Morphological features and protective functions of the fish integument

Physiological functions of fish skin and protective mechanisms of coloration

For fish farmers, the skin of a fish serves as a crucial indicator of stock health and a regulator of metabolic processes. Body coloration has an adaptive significance: in pelagic fish, a dark back and silvery-white belly make them inconspicuous from both above and below. Bottom-dwelling species, such as flatfish, gobies, and pipefish, are capable of visually adapting to the color and even the pattern of the bottom.

This process is regulated by the nervous system through visual receptors; therefore, blinded fish lose the ability to change their coloration. During the spawning period, under the influence of hormones, the fish's coloration changes, forming nuptial attire and emphasizing differences between males and females. This allows for a visual assessment of the readiness of broodstock for spawning.

Fish skin possesses high regenerative capacity. It is used to excrete metabolic end-products and absorb mineral elements from the water. Furthermore, the skin serves as a receptor surface due to thermo-, baro-, and chemoreceptors, and in its deep layer — the corium — the dermal bones of the skull and the pectoral girdle of the pectoral fins develop.

• Main elements absorbed by the skin: oxygen, carbon dioxide, water, sulfur, phosphorus, calcium
• Maximum scale size: several centimeters (in the Mahseer)
• Minimum scale size: microscopic (in eels)

Types of scales and anatomical features of the fish skeleton

Scales function as an external protective skeleton, although the degree of their development varies greatly. Catfish and rays secondarily lost their scales during the process of evolution, while in other species, their size ranges from microscopic in eels to several centimeters in the Mahseer. Several main types of scales are identified, determining the durability of the fish's integument.

In bony fish, the scale type may depend on gender. For example, in flatfish, males have ctenoid scales, while females have cycloid scales. The internal skeleton of fish consists of the axial section, skull, fins, and their girdles. The skull is immovably connected to the spine: the absence of a neck helps fish more easily overcome water resistance during movement.

The axial skeleton is represented by the notochord or spine. In jawless fish, sturgeons, and lungfish, the notochord persists throughout their entire life, while in other species, it is replaced by the spine in the adult state. Evolutionary complication of the skeleton can be clearly traced by comparing the structure of different groups of fish.

In jawless chordates (lampreys, hagfish), the axial skeleton is represented by an elastic notochord with vertebral rudiments, and jaws are absent. Cartilaginous fish (sharks, rays) have a completely cartilaginous skeleton with remnants of the notochord inside the vertebrae, and their jaws are equipped with real teeth. In bony fish, the skeleton ossifies and becomes more complex over time.

Chondrostean fish (sturgeons). The skeleton of these fish is mainly cartilaginous, but bone formations appear in it for the first time. In the sturgeon skeleton, there are only dermal bones. The spine is cartilaginous and continuous. The skull of sturgeons differs little from the skull of cartilaginous fish: it is a solid cartilaginous mass in the form of a box, but it has dermal bones on it. There are five pairs of gill arches in the head skeleton.

Bony fish already have an ossified spine. It is divided into the trunk and caudal sections. The trunk section is segmented into typical vertebrae – amphicoelous, in which a body, an upper arch with upper (neural) spines (protecting the spinal cord), and large lower arches with lower processes are distinguished.

• In the trunk region, ribs are attached to the vertebral column (to the transverse processes or the vertebral body).
• In the tail region, the transverse processes converge to form the lower (haemal) arch, which ends in a lower neural spine.
• The caudal artery and vein run through the haemal canal.
• The last caudal vertebra is flattened and serves for the attachment of the caudal fin rays; it often changes its normal shape, becoming elongated and bending upwards at the end to form the urostyle.

The number of vertebrae is determined by a number of internal and external factors and serves as a taxonomic character for fish. Within a species, there is a known dependence of the number of vertebrae (and rays in the pectoral and anal fins) on temperature: an increase in temperature during embryogenesis causes a decrease in their number.

In addition to ribs, thin intermuscular bones, which penetrate the muscles, perform a supporting function in bony fish. These bones are formed by ossified tendons. They are most numerous in cyprinids.

The skull of fish, just like the axial skeleton, gradually becomes more complex in the process of evolution. Cyclostomes lack a cranial box; they have separate cartilaginous brain capsules (olfactory, auditory, and ocular) connected by connective tissue.

Cartilaginous fish (sharks, rays) already have a cartilaginous skull formed by fused brain capsules, and jaws armed with teeth. The jaw apparatus is connected to the skull. In sturgeons, furthermore, a series of dermal overlay bones appears, covering the skull from above, and true bones that protect the skull from below.

Higher bony fish pass through all these stages in the embryonic period: first, a cartilaginous skull is laid down and develops, and later it ossifies, with dermal bones emerging alongside replacement bones.

In the skull of bony fish, two sections are distinguished: the cranial (axial) and the visceral. Numerous bones of the axial section are connected immovably. This ensures reliable protection for the brain. The visceral section of the skull is formed by the jaw and gill apparatus. It consists of the jaw, hyoid, and five gill arches, covered by the gill cover.

The gill cover consists of four bones: the preopercular, opercular, interopercular, and subopercular. They cover the five gill arches. On the inner side of the four arches are gill rakers, and on the outer side are gill filaments (organs of respiration).

ab – length of the lower part of the arch; bv – length of the upper part of the arch;

The bones of the jaw and gill apparatus are movably articulated with the skull and, being interconnected, move in a coordinated manner with the help of muscles.

There are no filaments on the 5th gill arch; in some fish, this arch transforms into the pharyngeal bone, on which pharyngeal teeth are formed, reaching their greatest development in cyprinids. Pharyngeal teeth are arranged in 1–3 rows. The shape and arrangement of the pharyngeal teeth are taxonomic characters.

Pharyngeal teeth in cyprinids are located on the pharyngeal bones on the fifth gill arch in one, two, or three rows. To examine them, the following actions must be performed:

• cut the muscles;
• extract the fifth gill arch through the gill opening.

The notation for the number of pharyngeal teeth for uniserial ones, for example, 6–5, means 6 teeth on the left side and 5 on the right (roach). For biserial ones, for example, 3.5–5.3, it means 3 teeth in one row on the left side and 5 in the other, and 5 teeth in one row on the right side and 3 in the other (rudd, asp). An example of triserial teeth: 1.1.3–3.1.1.

Cyprinids also possess a pharyngeal pad – a horn-like cushion-shaped formation in the upper part of the pharynx, which serves together with the pharyngeal teeth to grind food.

The pectoral (shoulder) girdle consists of three main bones: the cleithrum, scapula, and coracoid. Pectoral fins are attached to it. It is articulated to the skull via the supracleithrum bone.

The pelvic girdle in bony fish is represented by two fused bones to which the pelvic fin rays are attached. It lies autonomously in the muscles, so in some species, it can move far forward, even to the throat, and sometimes disappear altogether.

The fin skeleton acts as a support, allowing the fish to use the fins as a lever or keel. In bony fish, it is represented by bony rays that stretch the membranous fin web. The axial skeleton and the girdle skeleton perform a supporting function; in addition, the locomotor muscles are attached to them.

In general, fish musculature is represented by two types of muscles:

Muscle types and their influence on fish activity

The type of a fish's muscular system directly determines its activity and endurance in artificial water bodies. Muscles are divided into two main groups: "slow" red and "fast" white. Red muscles contain a lot of myoglobin and are constantly supplied with oxygen, which allows the fish to perform long monotonous swims without fatigue. White muscles operate through rapid glycolytic metabolism — they provide powerful sudden bursts of speed but quickly deplete their energy resource.

In some fish species, the musculature performs an additional thermoregulatory function. For example, in tuna hunting for squid in deep, cold waters, active muscle work allows the brain temperature to be maintained at a higher level than in other parts of the body. This helps the predator maintain high reaction speeds under unfavorable temperature conditions.

Gills and reserve respiratory organs in conditions of oxygen deficiency

The main volume of gas exchange in bony fish is provided by gills, protected by movable bony covers. At the base of each gill filament, supported by a thin cartilaginous strip, lies the afferent branchial artery, which branches into capillaries. The capillaries merge into the efferent branchial artery, carrying arterial blood to the root of the aorta. They form a countercurrent system: blood flows in the opposite direction to the surrounding water, which allows hemoglobin to absorb dissolved oxygen as efficiently as possible. To increase the contact area, the filaments are covered with tiny folds (15 or more per 1 mm), and in some species, a rudiment of an accessory hemibranch is preserved under the cover.

In active fish species, as well as in inhabitants of water bodies with constant or seasonal oxygen deficiency, the relative gill area is always larger. Their breathing process is carried out through an efficient suction mechanism. It consists of two sequential stages:

1. Inhalation. Gill covers and arches move apart. Under the influence of external pressure, the leathery edges of the covers close the gill slit, the pressure in the cavity drops, and water is sucked in through the mouth, washing over the gill filaments.
2. Exhalation. Gill covers move closer to the arches, and the pressure inside the cavity increases. Water pushes back the leathery edge of the cover and exits, while the closed gill filaments block its return flow into the oral cavity.

Besides respiration, the gills are responsible for the excretion of carbon dioxide, urea, and ammonia, and they also regulate salt and water balance. During the fish's development, its respiratory system transforms. In embryos and larvae, internal gills are not yet formed, so oxygen is absorbed through the capillary network of the yolk sac, the fin fold, and the skin. In the larvae of some species (lungfish, bichir, some loaches from the Cyprinidae family), external gills — vascular outgrowths of the skin — develop temporarily.

To survive in conditions of winterkill (hypoxia) or during migrations between water bodies, fish use reserve respiratory mechanisms. The main one is the skin, which efficiently absorbs oxygen when it is scarce in the water. The skin is also used to remove carbon dioxide — its release through the skin occurs even more intensively than oxygen absorption.

• Share of cutaneous respiration in normal conditions — less than 10% of oxygen
• Share of cutaneous respiration during O₂ deficiency — 20–30% of oxygen
• Cutaneous respiration of loaches and eels on land — up to 85% of oxygen

Loaches, mudskippers, and eels, which crawl from one water body to another on damp, dewy nights, receive up to 85% of the necessary oxygen through their skin. They cover the rest of their needs, presumably, at the expense of reserves in the swim bladder. In general, various systematic groups of fish develop specialized adaptations that allow them to effectively use atmospheric oxygen under unfavorable environmental conditions.

Structure and hydrostatic role of the swim bladder

The swim bladder is an important internal organ in most fish, located in the body cavity under the spine and kidneys. It is filled with a mixture of nitrogen, oxygen, and carbon dioxide, which reduces the average body density. The main task of the bladder is hydrostatic. It provides neutral buoyancy, allowing the fish to hover in the water column without constant fin movement and unnecessary energy expenditure.

In jawless fish and cartilaginous fish, such as lampreys and sharks, there is no swim bladder at all. It is also absent in some species that live at great depths or quickly change swimming horizons — such as flatfish, tuna, and mackerel. In bottom-dwelling species, this organ is highly reduced or absent entirely. For example, in the catfish, it is poorly developed, and the bullhead sculpin does not have it at all, moving along the bottom with the help of spread-out pectoral fins.

Besides hydrostatics, the swim bladder functions as a baroreceptor and acoustic resonator, helping to perceive sound and shock waves (for example, in carp and catfish). With the help of special muscles that cause the bladder walls to vibrate, some fish are capable of making sounds. Also, to a certain extent, this organ participates in gas exchange and can serve as an additional reservoir for breathing.

Types of swim bladders and buoyancy regulation

In the embryonic period, the swim bladder in all fish is connected to the gut by a thin tube. It is through this that larvae fill the organ with air during their first days of life. Based on the nature of this connection in the adult state, fish are divided into two main groups:

• Physostomes (sturgeons, salmonids, herrings, cyprinids) — retain a hollow duct throughout their entire life and regulate gas volume by swallowing air through the mouth.
• Physoclisti (percoids, sticklebacks, seahorses, zander, perch) — have an isolated bladder, as the connecting duct eventually closes.

Since physoclistous fish cannot swallow air, they regulate pressure in the bladder via gas secretion. For this, "rete mirabile" — dense clusters of capillaries — are located in the anterior part of the organ. Excess gas in physostomes is expelled through the mouth or blood vessels of the connecting duct, while in physoclisti, it is reabsorbed into the blood through a special pocket with capillaries (the oval) in the posterior part of the bladder.

• Volume of the eel "rete mirabile" — 64 mm³
• Venous capillaries of the eel — 88 thousand units
• Arterial capillaries of the eel — 116 thousand units
• Total length of eel capillaries — 352–464 m

In the first days of life, larvae of all fish species must rise to the surface and swallow atmospheric air for the initial filling of the swim bladder. If this does not happen within strictly defined timeframes, the fry are doomed to death or significant growth retardation. In aquaculture, this stage requires special control by the technologist.

The anatomy of a fish's digestive system directly determines the feeding regimen and efficiency in aquaculture. The structure of the oral apparatus indicates in which water layers the fish searches for food and what type of feed is suitable for it:

• Superior mouth is found in fish that stay near the water surface;
• Inferior mouth is characteristic of benthic species;
• Terminal mouth is developed in fish living in the middle water layers.

Barbels in the snout area serve as sensory organs and carry taste cells, helping benthic fish find food in turbid water. The oral cavity contains teeth made of dentin covered with enamel, and some species have pharyngeal teeth. A muscular tongue helps move food into the pharynx with gill slits, from where it enters the stomach through a short esophagus.

In fish with a distinct stomach, blind pyloric caeca are located at the border with the intestine, which increase the absorption surface. The length of the intestine depends on the feeding type: in predators, it is short and forms only one or two loops, while in herbivorous fish, it is long and highly convoluted. The large liver, which produces bile that accumulates in the gallbladder, actively participates in digestion. The spleen, responsible for hematopoiesis, is located in the intestinal loop. Decay products are filtered by the kidneys near the spine, discharged into the urinary bladder and further outward, while undigested food residues are removed through the anal opening.

Nervous system and adaptation to the habitat

The nervous system of fish is simpler than that of terrestrial animals, but it is precisely what determines the fish's reaction to stimuli, stocking density, and herd behavior. The brain has a linear arrangement of parts and a small size.

The brain consists of the forebrain, diencephalon, midbrain, cerebellum, and medulla oblongata. The development of these parts depends on the fish's lifestyle:

• In pelagic and predatory fish that lead an active lifestyle, the cerebellum and the visual lobes of the midbrain are more developed.
• In benthic fish, the forebrain and medulla oblongata, which are responsible for touch and smell, are more developed.

The forebrain regulates schooling behavior and is responsible for the sense of smell. At its base lie the basal ganglia (striatum), connected to the thalamus. The arrangement of olfactory bulbs varies by family: in salmonids, they are located next to the basal ganglia, while in cyprinids, silurids, and gadids, they are shifted toward the nasal pits and connected to the brain by long olfactory tracts.