largest of several bones in the lower jaw. It is referred to as the dentary bone, and forms the body of the outer surface of the jaw. It is bordered below by a number of splenial bones, while the angle of the jaw is formed by a lower angular bone and a suprangular bone just above it. The inner surface of the jaw is lined by a prearticular bone, while the articular bone forms the articulation with the skull proper. Finally a set of three narrow coronoid bones lie above the prearticular bone. As the name implies, the majority of the teeth are attached to the dentary, but there are commonly also teeth on the coronoid bones, and sometimes on the prearticular as well. [11]

This complex primitive pattern has, however, been simplified to various degrees in the great majority of vertebrates, as bones have either fused or vanished entirely. In teleosts, only the dentary, articular, and angular bones remain. [11] Cartilagenous fish, such as sharks, do not have any of the bones found in the lower jaw of other vertebrates. Instead, their lower jaw is composed of a cartilagenous structure homologous with the Meckel's cartilage of other groups. This also remains a significant element of the jaw in some primitive bony fish, such as sturgeons. [11]

Upper Edit

The upper jaw, or maxilla [12] [13] is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mandibular symphysis. In bony fish, the maxilla is called the "upper maxilla," with the mandible being the "lower maxilla". The alveolar process of the maxilla holds the upper teeth, and is referred to as the maxillary arch. In most vertebrates, the foremost part of the upper jaw, to which the incisors are attached in mammals consists of a separate pair of bones, the premaxillae. In bony fish, both maxilla and premaxilla are relatively plate-like bones, forming only the sides of the upper jaw, and part of the face, with the premaxilla also forming the lower boundary of the nostrils. [14] Cartilaginous fish, such as sharks and rays also lack a true maxilla. Their upper jaw is instead formed from a cartilagenous bar that is not homologous with the bone found in other vertebrates. [14]

Some fish have permanently protruding upper jawbones called rostrums. Billfish (marlin, swordfish and sailfish) use rostrums (bills) to slash and stun prey. Paddlefish, goblin sharks and hammerhead sharks have rostrums packed with electroreceptors which signal the presence of prey by detecting weak electrical fields. Sawsharks and the critically endangered sawfish have rostrums (saws) which are both electro-sensitive and used for slashing. [15] The rostrums extend ventrally in front of the fish. In the case of hammerheads the rostrum (hammer) extends both ventrally and laterally (sideways).

Sailfish, like all billfish, have a rostrum (bill) which evolved from the upper jawbone

The paddlefish has a rostrum packed with electroreceptors

Sawfish have an electro-sensitive rostrum (saw) which is also used to slash at prey

Jaw protrusion Edit

Teleosts have a movable premaxilla (a bone at the tip of the upper jaw) and corresponding modifications in the jaw musculature which make it possible for them to protrude their jaws outwards from the mouth. This is of great advantage, enabling them to grab prey and draw it into the mouth. In more derived teleosts, the enlarged premaxilla is the main tooth-bearing bone, and the maxilla, which is attached to the lower jaw, acts as a lever, pushing and pulling the premaxilla as the mouth is opened and closed. These protrusible jaws are evolutionary novelties in teleosts that evolved independently at least five times. [16]

The premaxilla is unattached to the neurocranium (braincase) it plays a role in protruding the mouth and creating a circular opening. This lowers the pressure inside the mouth, sucking the prey inside. The lower jaw and maxilla (main upper fixed bone of the jaw) are then pulled back to close the mouth, and the fish is able to grasp the prey. By contrast, mere closure of the jaws would risk pushing food out of the mouth. In more advanced teleosts, the premaxilla is enlarged and has teeth, while the maxilla is toothless. The maxilla functions to push both the premaxilla and the lower jaw forward. To open the mouth, an adductor muscle pulls back the top of the maxilla, pushing the lower jaw forward. In addition, the maxilla rotates slightly, which pushes forward a bony process that interlocks with the premaxilla. [17]

Teleosts achieve this jaw protrusion using one of four different mechanisms involving the ligamentous linkages within the skull. [18]

  • Mandibular depression mechanism: The depression of the lower jaw (mandible) pulls or pushes the premaxilla into protrusion via force transmission through ligaments and tendons connected to the upper jaws (e.g. Cyprinus, Labrus). [18] This is the most commonly used mechanism.
  • Twisting maxilla mechanism: The depression of the mandible causes the maxilla to twist about the longitudinal axis resulting in the protrusion of the premaxilla (e.g. Mugil). [18]
  • Decoupled mechanism: Protrusion of the premaxilla is accomplished through elevation of the neurocranium causing the premaxilla to move anteriorly. Movements of the neurocranium are not coupled with the kinematics of the upper jaw (e.g. Spathodus erythrodon), [18][19] allowing for more versatility and modularity of the jaws during prey capture and manipulation.
  • Suspensorial abduction mechanism: The lateral expansion of the suspensorium (a combination of the palatine, pterygoid series, and quadrate bones) pulls on a ligament which causes the premaxilla to protrude anteriorly (e.g. Petrotilapia tridentiger). [18][19]

Some teleosts use more than one of these mechanisms (e.g. Petrotilapia). [18]

Wrasses have become a primary study species in fish-feeding biomechanics due to their jaw structure. They have protractile mouths, usually with separate jaw teeth that jut outwards. [20] Many species can be readily recognized by their thick lips, the inside of which is sometimes curiously folded, a peculiarity which gave rise the German name of "lip-fishes" (Lippfische). [21]

The nasal and mandibular bones are connected at their posterior ends to the rigid neurocranium, and the superior and inferior articulations of the maxilla are joined to the anterior tips of these two bones, respectively, creating a loop of 4 rigid bones connected by moving joints. This "four-bar linkage" has the property of allowing numerous arrangements to achieve a given mechanical result (fast jaw protrusion or a forceful bite), thus decoupling morphology from function. The actual morphology of wrasses reflects this, with many lineages displaying different jaw morphology that results in the same functional output in a similar or identical ecological niche. [20]

The most extreme jaw protrusion found in fishes occurs in the slingjaw wrasse, Epibulus insidiator . This fish can extend its jaws up to 65% the length of its head. [22] This species utilizes its quick and extreme jaw protrusion to capture smaller fishes and crustaceans. The genus this species belongs to possess one unique ligament (vomero-interopercular) and two enlarged ligaments (interoperculo-mandibular and premaxilla-maxilla), which along with a few changes to the form of cranial bones, allow it to achieve extreme jaw protrusion.

Pharyngeal jaws are a second set of jaws distinct from the primary (oral) jaws. They are contained within the throat, or pharynx, of most bony fish. They are believed to have originated, in a similar way to oral jaws, as a modification of the fifth gill arch which no longer has a respiratory function. The first four arches still function as gills. Unlike the oral jaw, the pharyngeal jaw has no jaw joint, but is supported instead by a sling of muscles.

A notable example occurs with the moray eel. The pharyngeal jaws of most fishes are not mobile. The pharyngeal jaws of the moray are highly mobile, perhaps as an adaptation to the constricted nature of the burrows they inhabit which inhibits their ability to swallow as other fishes do by creating a negative pressure in the mouth. Instead, when the moray bites prey, it first bites normally with its oral jaws, capturing the prey. Immediately thereafter, the pharyngeal jaws are brought forward and bite down on the prey to grip it they then retract, pulling the prey down the moray eel's gullet, allowing it to be swallowed. [23]

All vertebrates have a pharynx, used in both feeding and respiration. The pharynx arises during development through a series of six or more outpocketings called pharyngeal arches on the lateral sides of the head. The pharyngeal arches give rise to a number of different structures in the skeletal, muscular and circulatory systems in a manner which varies across the vertebrates. Pharyngeal arches trace back through chordates to basal deuterostomes who also share endodermal outpocketings of the pharyngeal apparatus. Similar patterns of gene expression can be detected in the developing pharynx of amphioxus and hemichordates. However, the vertebrate pharynx is unique in that it gives rise to endoskeletal support through the contribution of neural crest cells. [24]

Cartilaginous fishes (sharks, rays and skates) have cartilaginous jaws. The jaw's surface (in comparison to the vertebrae and gill arches) needs extra strength due to its heavy exposure to physical stress. It has a layer of tiny hexagonal plates called "tesserae", which are crystal blocks of calcium salts arranged as a mosaic. [25] This gives these areas much of the same strength found in the bony tissue found in other animals.

Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five layers. [26] In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

In sharks and other extant elasmobranchs the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The arrangement of soft tissue and any additional articulations connecting these elements is collectively known as the jaw suspension. There are several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support. In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid. [27] [28] [29]

Jaws provide a platform in most fishes for simple pointed teeth. Lungfish and chimaera have teeth modified into broad enamel plates with jagged ridges for crushing or grinding. Carp and loach have pharyngeal teeth only. Sea horses, pipefish and adult sturgeon have no teeth of any type. In fish, Hox gene expression regulates mechanisms for tooth initiation. [30] [31]

However, sharks continuously produce new teeth throughout their lives via a drastically different mechanism. [32] [33] [34] Shark teeth form from modified scales near the tongue and move outward on the jaw in rows until they are eventually dislodged. [35] Their scales, called dermal denticles, and teeth are homologous organs. [36]

Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily moving forward as though on a conveyor belt. Some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months. In most species, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row, which is observed in the cookiecutter shark. [37]

Tooth shape depends on the shark's diet: those that feed on mollusks and crustaceans have dense and flattened teeth used for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are small and non-functional. [38]

Jaw reconstruction of the extinct Carcharodon megalodon, 1909

The thornback ray has teeth adapted to feed on crabs, shrimps and small fish.

The shortfin mako shark lunges vertically and tears flesh from prey

Tiger shark teeth are oblique and serrated to saw through flesh

The prickly shark has knife-like teeth with main cusps flanked by lateral cusplets

Salmon Edit

Male salmon often remodel their jaws during spawning runs so they have a pronounced curvature. These hooked jaws are called kypes. The purpose of the kype is not altogether clear, though they can be used to establish dominance by clamping them around the base of the tail (caudal peduncle) of an opponent. [39] [40]

Cichlids Edit

Fish jaws, like vertebrates in general, normally show bilateral symmetry. An exception occurs with the parasitic scale-eating cichlid Perissodus microlepis. The jaws of this fish occur in two distinct morphological forms. One morph has its jaw twisted to the left, allowing it to eat scales more readily on its victim's right flank. The other morph has its jaw twisted to the right, which makes it easier to eat scales on its victim's left flank. The relative abundance of the two morphs in populations is regulated by frequency-dependent selection. [41] [42] [43]

In cichlids generally, the oral and pharyngeal teeth differ with different species in ways that allow them to process different kinds of prey. Primary oral jaws contain teeth which are used to capture and hold food, while pharyngeal jaws have pharyngeal teeth which function as a chewing tool.

This allows for different nutritional strategies, and because of this, cichlids are able to colonize different habitats. The structural diversity of the lower pharyngeal jaw could be one of the reasons for the occurrence of so many cichlid species. Convergent evolution took place over the course of the cichlid radiation, synchronous with different trophic niches. [44] The pharyngeal jaw apparatus consists of two upper and one single lower plate, all of which have dentations that differ in size and type. [45] The structure of the lower pharynx is often associated with the species of food of the species. [46]

In order to crack shellfish considerable force must be generated, which is why cichlids that feed on molluscs (e.g. the cichlid bass, Crenicichla minuano), have molariform teeth and a strengthened jawbone bone. To grab and bite prey not armoured with shells, predators need conical, bent back teeth. [47] Herbivorous cichlids also have structural differences in their teeth. Cichlids that specialise in algae (e.g. Pseudotropheus) tend to have small conical teeth. Species that feed on pods or seeds require large conical teeth for chewing their food. [48]

Other Edit

Stoplight loosejaws are small fish found worldwide in the deep sea. Relative to their size they have one of the widest gapes of any fish. The lower jaw has no ethmoid membrane (floor) and is attached only by the hinge and a modified tongue bone. There are several large, fang-like teeth in the front of the jaws, followed by many small barbed teeth. There are several groups of pharyngeal teeth that serve to direct food down the esophagus. [49] [50]

Another deep sea fish, the pelican eel, has jaws larger than its body. The jaws are lined with small teeth and are loosely hinged. They open wide enough to swallow a fish larger than the eel itself.

Distichodontidae are a family of fresh water fishes which can be divided into genera with protractile upper jaws which are carnivores, and genera with nonprotractile upper jaws which are herbivores or predators of very small organisms. [51]

The appearance of the early vertebrate jaw has been described as "a crucial innovation" [53] and "perhaps the most profound and radical evolutionary step in the vertebrate history". [4] [5] Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period. However studies of the cyclostomes, the jawless hagfishes and lampreys that did survive, have yielded little insight into the deep remodelling of the vertebrate skull that must have taken place as early jaws evolved. [54] [55]

The customary view is that jaws are homologous to the gill arches. [56] In jawless fishes a series of gills opened behind the mouth, and these gills became supported by cartilaginous elements. The first set of these elements surrounded the mouth to form the jaw. The upper portion of the second embryonic arch supporting the gill became the hyomandibular bone of jawed fishes, which supports the skull and therefore links the jaw to the cranium. [57] The hyomandibula is a set of bones found in the hyoid region in most fishes. It usually plays a role in suspending the jaws or the operculum in the case of teleosts. [58]

It is now accepted that the precursors of the jawed vertebrates are the long extinct bony (armoured) jawless fish, the so-called ostracoderms. [59] [60] The earliest known fish with jaws are the now extinct placoderms [61] and spiny sharks. [62]

Placoderms were a class of fish, heavily armoured at the front of their body, which first appeared in the fossil records during the Silurian about 430 million years ago. Initially they were very successful, diversifying remarkably during the Devonian. They became extinct by the end of that period, about 360 million years ago. [63] Their largest species, Dunkleosteus terrelli, measured up to 10 m (33 ft) [64] [65] and weighed 3.6 t (4.0 short tons). [66] It possessed a four bar linkage mechanism for jaw opening that incorporated connections between the skull, the thoracic shield, the lower jaw and the jaw muscles joined together by movable joints. [67] [68] This mechanism allowed Dunkleosteus terrelli to achieve a high speed of jaw opening, opening their jaws in 20 milliseconds and completing the whole process in 50-60 milliseconds, comparable to modern fishes that use suction feeding to assist in prey capture. [67] They could also produce high bite forces when closing the jaw, estimated at 6,000 N (1,350 lbf) at the tip and 7,400 N (1,660 lbf) at the blade edge in the largest individuals. [68] The pressures generated in those regions were high enough to puncture or cut through cuticle or dermal armour [67] suggesting that Dunkleosteus terrelli was perfectly adapted to prey on free-swimming, armoured prey like arthropods, ammonites, and other placoderms. [68]

Spiny sharks were another class of fish which appeared also in the fossil records during the Silurian at about the same time as the placoderms. They were smaller than most placoderms, usually under 20 centimetres. Spiny sharks did not diversify as much as placoderms, but survived much longer into the Early Permian about 290 million years ago. [69]

The original selective advantage offered by the jaw may not be related to feeding, but rather to increased respiration efficiency. [70] The jaws were used in the buccal pump still observable in modern fish and amphibians, that uses "breathing with the cheeks" to pump water across the gills of fish or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws (to humans), in feeding, was selected for and became a very important function in vertebrates. Many teleost fish have substantially modified jaws for suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved. [71]

Jaws are thought to derive from the pharyngeal arches that support the gills in fish. The two most anterior of these arches are thought to have become the jaw itself (see hyomandibula) and the hyoid arch, which braces the jaw against the braincase and increases mechanical efficiency. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed (the Gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine.

Meckel's cartilage is a piece of cartilage from which the mandibles (lower jaws) of vertebrates evolved. Originally it was the lower of two cartilages which supported the first gill arch (nearest the front) in early fish. Then it grew longer and stronger, and acquired muscles capable of closing the developing jaw. [72] In early fish and in chondrichthyans (cartilaginous fish such as sharks), Meckel's cartilage continued to be the main component of the lower jaw. But in the adult forms of osteichthyans (bony fish) and their descendants (amphibians, reptiles, birds and mammals) the cartilage was covered in bone – although in their embryos the jaw initially develops as the Meckel's cartilage. In tetrapods the cartilage partially ossifies (changes to bone) at the rear end of the jaw and becomes the articular bone, which forms part of the jaw joint in all tetrapods except mammals. [72]


Two attributes of fin function in fishes that have received the least attention are (1) a precise description of the motion of surface elements of the fin and (2) an analysis of the effect that fin motions have on the water. Since the presence of fins as control surfaces in fishes is a prominent aspect of their biological design, it is at first glance surprising that so little is known about how fins move and what effect such movements have on fluid motion. But measuring both fin and fluid motion accurately and in a time-dependent manner is a difficult proposition. Fish fins are thin and often diaphanous and monochromatic, making identification of specific points difficult, while quantifying motion of a clear fluid is a difficult problem of long-standing ( Nakayama, 1988 Yang, 1989 Nieuwstadt, 1993 Moin and Kim, 1997). Fortunately, recent developments in experimental methodology have allowed the application of video and fluid dynamics techniques to the study of fin and fluid movement, and we are now in a position to generate new data on the function of fins.

Three-dimensional kinematics

Given that the vast majority of research on fish locomotion has involved analysis of body deformation and myotomal muscle function, it is perhaps not surprising that the most common images in the literature of fishes swimming are ventral or dorsal views. Such images are usually obtained by aiming a video camera at a mirror mounted either above or below the swimming fish, and quantifying deformation of the body by digitizing either the midline or the silhouette. But examination of the shape of the tail in these images reveals that changes in thickness occur which indicate that there are as yet unrecognized alterations in caudal fin shape that are not well revealed by ventral or dorsal views (e.g., Gray, 1933, 1968 Aleev, 1969). This suggests that a three-dimensional analysis is needed to capture the complex motions of fins.

A three-dimensional analysis would also alleviate the possibility of serious error when a two-dimensional analysis alone is used. One way in which such errors can arise is shown in Figure 4 which depicts a three-dimensional space defined by X, Y, and Z axes. Such a space may represent the working section of a flow tank, or the aquarium within which an experiment is conducted. The XZ plane represents the horizontal or frontal plane, the XY plane the vertical or parasaggital plane, and the YZ plane the transverse section. If a triangle is suspended within this space to represent the tail of a fish swimming in a flow tank, then water would flow through the YZ plane parallel to the XY plane. The video images obtained through the XY plane would represent a lateral view while images through the YZ plane a posterior view. By examining the projection of the triangle on the XZ plane and the locations of the vertices on the Z axis, it is possible to see that this triangle has been positioned so that it forms an acute angle to the XZ plane that is, it is inclined toward increasing Z values and vertex three leads the triangle as it moves toward the XY plane. Water influenced by motion of the tail in this way would be expected to move ventrally, so directed by the ventrally inclined surface of the triangle. However, if we rely on a posterior view alone, projection of the trailing edge (line segment 2-1) onto the YZ plane is inclined dorsally suggesting, erroneously, that fluid influenced by such a motion might be directed dorsally. Reliance on a lateral or ventral view alone provides similarly misleading information on motion in the other planes. Lauder and Jayne (1996) showed that angles of fin surfaces estimated from two-dimensional analyses can be in error by as much as 83° from the correct three-dimensional angle (and further details about 3D angle calculations can be found in that paper).

In order to record three-dimensional data on caudal fin movements during steady locomotion, I have used the experimental design illustrated in Figure 5. Two synchronized video cameras record orthogonal planar views of the fins at 250 images per second. One camera images a lateral (XY view) through the side of the flow tank while the second camera is aimed at a small mirror located in the flow posterior to the swimming fish. By aligning this mirror at a 45° angle to the flow, the camera images the posterior (YZ view) of the fins. Information from both cameras together provides X, Y, and Z coordinates for points on the fins. In order to facilitate repeated and accurate recognition of specific locations on the fin, fish are anesthetized prior to each experiment and small markers are glued bilaterally onto the fin. In the image shown in Figure 5, a triangular marker arrangement has been used on both the dorsal and ventral lobes of the tail. Such triangular patterns allow reconstruction of the surface orientation of fin regions through calculation of planar angles of intersection between triangular fin elements and the three reference planes ( Lauder and Jayne, 1996).

Location of the posterior-view mirror at least one to two body lengths posterior to the trailing edge of the tail and against the downstream flow grid (which restricts recirculatory vortices downstream from the mirror and hence their impact on flow immediately upstream from the mirror) minimizes any disturbance of the flow caused by the mirror in the region of the swimming fish. Analyses of variance conducted for leopard sharks swimming in this apparatus ( Ferry and Lauder, 1996) and similar analyses for bluegill showed that the presence of the mirror in the flow had no significant effect on either tail beat amplitude or frequency (P > 0.27), suggesting that the mirror has little impact on the kinematics of the tail beat.

Digital particle image velocimetry (DPIV)

While quantifying the three-dimensional motion of the caudal fin is one critical component of understanding caudal fin function, it is also necessary to evaluate the impact that movement of the fin has on the fluid. By understanding the fluid motion induced by action of the caudal fin, the forces exerted on the fluid and the direction of those forces can be estimated. While analyses of locomotion on land have traditionally used force plates to quantify the forces exerted by limbs during locomotion ( Cavagna, 1975 Biewener and Full, 1992), a technique allowing similar measurements has not been available until recently for the aquatic realm.

The technique of DPIV (digital particle image velocimetry) provides a means of quantifying fluid flow and of calculating forces exerted by fishes swimming in vivo. By visualizing flow in two or more dimensions, vortices formed by fin movement can be reconstructed and the orthogonal components of momentum and force calculated (e.g., Lauder et al., 1996 Drucker and Lauder, 1999 Wolfgang et al., 1999 Wilga and Lauder, 1999). Such measurements allow a direct test of functional hypotheses.

Figure 6 illustrates the basic principle of DPIV as used in our experiments visualizing flow in the wake of the caudal fin. Water in a flow tank is seeded with small (12 μ mean diameter) silver coated glass beads which reflect light from an argon-ion laser. The laser beam is focused via a series of lenses into a light sheet approximately 10 cm wide and 1–2 mm thick. The experimental arrangement is as shown in Figure 5 with the addition of a laser and light sheet extending into the flow tank. Movement of the optical components allows the laser light sheet to be oriented into three orthogonal planes (video cameras are also appropriately repositioned to provide an image of the light sheet), and video images are taken of the light reflected from the particles in the flow (also see Drucker and Lauder, 1999). The particles are carried through the light sheet with water movement, and as the flow is disturbed by movement of the tail particles move with the flow and their reflections are captured on video. By using two simultaneous video cameras, one camera can capture the particle reflections while the other images the position of the fish relative to the light sheet. This allows determination of precisely which portion of the tail is acting on the fluid. By repositioning the fish in the flow tank, images of the flow around different regions of the tail can be obtained.

Analysis proceeds by choosing pairs of video images (separated in time by 4 ms) that capture flow in the wake behind the tail. The area of interest in the wake (typically a 10 cm 2 region, see Fig. 6) is then selected and divided into a matrix of discrete smaller areas of interrogation. For the analyses presented here, a 20*20 matrix of areas of interrogation was used. A standard two-dimensional cross-correlation analysis is then used to compare the pixel intensities at one time to that Δt later, and each cross-correlation analysis yields a velocity vector that estimates the direction and speed of flow in that area of interrogation ( Raffel et al., 1998). Given a 20*20 matrix of areas, a regularly spaced array of 400 velocity vectors is obtained that provides a quantitative estimate of flow in the light sheet at that time. From this matrix of velocity vectors, fluid vorticity, momentum, circulation, and force can be calculated ( Drucker and Lauder, 1999) using standard methods ( Rayner, 1979 Spedding et al., 1984 Spedding and Maxworthy, 1986 Spedding, 1987).

Weakly electric fish are extraordinarily maneuverable swimmers, able to swim as easily forward as backward and rapidly switch swim direction, among other maneuvers. The primary propulsor of gymnotid electric fish is an elongated ribbon-like anal fin. To understand the mechanical basis of their maneuverability, we examine the hydrodynamics of a non-translating ribbon fin in stationary water using computational fluid dynamics and digital particle image velocimetry (DPIV) of the flow fields around a robotic ribbon fin. Computed forces are compared with drag measurements from towing a cast of the fish and with thrust estimates for measured swim-direction reversals. We idealize the movement of the fin as a traveling sinusoidal wave, and derive scaling relationships for how thrust varies with the wavelength, frequency,amplitude of the traveling wave and fin height. We compare these scaling relationships with prior theoretical work. The primary mechanism of thrust production is the generation of a streamwise central jet and the associated attached vortex rings. Under certain traveling wave regimes, the ribbon fin also generates a heave force, which pushes the body up in the body-fixed frame. In one such regime, we show that as the number of waves along the fin decreases to approximately two-thirds, the heave force surpasses the surge force. This switch from undulatory parallel thrust to oscillatory normal thrust may be important in understanding how the orientation of median fins may vary with fin length and number of waves along them. Our results will be useful for understanding the neural basis of control in the weakly electric knifefish as well as for engineering bio-inspired vehicles with undulatory thrusters.

Weakly electric knifefish have been studied for several decades to gain insights into how vertebrates process sensory information (for reviews, see Bullock, 1986 Turner et al., 1999). Knifefish continuously emit a weak electric field, which is perturbed by objects that enter the field and distort the field because their electrical properties differ from the surrounding fluid. These perturbations are detected by thousands of electroreceptors on the surface of the body. MacIver and coworkers (Snyder et al.,2007) have shown that the knifefish is able to both sense and move omnidirectionally. The primary thruster that weakly electric fish use in achieving their remarkable maneuverability is an elongated anal fin(Fig. 1), which we generically refer to as a ribbon fin.

Approximately 150 species of South American electric fish (family Gymnotidae) swim by using a ribbon fin positioned along the ventral midline. This fin is also used for swimming in one weakly electric species in Africa, Gymnarchus niloticus, where the fin is positioned on the dorsal midline, and in species of the non-weakly electric family Notopteridae, where the fin is positioned along the ventral midline. In the present study, we address the mechanical principles of force generation by the ribbon fin in the context of the South American weakly electric black ghost knifefish, Apteronotus albifrons (Linnaeus 1766)(Fig. 1A).

Knifefish swim by passing traveling waves along the ribbon fin. The waveform is often similar in overall shape to a sinusoid(Fig. 1B). The body is typically held straight and semi-rigid while swimming(Fig. 1B), i.e. body deformations are small compared with fin deformations. This may facilitate sensory performance as the body houses the electric field generator, and movement of the tail causes modulations of the field that are more than a factor of 10 larger than prey-related modulations(Chen et al., 2005 Nelson and MacIver, 1999). Knifefish frequently reverse the direction of movement without turning by changing the direction of the traveling wave on the fin and are as agile swimming backward as they are swimming forward(Blake, 1983 Lannoo and Lannoo, 1993 MacIver et al., 2001 Nanjappa et al., 2000).

The ability to switch movement direction rapidly (in ≈100 ms)(MacIver et al., 2001) is integral to several behaviors. Previous work by MacIver et al.(MacIver et al., 2001) has shown that prey are usually detected while swimming forward, after the prey has passed the head region, and the fish then rapidly reverses the body movement to bring the mouth to the prey during the prey strike. During inspection of novel objects, the fish are observed to engage in forward–backward scanning motions(Assad et al., 1999), which may be important for increasing spatial acuity(Babineau et al., 2007).

Ribbon-fin-based swimming is commonly referred to as the gymnotiform mode by Breder (Breder, 1926). In addition to being agile, prior research on ribbon-finned swimmers has also suggested that they are highly efficient for movement at low velocities(Blake, 1983 Lighthill and Blake, 1990). This claim is supported by the discovery that these fish use half the amount of oxygen per unit time and mass as non-gymnotid teleosts(Julian et al., 2003).

Two goals motivate the current study. First, in order to advance from the mature understanding we have of sensory signal processing in weakly electric knifefishes to an understanding of how these signals are processed to control movement, we need to characterize the hydrodynamics of ribbon-fin propulsion. Second, artificial ribbon fins may provide a superior actuator for use in highly maneuverable underwater vehicles for applications such as environmental monitoring (Epstein et al.,2006 MacIver et al.,2004).

Apteronotus albifrons, the black ghost knifefish of South America.(A) Photograph courtesy of Neil Hepworth, Practical Fishkeepingmagazine. Inset shows side view of the fish to illustrate the angle of the fin with respect to the body. (B) Frame of video of the fish swimming in a water tunnel, from ventral side. (C) Model of the ribbon fin, shown together with symbols for fin length (Lfin), fin height(hfin) and two of the kinematic state variables, angular deflection (θ) and the vector from the rotational axis to a point on the fin (rm). The Eulerian (fluid) reference frame (x, y, z positive direction indicated by inset) is shown together with names for velocities in the body frame fixed to the rigid fin rotation axis indicated by the red line (surge, heave, sway, roll, pitch, yawpositive direction indicated by arrows). In this work, simulations were performed with the body-fixed frame oriented to the Eulerian frame as illustrated, and forces are with respect to a traveling wave moving in the head-to-tail direction indicated.

Apteronotus albifrons, the black ghost knifefish of South America.(A) Photograph courtesy of Neil Hepworth, Practical Fishkeepingmagazine. Inset shows side view of the fish to illustrate the angle of the fin with respect to the body. (B) Frame of video of the fish swimming in a water tunnel, from ventral side. (C) Model of the ribbon fin, shown together with symbols for fin length (Lfin), fin height(hfin) and two of the kinematic state variables, angular deflection (θ) and the vector from the rotational axis to a point on the fin (rm). The Eulerian (fluid) reference frame (x, y, z positive direction indicated by inset) is shown together with names for velocities in the body frame fixed to the rigid fin rotation axis indicated by the red line (surge, heave, sway, roll, pitch, yawpositive direction indicated by arrows). In this work, simulations were performed with the body-fixed frame oriented to the Eulerian frame as illustrated, and forces are with respect to a traveling wave moving in the head-to-tail direction indicated.

We use computational fluid dynamics to examine the flow structures and forces arising from a sinusoidally actuated ribbon fin. We compare the computed flow structures with those measured from a robotic ribbon fin using digital particle image velocimetry (DPIV) and compare the computed surge force with the drag force measured from towing a cast of the fish. Whereas tow drag can provide a useful estimate of the thrust needed during steady swimming, for the impulsive motions modeled in this study, the thrust needed to undergo typical accelerations is more directly relevant. Thus, we also compare computed forces with the thrust that we estimate is needed for two different types of swimming direction reversals: reversals that occur during prey capture strikes from kinematic data collected in a previous study(MacIver et al., 2001) and reversals that occur during refuge tracking behavior, where fish placed in a sinusoidally oscillating refuge will move to maintain constant position with respect to the refuge.

For the present study, we idealize the fin kinematics as a traveling sinusoid on an otherwise stationary (i.e. non-translating, non-rotating)membrane (Fig. 1C). As a consequence, the top edge of the fin remains fixed at all times, and all points on the fin below this edge move in a sinusoidal manner. The fin deformation is specified in Eqn 1below. As indicated in Fig. 1C,positive surge is defined as the force on the fin from the fluid in the direction from the tail to the head. If the traveling wave passes from the tail to the head, then the force on the fin from the fluid would be from the head to tail, corresponding to negative surge. Positive heave is vertically upward.

We chose to characterize the hydrodynamics of a fixed fin in a stationary flow because this is most relevant for understanding forces arising from maneuvering movements that occur when the body is at near-zero velocity with respect to the fluid far away from the body. In future work, using this approach will also allow us to compare our simulated force estimates with those obtained empirically from a robotic ribbon fin placed on a linear track pushing against a force sensor (Epstein et al., 2006). In subsequent studies, we will be examining the hydrodynamics of a stationary fin under imposed flow conditions and when the fin and an attached body are allowed to self-propel through the fluid.

Flow visualizations from computational simulations and DPIV indicate that the mechanism of thrust generation is a streamwise central jet and associated attached vortex rings. We show that, despite the lack of cylindrical symmetry in the morphology of the fin, its peculiar deformation pattern – the traveling wave – produces a jet flow often found in highly symmetric animal forms, such as jellyfish and squid. Whereas previous research focused exclusively on the surge force (Blake,1983 Lighthill and Blake,1990), we find that the ribbon fin is also able to generate a heave force, which pushes the body up. This arises from the generation of counter-rotating axial vortex pairs that are shed downward and laterally from the bottom edge of the fin. We hypothesize that the slanted angle of the fin base with respect to the spine observed in many gymnotids (e.g. Fig. 1A) leverages this heave force for forward translation. We also find that, in certain cases, as the number of waves on the fin decreases to below approximately two-thirds, the heave force surpasses the surge force. This switchover from an undulatory parallel thrust mode to an oscillatory normal thrust mode may provide insight into how the position and orientation of median fins varies with the length of the fin and the number of wavelengths that can be placed on it.

We show how the surge force from the fin scales as a function of a few key parameters. We found that for a stationary fin without imposed flow: (1) the surge force is proportional to (frequency) 2 × (angular amplitude) 3.5 ×(fin height/fin length) 3.9 ×Φ(wavelength/fin length), where Φ is a function that approximates the variation of surge force as a function of wavelength normalized with fin length (2) for angular deflections aboveθ=10 deg., where θ is defined in Fig. 1C, previous analytical work (Lighthill and Blake,1990) underestimated the magnitude of surge force (3) surge force shows a peak when the wavelength is approximately half the fin length, similar to what is observed biologically (Blake,1983) and contrary to the monotonic increase in force with wavelength predicted by the analytical results of Lighthill and Blake(Lighthill and Blake, 1990)and (4) the computed surge force compares well with empirical estimates based on fin kinematics and accelerations during swimming direction reversals, as well as with body drag measurements.

What Characteristics Do All Vertebrates Share?

Characteristics common to all vertebrates include bilateral symmetry, two pairs of jointed appendages, outer covering of protective cellular skin, metamerism, developed coeloms and internal skeletons, developed brains, vertebrae and sensory organs. Vertebrates also have respiratory systems, closed circulatory systems, genital and excretory systems and digestive tracts.

The 12 features that define vertebrates look different in various species but perform the same basic functions. All vertebrates have bilateral lines of symmetry, although some vertebrates look more symmetrical than others. Vertebrates have two pairs of appendages that facilitate locomotion, although these organs appear as fins in some vertebrates but as forelimbs and hind limbs in others.

Vertebrates have outer protective coverings of cellular skin, which appears as scales in some creatures and as hair and feathers in others. Vertebrates have coeloms, or body cavities, that are lined entirely with epithelium, or cellular tissue. This tissue is then divided into two or four distinct compartments.

These organisms also have structured internal skeletons made of cartilage and bone, which are separated into an axial skeleton and appendicular skeleton. All vertebrates have brains enclosed by skulls and nerve cords surrounded by strong vertebrae, which protect these vulnerable organs from damage. Lastly, vertebrates have genital and excretory systems and digestive tracts with livers and pancreases.

Siphonophore Biology

Siphonophores are complex, highly polymorphic creatures, whose “colonies” are composed of many polypoid and medusoid “individuals”, and yet they function physiologically as single individuals. Curiosity about the paradoxical nature of these animals prompted attention in the past, particularly during the latter half of the nineteenth century, when many researchers provided detailed descriptions of siphonophore anatomy from animals collected in their entirety at the surface of the oceans. These authors attempted to make sense of these complicated animals in terms of the polypoid or medusoid origins of their component “members”, and their relations with other hydrozoans. They were interested in finding out how such composite organisms could function effectively, and compete on equal terms with unitary zooplankton forms. Much of this knowledge of siphonophore morphology and life cycles also dates from that time, and the early years of the present century. However, during this period little attention was paid to the ecology of siphonophores, and it was not until the introduction of the “quantitative approach” to marine biology that the large-scale distributional patterns of many groups of pelagic organisms began to be investigated. At present, thanks to both in situ observations and to improved net sampling techniques, one has a better appreciation of the importance of siphonophores in the marine environment. This growing understanding of the group's importance highlights the need for an up-to-date treatment of siphonophore biology, and that is the goal for the present chapter.

Embryonic Development in Fish (With Diagram)

The embryonic development starts with the penetration of sperm in the egg. The process is called as impregnation. The sperm enters the egg through micropyle. In some fishes, the micropyle is funnel-shaped. As soon as sperm penetrates, there occurs a cortical reaction which prevents further entry of sperms.

Even in those cases where polyspermy occurs, only one sperm fuses with the egg nucleus. After the completion of the cortical reaction, the vitelline membrane is known as fertilization membrane.

The fertilization in teleost is external, taking place in water outside the body. So in these eggs, water hardening takes place, when the water uptake completes, the egg is turgid. The gametes of fishes have the fertilization capability even after leaving the body.

The ability could be artificially maintained by using modern techniques of cryopreservation i.e., by deep freezing at – 196 C. The preservation of gametes will help in breeding fish and also for improving stock for market and for selective purposes.

Amongst elasmobranchs, 12 families of Squaliformes are entirely viviparous, 2 are oviparous and 2 are mixed. The dogfish, Squalus canicula, is oviparous species, which lay shelled egg. Latimeria chalumnalis the only living representative of lobe finned fishes, Coelocanthine, gravid female contains advance young. Each has a large yolk sac with no apparent connection to the surrounding oviduct wall.

During the fertilization the pronuclei of sperm and ovum unite together with the fusion of cytoplasm. At this stage the egg contains yolk in the centre and cytoplasm occupy the periphery.

The quantity of cytoplasm is slightly more where the egg nuclear material is present. The cytoplasm is completely separated from the yolk in Cyprinus carpio. Ophiocephalus punctatus and Gasterosteus aculeatus. The vitelline membrane is double layered.

Fish spermatozoa is divisible into head, middle piece and tail (Fig. 21.1a-g). The teleost spermatozoa lack a head cap, the acrosome. The holostean spermatozoa are also devoid of acrosome. The fishes in which head is present in sperm, the heads are often ovoid and the middle piece is small. The tail is relatively long and contains microtubules and forms cytoskeletal framework.

The microtubules have 9 + 2 arrangement (Fig. 21.2). However, a few investigators in Anguilliformes and Elopiformes, have reported that the central microtubule shows 9 + 0 pattern. In viviparous fishes, head and middle piece are elongated.

The middle piece contains a substantial number of mitochondria as found in Poe cilia reticulata. Biflagellate cells are found in Porichthys notatus, and Ectalurus punctatus and Poecilia retulata. The mobility of the sperm is limited to a period of second to minute in freshwater spawners because of lysis. Motility is considerably longer in saltwater spawner. It is 15 minutes in Atlantic cod or several days in Herring.

There is little doubt that K + ions from seminal plasma blocks motility and the spermatozoan motility can be enhanced in proper physiological solutions by controlling pH and dilution of K + and proper fish Ringers solutions. This technique is used in the preservation of the spermatozoa (Cryopreservation).

The egg is surrounded by tough layer called chorion, next to chorion is the plasma or vitelline membrane or pellicle. This layer surrounds the yolk and cytoplasm (ooplasm). The yolk is present in considerable quantity in lungfish, Neoceratodus and Lepidosiren.

The quantity of yolk is more in cartilaginous fishes such as Acipenser. Egg size and yolk contents are independent variables. The chorion of teleost fish arises entirely from oocytes and made up of proteins and polysaccharide.

The unfertilized teleost eggs are gener­ally opaque and heavier than water. According to Swarup (1958), the eggs of newly caught female are light orange but if the female stickleback have been previously kept in aquarium and fed on tubifex, they are colourless. The eggs of Cyprinus carpio are yellow.

The eggs of some culturable variety of fishes are classified as non-floating and floating. The non-floating eggs are further divisible as non-adhesive and adhesive. The eggs of Catla catla are light red, Cirrhinus mrigala are brownish and Labeo rohita are reddish, but the eggs of Labeo calbasu are bluish. These eggs are non-floating and non-adhesive.

The eggs of Clarias batrachus and Heteropneustes fossilis are adhesive and non-filamentous and are green in colour. The eggs of Notopterus notoptorus and Notopterus chitala are yellowish. The floating eggs are of Channa punctatus and C. striatus. Their colour are amber.

Fertilized Eggs:

The fertilized eggs become gradually more and more transparent and the vitelline membrane separates from egg proper and develops a space known as perivitelline space, which is filled with fluid (Fig. 21.3a).

Formation of Blastodisc:

Just after fertilization, the cytoplasm which is present on the periphery, starts flowing towards the area where the sperm has probably entered the egg. The accumulation of cytoplasm is due to the contraction wave which is set in vegetal pole, pass through the equator and at the animal pole. The polarity at this stage is set up.

The completion of one contraction cycle takes about 2 minutes. About twenty of these cycles follows one another, each cycle adding more and more cytoplasm at the animal pole and soon forms a cap-like structure, the blastodermic cap or blastodisc (Fig. 21.3b).

The blastodisc in teleost is disc shape. Most of the eggs have two principal regions in common, a centre, which is relatively stable to centrifugation and an endoplasm which is displaceable containing yolk and other inclusion.

The further development in major teleosts are almost identical, followed by the process of cleavage. The teleost egg has blastoderm in the form of blastodisc, the cleavage is meroblastic, i.e., limited to blastodisc, the entire zygote is not divided.

The segmentation starts from 1 to 1 3/4 hours after fertilization. The factors which bring the cleavage are many but the major changes are the orientation of nuclear spindle and viscosity visible. They are parallel and on either side of the second cleavage plane and right angles to the first and third. In this way 16 cells are formed (Fig. 21.3f).

The 16-cell stage undergoes further division, but from now, the cleavage furrows are horizontal as well as vertical. The four central cells are divided by a horizontal division into 8 cells which are arranged in two layers of four cells each.

With the exception of the four coiner cells in which the division is more or less diagonal, the rest of the cells are divided by vertical division. These are either parallel to the first or to the second cleavage furrow. In this way the 32-cell stage is formed.

At the end of cleavage, a ball of cells the morula is formed. The total area occupied by the cells of the early morula is more or less the same as that of the original blastodermic disc (Fig. 21.3g). The superficial view of egg showing cleavage and formation of morula is given in diagrams (Fig. 21.4 A-K, A-F).

The cells of morula divide further and becomes smaller in size. In a side view, this stage appears as a mass of cells with prominent hemispherical projection and the convex base which rests in hollow concavity of the yolk (Fig. 21.3h). A large number of oil granules pass out from the cell mass into the yolk, where they combine to form bigger globules.

The cells of morula loose and they become separated from one another under slight pressure. A syncytial layer is formed between yolk and the convex base of the cell mass. This syncytium is called periblast. Cleavages result in the formation of two kinds of cells, blastoderm or periblast.

The blastoderm cells are distinct and produce the embryo. The periblast or trophoblast cells lie between the yolk and cells of blastoderm and cover the entire yolk mass, having originated from the most marginal and outlying blastomeres. This syncytial layer helps in the mobilization of yolk reserves.

The nuclei arise at the edge of the blastoderm from division of marginal cell nuclei, each resultant nuclei is drawn into the divided yolk protoplasm or the periblast. The periblast is syncytial i.e., multinucleated cytoplasm.

These nuclei resemble the nuclei of the blastomeres and since spindle, asters and chromosomes were observed, it is concluded that they divide mitotically. According to Beer’s law, percent transmission of a nucleus would be inversely proportional to the number of absorbing molecules in that nucleus, and the relationship would be logarithmic rather than linear.

The cleavages or segmentations results in the formation of two kinds of cells, the ‘blastoderm’ and ‘periblast’ (Fig. 21.5a-g). The embryo is formed by the blastoderm, while the periblast or trophoblast cells, which lie between yolk and cells of blastoderm, which is syncytial in nature helps in mobilization of yolk reserves.

There are substantial cohesive forces between developing blastomeres and the surrounding periblast which are important in the subsequent morphogenetic movement. It is suggested that periblast acts as an intermediary between two ‘non-wettable’ components—blastoderm and yolk. When the blastoderms diameter is 4/5 of the egg diameter, it is converted into blastula.

The hemispherical mass of cells of morula project from the yolk. The cells then flattened and extend outwards. The periphery of the blastodisc lines with the periphery of the yolk. The marginal or peripheral cells remain in close contact with the periblast. Whereas the central cells of the floor of the blastodisc are raised.

As these cells are raised, a space is developed. This space is called as segmentation cavity or blastocoel (Fig. 21.5a). Soon blastocoel becomes well developed. The blastula formation starts 15 hours after fertilization in stickleback while it takes 8 hours after fertilization in Cyprinus carpio.

At the end of segmentation, the blastodisc becomes radially symmetrical. The radial symmetry changes to bilateral symmetry because the flattening of cell mass is expressed in one sector and thus this region becomes thicker.

The thicker sector is very important because it is embryonic material and future embryo develops from it, and its median plane becomes the median plane of the embryo. At this stage the anterior and posterior sides of future embryo are also fixed. The distal part of the thicker sector is the prospective posterior end of the embryo and its central part corresponds to the prospective anterior end of the embryo.

The appearance of distinct primitive streak on the embryonic shield is the beginning of gastrula. The gastrulation generally ends with the closure of blastopore. According to Riley (1974), this distinction is arbitrary. Both epiboly and emboly actively take part in the formation of gastrula.

Invagination or Emboly:

It takes place at about 21-26 hours after fertilization, the cells of the thicker sector invaginate at the limit of cytoplasm and yolk. This marks the beginning of gastrulation (Fig. 21.6a-d). The invagination which originally starts at one point, then extends laterally around the edge of the blastoderm and soon spread to the periphery of the entire blastodisc.

The invaginated layer does not extend over the floor of the sub-germinal cavity but is confined to the edges of the blastoderm thus forming a prominent ring, known as ‘germ ring’ (Fig. 21.5b). The only part which shows further invagination is the region of the germ ring which is formed by the thick sector of blastoderm disc.

As soon as the germ ring becomes established, it moves towards the yolk of the egg (Fig. 21.5b). The width remains constant but it increases in its circumference. Regarding further invagination, it advances more rapidly at one place than round the rest of the periphery of the blastoderm making it in triangular shape (Fig. 21.5c). The apex is pointing towards the animal pole of the egg.

The embryo loses its triangular shape and becomes elongated. If blastoderm is seen from above, the posterior pole is roughly triangular which is thicker than the adjacent area. This makes the embryonic shield more prominent. The embryonic shield has been differentiated as embryonic and extra embryonic area.

The embryonic shield is made up of an epiblast of polygonic cells covered by epidermic stratum and a complex lower layer which is known as entochordamesoblast. This entochordamesoblast is the analogue of mesoderm and endoderm. The thickened median portion will become the prechordal plate and chorda, while somewhat loosely arranged cells will form entoderm.

In the extra embryonic region an elongated sub-germinal cavity bounded laterally by germ ring extends between the periblast and epiblast.

The presumptive mesoderm in the meantime has coverage towards the dorsolateral edges of the blastodisc where it involutes, passing to the inside between the entoderm and ectoderm. The mesoderm becomes arranged on either side of the median notochordal material in developing embryo.

The notochord, prechordal plate and mesoderm which are differentiated but continue with entochordamesoblast, later ectoderm differentiates and all three germinal layers are distinguished ectoderm, mesoderm and entoderm. With the advancement of development notochord, Kuffer’s vesicle and neural plate are differentiated.

Simultaneously with emboly, the epiboly also starts and the cells overgrow the yolk and at the same time migrate at its periphery. The blastoderm become flattened. The flattening of blastoderm causes it to spread over yolk.

Eventually the rim of blastoderm converges at or near the opposite side of the yolk and the opening closes by the contraction of rim. The rim of the blastodisc corresponds to the lip of the blastopore. Later before the blastopore closes, a yolk plug is seen projecting from the blastopore (Fig. 21.5d).

Organisation of Fish Embryo:

Presumptive areas can be mapped out in the wall of the gastrula. The fate map for gastrula of Cyprinus carpio has been given by Verma (1971) (Figs. 21.7A-F & Fig. 21.8).

About 27 to 50 hours after fertilization, due to further contraction of the lips, the blastopore closes

The presumptive notochordal cells migrate inwards and roll up along the posterior edge of the blastoderm and thus form a solid string like notochord.

The presumptive neural plate sinks down to the space vacated by the inwardly migrated notochordal cells. The edges of neural plates rise up and fuse with each other at the middle line enclosing a cavity, ‘neurocoel’. Thus a hollow tube like neural tube is formed just above the notochord. The anterior part of neural tube swells to form the brain whereas the part behind it remains as such and forms the spinal cord.

By the two consecutive invaginations in the brain, it is differentiated into fore, mid and hind parts. The optic lobes appear by lateral outgrowth from the forebrain. The un-segmented parts of the embryo converge towards the embryonic axis.

This convergence along with the development of the central nervous system causes a thickening of the embryo proper, which now protrudes from the surface of the egg extending approximately half way round the circumference of the yolk sphere.

In a few hours, after further contraction of lips, the blastopore closes 60 hours after fertilization in stickleback and 21 hours after fertilization in Cyprinus carpio, 5-10 parts of somites appear in the middle of embryo on either lateral side of the nerve cord (Fig. 21.5g). Each pair of somite is formed by lateral mesoderm. Later the somites give rise to the muscle of the trunk, appendages and their skeleton.

Simultaneously blastopore closes, the whole germ ring fuse with embryo proper which now appears elevated and well demarcated from the yolk. By the development of central cavities in optic lobes, they become optic vesicles (Figs. 21.9a-e).

The appearance of optic capsule and Kuffer’s vesicle developed after 30 hours of fertilization in Cyprinus carpio. The head of embryo is differentiated further. The optic vesicles are converted into optic cups and the lenses are also formed. The brain develops as a median dorsal furrow which widens in fore and mid brain to ventricle which opens dorsally.

Lateral to the hind brain a pair of optic capsules can be recognized. Ventral to the posterior part of brain the pericardium appears although heart is not yet visible. The somites increase in number, Kuffer’s vesicle is now visible ventrally at the posterior end of the body.

Heart and tail develop at 88 hours of development in stickleback and 55 hours in Cyprinus carpio. Before this at 70 hours of development the pectoral fins and ventricle are formed and forebrain closes.

After the development of the various organs in the embryo, its body becomes cylindrical and bilaterally symmetrical. The connection between the body and yolk sac gradually narrows to form a stalk. The yolk sac gradually decreases in size as the embryo grows. Finally embryo hatches into a small free swimming larva.

The freshly developed larva of Cyprinus carpio measures about 4.5 mm in length which is characterised by (a) the head slightly bent on the yolk, (b) mouth is open but no alimentary canal, eyes are large, pectoral fins are rudimentary and the tail is heterocercal (Fig. 21.10a-e).

One day old larva increases to 5.5 mm in length. The head becomes straight than the preceding stage where the head is slightly bent. Eyes become dark black, heart enlarges in size and the alimentary canal differentiated above the yolk sac. Mouth is bounded by the jaws but covered by a thin membrane. Gill arches with rudimentary gill filaments are developed which are not yet covered over by an operculum.

In two days the mouth of larva opens and becomes slit like, alimentary canal opens through anus. At this stage larva starts respiration with gills and feeding with mouth. There is complete absorption of yolk at 7 mm stage larvae which is almost 4 days old. At 10 days, the larva assumes the shape of a fish with a convex dorsal profile.

Feeding, starvation and weight change of early fish larva of Tilapia sparmanii and Paralichthys oliyaceus (marine) have been studied by Ishibashi (1974). According to him, incubation time for Tilapia was 48 hours at 27 °C. The total length was 4.2 mm at hatching, the larva being inactive and without a functional mouth.

After two days mouth opens, caudal fin starts to move actively, and after three days the larva begins to swim. Weight increases rapidly after hatching. The weight on third day was 0.65 mg heavier than at hatching.

At this stage the larva began to take food and weight of 8.80 mg was increased and on ninth day, the unfed larvae were inactive and weight was 1.24 mg, 25% less than that on third day. Only 1% of the unfed larvae survived to day 12.

Factors influencing Early Survival:

Light, oxygen, temperature and feeding are some important factors which are responsible for survival during embryonic development. According to Pinus (1974), tiulka (Culpeonella delicatula) is most abundant fish of the sea of Azon with catches amounting to 40-50% of the total fish landed from this sea. He found optimum condition for the survival or eggs of this fish, when the temperature reaches up to 15-18 °C.

Biochemistry of Egg of Fishes:

The fish eggs with its relatively bulky yolk is the most difficult subject for chemical analysis. Information has been sought from the techniques of cytochemistry and more refined modern methods of electrophoresis and chromatography. The analysis of 100 mg egg of Salmo irideus is as follows.

Young and Inman (1938) found 0.4% of ash and about 4.38% of volatile material. However, arginine, histidine and lysine present in respective ratio of 4 : 1 : 3. Hayes (1930) found very little glucose in the salmon egg, only 0.049 mg for 100 mg of egg. The rest of carbohydrate probably is bound to protein.

Amino acid composition of Salmo gairdneri egg during development:

Enzyme in Fishes:

The chorion contains an enzyme known as chorionase. The chorion also resists digestion by trypsin and pepsin, it possesses pseudo-keratin. The hardening of chorion is due to the enzyme in the perivitelline fluid. Ca ++ affects the enzyme rather than chorion itself.

Hardening resulted from oxidation of SH to SS groups by means of aldehydes produced by polysaccharide with a glycol groups. Hatching enzyme works best under alkaline condition, pH 7.2-9.6 and temperature of 14-20 °C.

Acetyl cholinesterase, the enzyme associated with nervous stimulation of the muscles, has been detected on 10th day after fertilization in Salmo gairdneri eggs. In the eye stage increase in activity is almost coinciding with the development of excitable tissue. Ornithine transcarbamylase and arginase of the five O-U cycle enzymes were reported in the egg of Salmo which were capable of excreting nitrogen.

Metabolism of Nitrogenous Wastes in Fishes:

Metabolism and nitrogenous wastes in the eggs and alevins of Rainbow trout, Salmo gairdneri were studied by Rice and Stoke (1974). Ammonia, urea, uric acid and total protein and free arginine were recorded in eggs in alevines.

According to them, ammonia and concentrations through hatching and greatest concentrations were found in alevines. Ammonia level increases for the first few days, then declines as the yolk was absorbed. Uric acid concentration did not change dramatically during development, but concentration before and after hatching period were lower than just shortly after fertilization and when the yolk was nearly absorbed.

Urea and free arginine both increased in concentration while the developing embryo was still within chorion. Peak concentration of urea and arginine occurred soon after hatching but thereafter concentration of both compounds decrease. Protein concentrations were relatively constant during first 25 days of embryonic development, steadily decline thereafter.

Production of ammonia in salmon embryo is expected even though metabolism of fat is the predominant source of energy. The proteins in the yolk are broken down to amino acids. Before they are resynthesized into protein in the embryo, an amino acid pool is available for catabolism.

However, most amino acids are resynthesized into protein rather than catabolized, because total egg protein values remain stable until the day 21, after which catabolism leads to a decrease in total protein.

Urea appeared to be synthesized when from yolk proteins were degraded by arginine. Rice and Stoke (1974) supported the hypothesis that O-U cycle enzymes can function to produce intermediates in other metabolic pathways.

Swarup (1958, 1959 a, b, c, d) found twin forms if the freshly fertilized egg of G. aculeatus were subjected to heat and cold (32.5 to 37°C and 0 to 1/2°C). The malformation includes synophthalmia, monophthalmia, microphthalmia and anoph­thalmia. Not only above mentioned changes occur but abnormality in blastodisc also occur due to high and low temperature.


I've been thinking about this off an on.
You could have several species of these evolve. I'm picturing a predator which extends its head and body out in front of the propeller part. It's many legs have evolved to fit inside the plant and spin it around. The whole assemblage is torpedo like.
Eyes and mouth are forward facing like a shark, and it steers by flexing its body.

Macro animals aren't going to do this.

Single-celled creatures can do this because they don't need to supply the rotating body component with nutrients, those come in from the surrounding medium. The flagellum used by some bacteria is essentially a biological rotary engine. It likely evolved from a type three secretion system, which has very similar components.

Macro scale is much harder to accomplish. Spinning parts don't exist on the macro scale. Having the entire body as the corkscrew doesn't work well because they still need some way to push to get spinning in the first place. You've probably seen octopuses doing something that looks like corkscrewing through the water, but they use jet-propulsion to get themselves going and sometimes spin while doing it.

To have a body part that spins freely is a problem. It's not well connected to the rest of the body. Blood vessels, tendons, muscles, and whatever can't be used between the main body and spinning portion.

It's not more efficient to rotate for macro animals. If you look at gymnoformes, they are sort of (from one frame of reference) partially rotating to achieve locomotion. But rotating oneself in order to propel forward is like adding an unnecessary energy conversion step.

I can imagine some small animals that might use a external component, like a shell, which they would spin to propel themselves through the water. It wouldn't be very efficient and would probably only work in calm waters.

I can't see an evolutionary path to why this would happen, but I believe that physiologically, it is possible.

Consider the human arm. The ball and socket joint at the shoulder allows for full circumduction of the arm (imagine winding up and pitching a softball or bowling a cricket ball). If we have an underwater creature with a couple of appendages of the right shape and in the right places, and joints with the correct articulation, I don't see why it isn't possible.

However, I don't see what the evolutionary benefit of this would be. This seems to be something more like a mad scientist would piece together. I don't know what the thrust of such a mechanism would be, but I can imagine the energy requirements for sustained locomotion of the Frankenfish would be quite high.

I hate saying that something is not possible. After all, this is worldbuilding. So, I'll try to make a case for evolution of rotational locomotion in a macro-organism. Incidentally, the mechanism of movement is different than what I described above.

As sea levels have been steadily rising and land area has been steadily decreasing, land mammals, which had previously evolved onto land, are now evolving back into water. Some animals (amphibians, reptiles) are obviously better equipped to deal with this than others. But there is one interesting case to consider: the field mouse.

It started as field mice had to swim more often than they used to, from one piece of land to another. The mice that ended up surviving more often than not were the ones that were better swimmers - a trait that became primarily determined by their tails. Eventually, mice ended up spending more time in water than out of water. Their physical features changed over the centuries to reflect their aquatic lifestyles: their feet shrank to reduce drag, they could hold their breath for incredible amounts of time, they had special eyelids so they could see well underwater, and their tails grew powerful as a flagellum-like appendage for movement.

Watch the video: Find the axis of symmetry and your vertex (November 2022).