The Anatomy of Fish

anatomy of fish
Illustration by Marco MilanesiIndex Hide 1 Scales2 bones3 teeth4 fins5 Motility6 The musculature7 Colors and pigments8 The digestive system
ScalesSkin scales or flakes are dermal formations located in deep layers of the epidermis.
Such scales effectively protect animals from wounds; various types of scales have appeared over the course of evolution.

A scale seen under a microscopeThose called placoids, of the sharks and related species of the group of the Chondrichthyes are, effectively, epidermal teeth, with an enamel surface covering a dentine pulp; in the buccal cavity, large placoid scales form rows of teeth.
Other different types of scales are recorded: cycloidsctenoids and ganoids (the types concerning freshwater fish)   

Various types of scales. The cycloid scales are quite widespread and of primitive origin and are also called for their rounded shape, characterized by a smooth edge; they are present in cyprinoforms.
The ctenoid scales are characterized by a rough surface and a margin equipped with teeth, whose shape resembles that of the teeth of a comb; they are traceable in Perciformes.

The 2 aforementioned typologies reveal on their surface an interweaving of concentric formations which represent an essential tool for establishing the age of the animal.
The ganoid scales represent a primitive typology divided into 2 main branches: the paleoniscoidpertaining to the extinct Paleoniscus, but also existing in current species; the second concerns an evolved typology called the epis osteoid. A) Scaglia cyclone.

B) Ctenoid scale
(from U.D’Ancona) The cycloid scale of a 4-year-old salmon. Roman numerals indicate annual growth rings: These scales are often characterized by a rhomboidal shape and are partially superimposed on each other, and are found in polyptera, sturgeons and sea breams.
The top layer of these scales is covered with a layer of ganoin which gives them a glassy and shiny appearance and which has the same origin as dentin. Elasmoid scales within pockets of the dermis.
(from U.D’Ancona)The scales of bony fish ( teleosts ) have the shape of thin oval plates, devoid of enamel and dentin.

The cosmid scales are a modification of the ganoids, and resemble the placoid scales, found only in the coelacanth in modern fish. Evaluation of length as a function of the growth of cycloid scales.
(from L.Bertin)BoneThe skeleton consists of 2 different parts: the appendicular skeleton includes the skeletal parts of the fins and the pectoral and pelvic girdles; the axial skeleton includes the braincase and the vertebral column.

The neurocranium consists of a rather complex skeletal architecture which is based on compact blocks of bones of various types called auto stores (otic, temporal, mesethmoid, sphenoid) and allostasis ( nasal septa, the vomer, the frontal, parietal, frontal, orbital, lacrimal, jugal, squamous ).

Another part that contributes to the formation of the skull is the splanchnocranium, composed of a large number of mobile bones, of opercular origin that covers the gill slits and is related to the visceral arches.

The skull of sharks, rays and chimaeras is cartilaginous, however, in some species, there are secondary encrustations of calcium salts, which give them a bony appearance.
Sturgeons have a cartilaginous skull but are classified with bony ones, as their ancestors had an ossified skeleton.

Illustration by Marco MilanesiCharacteristics are the bony shapes called premaxillary, which with certain mobility, allow the mouth to protrude externally; the maxillary bones complete the formation of the upper jaw (they can also act as support for a dental structure).

But the lampreys, similarly to the hagfish, do not have jaws, as they are replaced by a sucker; the gill arches are supported by autonomous cartilaginous supports.

The vertebral column is ossified as a whole composed of arch-like vertebrae, developed from the bases of neural arches and adjacent in the caudal part to blood arches, which can be described as “portals” in which the caudal artery and vein wind.
In lampreys and hagfish, it is still a dorsal chord devoid of cartilaginous or bone tissue.

Nothing announces the vertebrae yet, and the tail is simply topped with small cartilaginous formations.
In cartilaginous fishes, the vertebrae are evident; they are cartilaginous and wrap around the internal cord; are joined by apophyses. 

Rays and sharks are cartilaginous fish (Photo Roberto Sozzani).On the dorsal side, there is a canal which contains the spinal cord; in the general cavity, the ribs attach to the ventral side of the vertebrae.
In the caudal area, the base of the vertebrae includes a canal which protects the caudal vein and artery.
Most bony fish have vertebrae shaped in the same way but ossified, however, there are exceptions, as in the case of sturgeon.

The ribs are elements recognizable in the vertebrae, which have the function of separating the viscera from the muscle mass.
In some genera there is a dorsal rib which performs a similar function, but between different parts of muscular origin, in this case, dorsal and ventral.

The skeleton of the fins is uneven, and is formed by bony and cartilaginous rays; the pectoral fins of the bony fishes ( teleosts ) are welded to the skull, while in the cartilaginous animals, they are connected with the skeleton of the branchial apparatus. 

In both groups, the pelvic fins are loosely attached to the musculature, with the exception of some groups, such as the gadiform: their ventral fins are in front of the pectorals and connected to the skull.TeethThe anterior dentition is composed of the Vomerini, located in the upper part of the mouth, the Palatines, located laterally, and the Premaxillaries.

The posterior dentition is characterized by pharyngeal teeth deriving from formations of the gill arches.  
The jaws of a shark and a piranhaThanks to these particular teeth, Cyprinids, for example, can make up for the lack of real teeth.

These teeth can take different shapes depending on the gender; we point out the characteristic spoon shape of B. brachycephalic caspius, the grinding tooth of the carp, and the canine-like tooth of Aspius aspires, used mainly for catching the prey, the long and sharp teeth of Bathybates leo or Belonesox Belarus.FinsThere is 2 types of fins: even fins and unequal fins.

The former is supported by skeletal support based on certain arc-shaped structures (pectoral girdle and pelvic girdle) and include the pectoral and pelvic fins.
These fins are supported by cutaneous rays ( lepidotrichous ) belonging to the membrane of the limb, and by deep endoskeletal rays.

The lepidotrichous rays are positioned on each side of the fin and are characterized by a covering under the skin of small bone plates separated from each other, similar to skin scales, which give the rays flexibility at the sides.

Different aspects of the caudal fin. By virtue of these characteristics, these cutaneous rays are soft; however, if the bony plates form a single body, the ray takes the shape of a sting, which can constitute a powerful weapon for the animal that uses it.

The pectoral girdle (or scapular ) is marked by a more or less evident ossification process and is formed by the scapula, positioned in the dorsal region, the coracoid, present in the ventral area, which in lateral position comes into contact with each other and with part of the limb. The shoulder girdle is connected to the back of the shoulder
via the clavicle and cleithrumneurocranium.

The pelvic girdle, on the other hand, has a significantly smaller development, marked by compact skeletal support which however does not create any connection between the fins and the vertebral column.

The paired fins allow the fish to move in the aquatic environment; in bony fish, the pectoral fins are connected to the skull, while in cartilaginous animals they are anchored in the musculature by autonomous cartilaginous elements.

In general, the pelvic fins are welded, similarly to the pectoral fins of cartilaginous fishes, in the musculature beautiful fins of a Betta Splendes.In some groups, such as the cuneiforms and the cyprinoforms, the pelvic fins are found in a ventral or abdominal position; in the preforms, they are located under the pectorals and in the gadiform they are positioned just in front of the pectorals, however in some groups they are completely absent, as in the case of the eels.
The unpaired fins include one or more dorsal limbs, a caudal fin and one or more anal fins, and are composed of mobile endoskeletal rays that fork each other.

These rays are bordering the hemospines of the vertebral column, for this reason, it is uncertain whether these rays originate from vertebral pieces, or represent a separate development.
The caudal fin is homocercal, symmetrical in shape (but internally asymmetrical), as it is characterized by a division into 2 equal parts, in this case, the dorsal and ventral lobes.The everted dorsal fin of a triggerfish.

(Photo Roberto Sozzani).The fusion of the 2 aforementioned parts determines the formation of the urostyle, in which modified hemospines called epidural are evident. The presence of caudal fins of heterocercal shape (with the 2 aforementioned parts having an asymmetrical shape), diphycercal ( heterocercal limbs transformed into homocercal fins, also as regards the internal skeleton), and homocercal fines were recorded.

With the exception of the perched fin, all the other fins are made solid by the bony or cartilaginous rays and can be modified into copulatory organs which allow internal fertilization (for example, the ventral fins can transform into myxopterygians or pterygoids in sharks, or into suckers, as is the case with the ventral fins of gobies ).MotilityGenerally, the fish swim by means of undulations of the whole body (eels), or only the caudal peduncle (as occurs for the majority of species).

Some fish also perform undulatory movements of the paired fins (bonnet fish, notopterids, etc.).
Even the synchronous or asynchronous movements of the paired fins produce remarkable results: some species of the Gasteropelecidae family are even able to get out of the water and make flights accompanied by the active beating of the pectoral fins.

Conversely, the flight of marine fish of certain species ( Exocoetidae and Hemirhampidae) is absolutely passive: their pectoral fins, which reach 80% of the length of the body, allow them to glide in the air, carrying out displacements which can exceed 400 m, when the force of the wind is favourable.
The shape of the body and its smooth surface is key to reducing water resistance; of no less importance also the morphology of the caudal fin.

Inexperienced swimmers, the lobes overcome the zone of currents with turbulent reactions forming on the back of the maximum dimension of the body and take an active part in the orientation of the moving fish.
The maximum speed that the fish reaches in short periods of time varies from species to species: from 3 m per second, covered by zander, up to 36 of swordfish.
However, these values ​​are bound to change when considering the average speed achieved by each species.

The musculature of fish does not differ much from one species to another.
Among the muscles there are numerous thin bones, often forked, not connected to the skeleton; these are characteristic of fish, and appear by ossification within connective cases of isolated muscle fibres. To find more differentiated muscles one must examine the head; the organs equipped with autonomous muscles are the eyeballs, the branchial apparatus and the jaws.

The musculature of the trunk is concentrated above all in the dorsal part of the body, and the horizontal septum divides it into 2: superior musculature and inferior musculature.

In turn, the muscles are divided into striated skeletal muscles, actively managed by the animal, and smooth visceral, independent of the will; the heart muscle, although striated, does not respond to voluntary commands.

Part of the skeletal musculature denotes a brighter red colour than the rest and is located just under the epidermis, in the median part of the body, at the level of the horizontal septum, and in the caudal peduncle.
The red colouration of muscles is due to an elevated concentration of haemoglobin and myoglobin.
Red muscles can work for long periods without tiring, which is why species that travel long distances have a high proportion of red muscles.

Tight muscles, less oxygenated, get tired quickly but can make great efforts on short journeys, which are paid for, however, a lack of oxygen leads to an increase in the concentration of lactic acid.
It is only when the latter is eliminated that the white muscles are ready to operate again; these muscles constitute the majority (very few, instead of the red ones) in fish with poor mobility.

Colours and pigmentsThe epidermis is composed of a multilayered covering, characterized by a variable thickness in relation to a specific species or to a particular area of ​​the body.
In these epidermal layers, unicellular and muciparous glands abound; the latter seems to fulfil the protection of the animal from the onset or otherwise of any infections of bacterial, fungicidal and other origins.

Furthermore, the presence of venom glands can be recorded, attributable to blades located at the base of the fins, or of photoshophores, luminous organs used by the fishes that live in very deep areas cloaked in darkness or scarcely illuminated.

In the dermis (the part underneath the epidermis) there are pigment cells ( chromatophores and iridocytes) which, based on certain environmental conditions perceived by the fish, can change the colour or decoration of the skin.

The chromatophores are divided into erythrophores, characterized by red and orange pigments, xanthophores, equipped with yellow pigments and melanophores, whose pigmentation is black.
The diet to be administered to the fishes in captivity is also important for safeguarding the chromatic liveliness of their liveries, in fact, some pigments are contained in the food substances ingested by the fishes.
Skin structure and Melanophores (from U.D’Ancona)Naturally the suggestive colours that we admire in some species are the result of the chromatic combination of the types of chromatophores mentioned above.
The iridocytes made up of guanine substances (a by-product of protein digestion), combine to transform the basic colour of a livery into silvery and iridescent chromatism.
The animal can change its colouring according to different solicitations: the need to blend in with the environment to face dangerous situations or wait for passing prey, creates the premises for this mutation; Betta splendens, for example, can do this when attacked by a contender; the colours take on different shades during the reproductive period.

Fish change their colouring during their development, during migration periods, according to the seasons, and according to their mood and psychological state.
Generally, the young specimens have a different colouring from that of the adults, and this corresponds to the difference in the medium in which they live. 

Pomacantus imperator in juvenile and adult livery. However, it is during the rut that the most spectacular changes are witnessed: the males take on dazzling colours, without worrying about their visibility, but this chromatic enrichment is only recognizable from very close potions.
Some fish can react very quickly to the chromatic variations of the support on which they are located, in fact, they can adapt to the predominant colour of the environment. A sole: the pigmentation can change quickly.
(Photo Roberto Sozzani).Even the premonitory colours of certain animals living in the coralline habitat serve to warn the other specimens that the surrounding territory cannot be violated.
The intense colours of the reef species derive from their life among thickets of colourful corals.
The secretion of certain substances (originating from the endocrine glands), caused by hormonal stimulation, contributes to the chromatic variation of the livery.

The digestive systemThe organic structure of this apparatus consists of various organs, in this case, the pharynx, oesophagus, oral cavity, stomach and intestine, to which other organs are connected which work in synergy with those mentioned above: teeth, tongue, pancreas, liver and gallbladder.

There are no salivary glands in the mouth, only mucous glands.
The tongue is not equipped with muscles, as it constitutes a simple protuberance of the buccal floor placed between the dental rows; the oral cavity does not have an accentuated development as it ends in correspondence with the pharynx, therefore it is rather short.

It is important to note that the tongue is not particularly developed in bony fishes, unlike what happens for cyclostomes.
Teeth, depending on the family they belong to, can move or be solidly welded to the underlying bone structure.

The oesophagus is rather wide, but has a short and linear development; the stomach is characterized by ascending and descending sections which give it the shape of a pouch.
The walls of this organ are dilatable to allow the animal (usually predatory species) to ingest rather voluminous prey. Pyloric appendages.

(from U.D’Ancona)However, some genera of fishes ( Protopterus and Cyprinus, for example) lack stomachs, thanks to their efficient masticatory apparatuses which allow them to abundantly chop up the ingested food, and therefore to easily complete the digestive process with the aid of the enzymes produced from the intestine.

The intestine is particularly elongated in phytophagous species, thus formed to facilitate the assimilation of vegetable substances, and vice versa in carnivorous animals it has a short morphology.
In Salmonids or Gadacians, this organ forms a large number of diverticula which increase the digestive surface, and probably also contribute to neutralizing the acid reaction of the food bolus as it exits the stomach.

Digestive system
A) herbivorous fish (much longer).
B) carnivorous fish 1-oesophagus
4-common bile duct
5-anusUnlike digestive enzymes in the stomach, which are acidic in nature, intestinal enzymes require a slightly basic environment.
It is in the intestine that proteins are digested, with the help of trypsin, while other enzymes ( lipase, carboxylase ) facilitate the digestion of fatty substances and sugars.

The size of the liver is quite significant, and is characterized by an abundant concentration of fats; the pancreas is widespread in it (important for the secretion of insulin, necessary to maintain blood sugar at its optimal rate), whose cellular distribution is capillary, as it is not concentrated in a specific point.
In addition, the liver produces bile, which aids in the digestion of lipids and increases the pH of the intestinal contents and conserves blood sugar, the aforementioned lipids, and vitamins A and D.
Elizabeth Canales is fond of marine life since childhood and got an aquarium as a gift from her father on her 6th birthday. Since then she started to take care of Coralia (her goldfish). Her love for fish made receive her B.S. in Marine Biology from the University of Washington and DVM from Delaware State University.

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