Fish Anatomy

Fish anatomy is primarily governed by the physical characteristics of water, which is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs more light than air does.

External fish anatomy http://commons.wikimedia.org/wiki/File:Fish_mouths.orizz.png. Public domain 14/03/11
(A) caudal fin - (B) dorsal fin - (C) lateral line - (D) nostril - (E) barbel
(F) operculum - (G) pectoral fin - (H) pelvic fin - (I) vent - (J) anal fin

Fins

The fins are the most distinctive features of a fish, composed of bony spines protruding from the body with skin covering them and joining them together, either in a webbed fashion, as seen in most bony fish, or more similar to a flipper, as seen in sharks. These usually serve as a means for the fish to swim. Fins can also be used for gliding or crawling, as seen in the flying fish and frogfish. Fins located in different places on the fish serve different purposes, such as moving forward, turning, and keeping an upright position.

Spines and rays
In bony fish, most fins may have spines or rays. A fin can contain only spiny rays, only soft rays, or a combination of both. If both are present, the spiny rays are always anterior. Spines are generally stiff and sharp. Rays are generally soft, flexible, segmented, and may be branched. This segmentation of rays is the main difference that separates them from spines; spines may be flexible in certain species, but they will never be segmented.

Spines have a variety of uses. In catfish, they are used as a form of defense; many catfish have the ability to lock their spines outwards. Trigger fish also use spines to lock themselves in crevices to prevent them being pulled out.

Types of fin
Dorsal fins are located on the back. A fish can have up to three of them. The dorsal fins serve to protect the fish against rolling, and assists in sudden turns and stops. In anglerfish, the anterior of the dorsal fin is modified into an illicium and esca, a biological equivalent to a fishing pole and a lure. The bones that support the dorsal fin are called Pterygiophore. There are two to three of them: "proximal", "middle", and "distal". In spinous fins the distal is often fused to the middle, or not present at all.

types of caudal fin http://en.wikipedia.org/wiki/File:PletwyRyb.svg. Public domain 14/03/11
Types of caudal fin: (A) - Heterocercal, (B) - Protocercal, (C) - Homocercal, (D) - Diphycercal

The caudal fin is the tail fin, located at the end of the caudal peduncle and is used for propulsion. The tail can be:

  • Heterocercal, which means that the vertebrae extend into a larger lobe of the tail or that the tail is asymmetrical
  • Epicercal means that the upper lobe is longer (as in sharks)
  • Hypocercal means that the lower lobe is longer (as in flying fish)
  • Protocercal means that the caudal fin extends around the vertebral column, present in embryonic fish and hagfish. This is not to be confused with a caudal fin that has fused with the dorsal and anal fins to form a contiguous fin.
  • Diphycercal refers to the special, three-lobed caudal fin of the coelacanth and lungfish where the vertebrae extend all the way to the end of the tail.
  • Most fish have a homocercal tail, where the vertebrae do not extend into a lobe and the fin is more or less symmetrical. This can be expressed in a variety of shapes.
    • The tail fin may be rounded at the end.
    • The tail fin may be truncated, or end in a more-or-less vertical edge (such as in salmon).
    • The fin may be forked, or end in two prongs.
    • The tail fin may be emarginate, or with a slight inward curve.
    • The tail fin may be lunate, or shaped like a crescent moon.

The anal fin is located on the ventral surface behind the anus. This fin is used to stabilize the fish while swimming.

The paired pectoral fins are located on each side, usually just behind the operculum, and are homologous to the forelimbs of tetrapods. A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish, such as sharks, in maintaining depth and also enables the "flight" for flying fish.

In some fish, the pectoral fins aid in walking, especially in the lobe-like fins of some anglerfish and in the mudskipper. Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robins and flying gurnards.

The "horns" of manta rays and their relatives are called cephalic fins; this is actually a modification of the anterior portion of the pectoral fin.

The paired pelvic or ventral fins are located ventrally below the pectoral fins. They are homologous to the hindlimbs of tetrapods. The pelvic fin assists the fish in going up or down through the water, turning sharply, and stopping quickly. In gobies, the pelvic fins are often fused into a single sucker disk. This can be used to attach to objects.

The adipose fin is a soft, fleshy fin found on the back behind the dorsal fin and just forward of the caudal fin. It is absent in many fish families, but is found in Salmonidae, characins and catfishes. It's function is unknown, and it is frequently clipped off to mark hatchery-raised fish, though recent data shows that trout with their adipose fin removed have an 8% higher tailbeat frequency.

Some types of fast-swimming fish have a horizontal caudal keel just forward of the tail fin. This is a lateral ridge on the caudal peduncle, usually composed of scutes (see below), that provides stability and support to the caudal fin. There may be a single paired keel, one on each side, or two pairs above and below.

Finlets are small fins, generally behind the dorsal and anal fins (in bichirs, there are only finlets on the dorsal surface and no dorsal fin). In some fish such as tuna or sauries, they are rayless, non-retractable, and found between the last dorsal and/or anal fin and the caudal fin.

Skin & Scales

The outer body of many fish is covered with scales. Some species are covered instead by scutes. Others have no outer covering on the skin; these are called naked fish. Most fish are covered in a protective layer of slime (mucus).

There are four types of fish scales.

  • Placoid scales, also called dermal denticles, are similar to teeth in that they are made of dentin covered by enamel. They are typical of sharks and rays.
  • Ganoid scales are flat, basal-looking scales that cover a fish body with little overlapping. They are typical of gar and bichirs.
  • Cycloid scales are small oval-shaped scales with growth rings. Bowfin and remora have cycloid scales.
  • Ctenoid scales are similar to the cycloid scales, with growth rings. They are distinguished by spines that cover one edge. Halibut have this type of scale.

Another, less common, type of scale is the scute, which is:

  • an external shield-like bony plate, or
  • a modified, thickened scale that often is keeled or spiny, or
  • a projecting, modified (rough and strongly ridged) scale, usually associated with the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile. Some fish, such as pineconefish, are completely or partially covered in scutes.
Skeleton
Skeleton: (A) hypural - (B) neural spine - (C) vertebra - (D) posteria dorsal fin ray - (E) radial cartilage - (F) anterior dorsal fin ray
(G) opercular - (H) skull - (I) orbit - (J). upper jaw - (K) lower jaw - (L) clavicle - (M) pelvic girdle - (N) pectoral fin ray
(O) pelvic fin ray - (P) rib - (Q) radial cartilage - (R) anal fin ray - (S) hemal spine - (T) caudal fin ray

Vertebrae

The vertebrae of lobe-finned fishes consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates. Just beneath the arch lies a small plate-like pleurocentrum, which protects the upper surface of the notochord, and below that, a larger arch-shaped intercentrum to protect the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in primitive tetrapods, but, in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body.

In most ray-finned fishes, including all teleosts, these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibians, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods.

In cartilagenous fish, such as sharks, the vertebrae consist of two cartilagenous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilagenous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification.

Lampreys have vertebral arches, but nothing resembling the vertebral bodies found in all higher vertebrates. Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. Hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in the tail.

Positions of the fish's mouths http://commons.wikimedia.org/wiki/File:Fish_mouths.orizz.png. Public domain 14/03/11
Mouth types: (a) - final, (b) - upper, (c) - bottom

Mouth

The position of the mouth gives an indication of the feeding habits of a species. Mid water feeding species usually have a forward pointing mouth, surface feeding species generally have an upturned mouth and bottom feeding species usually have a down turned mouth, often with barbels.

Barbels
A barbel on a fish is a slender, whiskerlike tactile organ near the mouth. Fish that have barbels include the catfish, the carp, the goatfish, sturgeon, the zebrafish (Danio rerio) and some species of shark. They house the taste buds of such fish and are used to search for food in murky water.

Barbels may be located in a variety of places. Maxillary barbels refer to barbels on either side of the mouth. Barbels may also be nasal, or extended from the nostrils. Also, barbels are often mandibular or mental, or located on the chin.

Jaw
Linkage systems are widely distributed in animals. The most thorough overview of the different types of linkages in animals has been provided by M. Muller, who also designed a new classification system, which is especially well suited for biological systems.

Linkage mechanisms are especially frequent and manifold in the head of bony fishes, such as wrasses, which have evolved many specialized feeding mechanisms. Especially advanced are the linkage mechanisms of jaw protrusion. For suction feeding a system of linked four-bar linkages is responsible for the coordinated opening of the mouth and 3-D expansion of the buccal cavity. Other linkages are responsible for protrusion of the premaxilla.

The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian. 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.

It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increased respiration efficiency. The jaws were used in the buccal pump (observable in modern fish and amphibians) that pumps 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.

Internal Organs

Internal organs
Internal organs: (A) lateral line - (B) spleen - (C) kidney - (D) swim bladder - (E) Weberian ossicles - (F) inner ear - (G) brain
(H) eye - (I) gills - (J) heart - (K) liver - (L) stomach - (M) gall bladder - (N) intestine - (O) gonads: testes/ovaries

Lateral line
The lateral line is a sense organ in aquatic organisms (chiefly fish), used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail. Sometimes parts of the lateral organ are modified into electroreceptors, which are organs used to detect electrical impulses. It is possible that vertebrates such as sharks use the lateral organs to detect magnetic fields as well (along with Ampullae of Lorenzini). Most amphibian larvae and some adult amphibians also have a lateral organ. Some crustaceans and cephalopods have similar organs.

The receptors in the lateral line are neuromasts, each of which is composed of a group of hair cells. The hairs are surrounded by a protruding jelly-like cupula, typically 1/10 to 1/5 mm long. The hair cells and cupolas of the neuromasts are usually at the bottom of a visible pit or groove in the fish. The hair cells in the lateral line are similar to the hair cells inside the vertebrate inner ear, suggesting that the lateral line and the inner ear share a common origin.

Teleosts and elasmobranchs usually have lateral-line canals, in which the neuromasts are not directly exposed to the environment, but communicate with it via canal pores. Additional neuromasts may appear individually at various locations on the body surface.

The development of the lateral-line system depends on the organisms's mode of life. For instance, fish that are active swimming types tend to have more neuromasts in canals than they have on their surface, and the line will be farther away from the pectoral fins, which probably reduces the amount of "noise" that is generated by fin motion.

The lateral-line system helps the fish to avoid collisions, to orient itself in relation to water currents, and to locate prey. For instance, the blind, cave-living Mexican tetra have rows of neuromasts on their heads, which appear to be used to precisely locate food without the use of sight; killifish are able to use their lateral line organ to sense the ripples made by insects struggling on the water's surface. Experiments with pollock have shown that the lateral line is also a key enabler for schooling behavior.

It has also been suggested that the lateral line may give sharks advanced warning of frontal pressure systems and that they use it to avoid severe weather conditions that may result in injury. It was observed that during Hurricane Gabrielle, which struck Florida in 2001, juvenile black tip sharks moved to deeper waters as the storm approached.

Ampullae of Lorenzini
The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled canals. They are mostly discussed as being found in cartilaginous fishes (sharks, rays, and chimaeras); however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon.Lungfish have also been reported to have them. Teleosts have re-evolved a different type of electroreceptors. They were first described by Stefano Lorenzini in 1678.

These sensory organs help fish to sense electric fields in the water. Each ampulla consists of a jelly-filled canal opening to the surface by a pore in the skin and ending blindly in a cluster of small pockets full of special jelly. The ampullae are mostly clustered into groups inside the body, each cluster having ampullae connecting with different parts of the skin, but preserving a left-right symmetry. The canal lengths vary from animal to animal, but the distribution of the pores is generally specific to each species. The ampullae pores are plainly visible as dark spots in the skin. They provide fish with a sixth sense capable of detecting electromagnetic fields as well as temperature gradients.

The ampullae detect electric fields in the water, or more precisely the difference between the voltage at the skin pore and the voltage at the base of the electroreceptor cells. A positive pore stimulus would decrease the rate of nerve activity coming from the electroreceptor cells, and a negative pore stimulus would increase the rate of nerve activity coming from the electroreceptor cells.

Sharks may be more sensitive to electric fields than any other animal, with a threshold of sensitivity as low as 5 nV/cm. That is 5/1,000,000,000 of a volt measured in a centimeter-long ampulla. Since all living creatures produce an electrical field by muscle contractions, it is easy to imagine that a shark may pick up weak electrical stimuli from the muscle contractions of animals, particularly prey. On the other hand, the electrochemical fields generated by paralyzed prey were sufficient to elicit a feeding attack from sharks and rays in experimental tanks; therefore muscle contractions are not necessary to attract the animals. Sharks and rays can locate prey buried in the sand, or DC electric dipoles simulating the main feature of the electric field of a prey buried in the sand.

The electric fields produced by oceanic currents moving in the magnetic field of the earth are of the same order of magnitude as the electric fields that sharks and rays are capable of sensing. This could mean that sharks and rays can orient to the electric fields of oceanic currents, and use other sources of electric fields in the ocean for local orientation. Additionally, the electric field they induce in their bodies when swimming in the magnetic field of the earth may enable them to sense their magnetic heading.

At least one company sells a shark repellent that makes use of shark electroreceptors. It uses large magnets formed into an ankle bracelet and it is reported that field tests show sharks moving away when coming near them.

Early in the 20th century, the purpose of the ampullae was not clearly understood, and electrophysiological experiments suggested a sensibility to temperature, mechanical pressure and possibly salinity. It was not until 1960 that the ampullae were clearly identified as specialized receptor organs for sensing electric fields. The ampullae may also allow the shark to detect changes in water temperature. Each ampulla is a bundle of sensory cells containing multiple nerve fibres. These fibres are enclosed in a gel-filled tubule which has a direct opening to the surface through a pore. The gel is a glycoprotein based substance with the same resistivity as seawater, and it has electrical properties similar to a semiconductor. This has been suggested as a mechanism by which temperature changes are transduced into an electrical signal that the shark may use to detect temperature gradients, although it is a subject of debate in the scientific literature.

Operculum
The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body.

The operculum is composed of four fused bones; the opercle, preopercle, interopercle, and subopercle. These appear to be derived from the separate gill-slit covers of an elasmobranch ancestor of the teleost fishes. The posterior rim of the operculum is equipped with a flexible, ribbed structure which acts as a seal to prevent reverse water flow during respiration. The morphology of this anatomical feature varies greatly between species. For example, the bluegill (Lepomis macrochirus) has a posteriorly and dorsally oriented rounded extension with a small black splotch present. In some species, the operculum can push water from the buccal cavity through the gills.

For some fish, opercula are vital in obtaining oxygen. They open as the mouth closes, causing the pressure inside the fish to drop. Water then flows towards the lower pressure across the fish's gill lamellae, allowing some oxygen to be absorbed from the water.

Gills
The gills, located under the operculum, are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. They are not usually visible, but can be seen in some species, such as the frilled shark.

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist. The microscopic structure of a gill presents a large surface area to the external environment.

Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area. A high surface area is crucial to the gas exchange of aquatic organisms as water contains only 1/20 the dissolved oxygen that air does.

With the exception of some aquatic insects, the filaments and lamellae (folds) contain blood or coelomic fluid, from which gases are exchanged through the thin walls. The blood carries oxygen to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water. Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, crustaceans, insects, fish, and amphibians.

In bony fish, the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so that bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside the mouth keep the water from escaping.

The gill arches of bony fish typically have no septum, so that the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers. Though all but the most primitive bony fish lack a spiracle, the pseudobranch associated with it often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.

Marine teleosts also use gills to excrete electrolytes. The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids. Saltwater is less dilute than these internal fluids, so saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of seawater and excrete the salt. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills.

Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six or seven pairs. Adjacent slits are separated by a cartilaginous gill arch from which projects a long sheet-like septum, partly supported by a further piece of cartilage called the gill ray. The individual lamellae of the gills lie on either side of the septum. The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water.

A smaller opening, the spiracle, lies in front of the first gill slit. This bears a small pseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills. The spiracle is thought to be homologous to the ear opening in higher vertebrates.

Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.

Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an operculum, developed from the septum of the gill arch in front of the first gill.

Lampreys and hagfish do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally. In adult lampreys, a separate respiratory tube develops beneath the pharynx proper, separating food and water from respiration by closing a valve at its anterior end.

Swim bladder
The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming. The swim bladder is also of use as a stabilizing agent because in the upright position the center of mass is below the center of volume due to the dorsal position of the swim bladder. Another function of the swim bladder is the use as a resonating chamber to produce or receive sound. The swim bladder is evolutionarily homologous to the lungs, and Charles Darwin himself remarked upon this in On the Origin of Species.

Swim bladders are only found in ray-finned fish. In the embryonic stages some species have lost the swim bladder again, mostly bottom dwellers like the weather fish. Other fishes like the Opah and the Pomfret use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom dwelling sea robin can use their pectoral fins to produce lift while swimming. The cartilaginous fish (e.g. sharks and rays) and lobe-finned fish do not have swim bladders. They can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.

The swim bladder normally consists of two gas-filled sacs located in the dorsal portion of the fish, although in a few primitive species, there is only a single sac. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. By adjusting the gas pressure using the gas gland or oval window the fish can obtain neutral buoyancy and ascend and descend to a large range of depths. Due to the dorsal position it gives the fish lateral stability.

In physostomous swim bladders, a connection is retained between the swim bladder and the gut, the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air and filling the swim bladder. Excess gas can be removed in a similar manner.

In more derived varieties of fish, the physoclisti, the connection to the digestive tract is lost. In early life stages, fish have to rise to the surface to fill up their swim bladders, however, in later stages the connection disappears and the gas gland has to introduce gas (usually oxygen) to the bladder to increase its volume and thus increase buoyancy. In order to introduce gas into the bladder, the gas gland excretes lactic acid and produces carbon dioxide. The resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffuses partly into the swim bladder. The blood flowing back to the body first enters a rete mirabile where virtually all the excess carbon dioxide and oxygen produced in the gas gland diffuses back to the arteries supplying the gas gland. Thus a very high gas pressure of oxygen can be obtained, which can even account for the presence of gas in the swim bladders of deep sea fish like the eel, requiring a pressure of hundreds of bars. Elsewhere, at a similar structure known as the oval window, the bladder is in contact with blood and the oxygen can diffuse back. Together with oxygen other gases are salted out in the swim bladder which accounts for the high pressures of other gases as well.

The combination of gases in the bladder varies. In shallow water fish, the ratios closely approximate that of the atmosphere, while deep sea fish tend to have higher percentages of oxygen. For instance, the eel Synaphobranchus has been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and 0.4% argon in its swim bladder.

Physoclist swim bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. Physostomes can "burp" out gas, though this complicates the process of re-submergence.

In some fish, mainly freshwater species (e.g. common carp, wels catfish), the swim bladder is connected to the labyrinth of the inner ear by the Weberian apparatus, a bony structure derived from the vertebrae, which provides a precise sense of water pressure (and thus depth), and improves hearing.

Swim bladders are evolutionarily closely related (i.e. homologous) to lungs. It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g. lungfish, gar, and bichir) and into the swim bladders of the ray-finned fish. In embryonal development, both lung and swim bladder originate as an outpocketing from the gut; in the case of swim bladders, this connection to the gut continues to exist as the pneumatic duct in the more "primitive" ray-finned fish, and is lost in some of the more derived teleost orders. There are no animals which have both lungs and a swim bladder.

The cartilaginous fish (e.g. sharks and rays) split from the other fishes about 420 million years ago and lack both lungs and swim bladders, suggesting that these structures evolved after that split. Correspondingly, these fish also have a heterocercal fin which provides the necessary lift needed due to the lack of swim bladders. On the other hand, teleost fish with swim bladders have neutral buoyancy and have no need for this lift.

Weberian apparatus
The Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system in fishes belonging to the Superorder Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes collectively referred to as the Weberian ossicles. The presence of the structure is one of the most important and phylogenetically significant distinguishing characteristics of the Ostariophysi. The structure itself consists of a set of minute bones that originate from the first few vertebrae to develop in an embryonic ostariophysan. These bones grow to physically connect the auditory system, specifically the inner ear to the swim bladder.

The generalized structure of the Weberian apparatus is akin to a skeletal complex of bones and ossicles that are physically connected to the labyrinth auditory complex anteriorly and the anteriormost region of the swim bladder posteriorly. The entire structure is derived from skeletal elements of the first four vertebrae. The involved elements include: The supraneural bones of the skull; modified neural arch bones, specifically the paired claustra and the scaphia; The intercalarium and the lateral processes; The tripus; The os suspensorium from the fourth vertebrae; The parapophysis of vertebrae #5 including the vertebrae itself, plus the vertebrae's corresponding pleural rib. In addition, a structure composed of fused neural spines form the dorsalmost part of the Weberian apparatus. Together, the structure interacts anteriorly with the lagenar otolith set within the skull and posteriorly with the swim bladder via the pleural rib. Postero-ventrally, it is the tripus, the os suspensorium and the 3rd rib that interact directly with the anterior chamber of the swim bladder.

The Weberian apparatus functions by transmitting auditory signals straight from the gas bladder, through the Weberian ossicles and then straight into the labyrinth structures of the inner ear. The structure essentially acts as an amplifier of sound waves that would otherwise be only slightly perceivable by the inner ear structure alone. With the added function of the swim bladder as a resonating chamber, signals are amplified to noticeable levels.

Embryonic analysis of Weberian apparati of the taxon Brycon has shed some light on the development of the structure itself. The Weberian apparatus elements form from the fully distinguishable first five vertebrae of the individual. The supraneural starts as an element of the skull. The claustra and the scaphia develop from expanded elements of the neural arch of the first vertebrae (V1). From the second vertebrae (V2), the intercalarum and the vertebrae's lateral process are reduced and clump together. The plural rib (R1) of the third vertebrae (V3) shrinks and moves somewhat ventrally, forming the tripus from a vertebral parapophysis fusing with the pleural rib. The os suspensorium bone of the fourth vertebrae (V4) somewhat retains its shape, developing from the pleural rib of the vertebrae (R2). The remaining elements of the fifth vertebrae (V5), the parapophysis and the articulating rib (R3), including the vertebrae itself form the posterior structure of the Weberian apparatus. The neural spines of the first four vertebrae fuse and compress, forming one of the major structures of the apparatus.

Study of the embryology of the Weberian apparatus has since been conducted on various other ostariophysan species. The results of which have resulted in various interpretations of the development (and thus the homology) of the structures that form the structure. Specific studies have been done on the Weberian apparati of a few select taxa, including Danio rerio, Rhaphiodon vulpinus and Corydoras paleatus.

The earliest recorded incidence of a Weberian apparatus is from the fossil fish Santanichthys diasii dating from the Early Cretaceous of Northeastern Brazil. In the aforementioned taxon, the Weberian apparatus is fairly developed; There is a distinguishable intercalarium and a tripus which articulate with the second and third vertebrae respectively. A scaphium can be seen in at least two specimens. The neural arch of the third vertebrae has already broadened, almost similar to that of modern ostariophysans. The claustrum, an element in modern apparati, is noticeably absent from the Weberian apparatus of S. diasii. Only the first four vertebrae are involved in the Weberian apparatus of Santanichthys; There are no signs of involvement from the elements of the fifth vertebrae unlike in modern otophysans.

Reproduction

Spawning
Most fish reproduce by spawning, and so do most other aquatic animals, including crustaceans such as crabs and shrimps, molluscs such as oysters and squid, echinoderms such as sea urchins and sea cucumbers, amphibious animals such as frogs and turtles, aquatic insects such as mayflies and mosquitoes, and corals (which are small aquatic animals and not plants). Fungi, such as mushrooms, are also said to "spawn" a white fibrous matter that forms the matrix from which they grow.

Spawn consists of the reproductive cells (gametes) of aquatic animals, some of which will become fertilized and produce offspring. The process of spawning typically involves females releasing ova (unfertilized eggs) into the water, often in large quantities, while males simultaneously or sequentially release spermatozoa to fertilize the eggs.

There are many variations in the way spawning occurs, depending on sexual differences in anatomy, on how the sexes relate to each other, on where and how the spawn is released, and on whether or how the spawn is subsequently guarded.

Internally, the sexes of most fish can be determined by looking at the gonads. Male testes of spawning fish are smooth and white and account for up to 12% of the mass of the fish, while female ovaries are granular and orange or yellow, accounting for up to 70% of the fish's mass. Male lampreys, hagfish and salmon discharge their sperm into the body cavity where it it is expelled through pores in the abdomen. Male sharks and rays can pass sperm along a duct into a seminal vesicle, where they store it for a while before it is expelled, while teleosts usually employ separate sperm ducts.

Externally, many fish, even when spawning, show little sexual dimorphism (difference in body shape or size) or little difference in colouration. Where species are dimorphic, such as sharks or guppies, the males often have penis-like intromittent organs in the form of a modified fin.

A species is semelparous if its individuals spawn only once in their lifetime, and iteroparous if its individuals spawn more than once. The term semelparity comes from the Latin semel, once, and pario, to beget, while iteroparity comes from itero, to repeat, and pario, to beget.

Semelparity is sometimes called "big bang" reproduction, since the single reproductive event of semelparous organisms is usually large and fatal to the spawners. The classic example of a semelparous animal is the Pacific salmon, which lives for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and then dying. Other spawning animals which are semelparous include mayflies, squid, octopus, smelt, capelin and some amphibians. Semelparity is often associated with r-strategists. However, most fish and other spawning animals are iteroparous.

When the internal ovaries or egg masses of fish and certain marine animals are ripe for spawning they are called roe. Roe from certain species, such as shrimp, scallop, crab and sea urchins, are sought as human delicacies in many parts of the world. Caviar is a name for the processed, salted roe of non-fertilized sturgeon. The term soft roe or white roe denotes fish milt. Lobster roe is called coral because it turns bright red when cooked. Roe (reproductive organs) are usually eaten either raw or briefly cooked.

The reproductive behaviour of fishes is remarkably diversified: they may be oviparous (lay eggs), ovoviparous (retain the eggs in the body until they hatch), or viviparous (have a direct tissue connection with the developing embryos and give birth to live young). All cartilaginous fishes, the elasmobranches (e.g., sharks, rays, and skates), employ internal fertilization and usually lay large, heavy-shelled eggs or give birth to live young. The most characteristic features of the more primitive bony fishes is the assemblage of polyandrous (many males) breeding aggregations in open water and the absence of parental care.

There are two main reproduction methods in fish. The first method is by laying eggs and the second by live-bearing (producing their young alive).

  • In the first method, the female fish lays eggs either on the sea floor or on the leaves of an aquatic plant. A male fish fertilizes the eggs, and both then work together to protect the eggs/babies from danger until they can defend themselves.
  • In the second method, the male fish uses its anal fin to transmit sperm into the female fish and fertilize the fish eggs. Later, the female gives live birth to her fry.

Internal fertilization
In many species of fish, fins have been modified to allow internal fertilization. A gonopodium is an anal fin that is modified into an intromittent organ in males of certain species of live-bearing fish in the families Anablepidae and Poeciliidae. It is movable and used to impregnate females during mating. The male's anal fin's 3rd, 4th and 5th rays are formed into a tube like structure in which the sperm of the fish is ejected. In some species, the gonopodium may be as much as 50% of the total body length. Occasionally the fin is too long to be used, as in the "lyretail" breeds of Xiphophorus helleri. Hormone treated females may develop gonopodia. These are useless for breeding. One finds similar organs having the same characteristics in other types of fish, for example the andropodium in the Hemirhamphodon or in the Goodeidae.

When ready for mating, the gonopodium becomes 'erect' and points forward, towards the female. The male shortly inserts the organ into the sex opening of the female, with hook-like adaptations that allow the fish to grip onto the female to ensure impregnation. If a female remains stationary and her partner contacts her vent with his gonopodium, she is fertilized. The sperm is preserved in the female's oviduct. This allows females to, at any time, fertilize themselves without further assistance of males.

Male cartilaginous fish have claspers modified from pelvic fins. These are intromittent organs, used to channel semen into the female's cloaca during copulation.