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Structural and Functional Adaptations of Fishes

In this structural and functional adaptations of fishes post we have briefly explained about adaptations of fishes, structural and functional adaptations.

Adaptations of Fishes

Fish physiology is the scientific study of how the component parts of fish function together in the living fish. It can be contrasted with fish anatomy, which is the study of the form or morphology of fishes. In practice, fish anatomy and physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the later dealing with how those components function together in the living fish.

Structural and Functional Adaptations of Fishes

Locomotion in Water

To the naked eye, some fish appear to be capable of swimming at high rates of speed. However, our judgement is unconsciously tempered by our own experience, which has taught us that water is a highly resistant medium through which to move. Most fishes, such as trout and minnows, can swim at a maximum speed of about 10 body lengths per second, which is obviously impressive by human standards. When these speeds are converted to kilometres per hour, a 30 cm (1 foot) trout can only swim 10.4 km (6.5 miles) per hour. As a general rule, the faster a fish can swim, the larger it is.

A fish’s trunk and tail musculature serve as its propulsion mechanism. The axial locomotory musculature is made up of zigzag bands known as myomeres. Muscle fibres are relatively short in each myomere and connect the tough connective tissue partitions that separate each myomere from the next (Figure: 1 and 2). On the outside, the myomeres resemble a W lying on its side, but on the inside, the bands are intricately folded and nested, allowing each myomere’s pull to extend across several vertebrae. Because many myomeres are involved in bending a given segment of the body, this arrangement produces more power and finer control of movement.

Adaptations of Fishes

Figure 1: Trunk musculature of a teleost fish, partly dissected to show internal arrangement of the muscle bands (myomeres). The myomeres are folded into a complex, nested grouping, an arrangement that favors stronger and more controlled swimming. Adaptations of Fishes

Adaptations of Fishes

Figure 2:  Bluefin tuna, showing adaptations of fishes for fast swimming. Powerful trunk muscles pull on the slender tail stalk. Since the body does not bend, all of the thrust comes from beats of the stiff sickle-shaped tail.

Studying the motion of a very flexible fish, such as an eel, can help us understand how fish swim (Figure 3). The movement is serpentine, similar to that of a snake, with waves of contraction moving backward along the body caused by alternate contraction of the myomeres on either side. Because the anterior end of the body bends less than the posterior end, the amplitude of each undulation increases as it travels along the body. While the undulations move backward, the body’s bending pushes laterally against the water, producing a reactive force that is directed forward but at an angle. It is divided into two components: thrust, which overcomes drag and propels the fish forward, and lateral force, which causes the fish’s head to “yaw,” or deviate from the course in the same direction as the tail. In a swimming eel or shark, this side-to-side head movement is obvious, but many fishes have a large, rigid head with enough surface resistance to minimise yaw (Adaptations of Fishes).

An eel’s movement is relatively efficient at low speeds, but its body shape generates too much frictional drag for fast swimming. Fast-swimming fish, such as trout, are less flexible and restrict body undulations to the caudal region. The large anterior muscle mass generates muscle force, which is transferred via tendons to the relatively non-muscular caudal peduncle and tail, where thrust is generated. This type of swimming is most developed in tunas, whose bodies do not flex at all. Almost all thrust is generated by powerful tail fin beats. Many fast oceanic fishes, such as marlin, swordfish, amberjacks, and wahoo, have sickle-shaped sweptback tail fins. These fins are the aquatic equivalent of the fastest birds’ high-aspect ratio wings (Adaptations of Fishes).

Adaptations of Fishes

Figure 3: Adaptations of fishes, movements of swimming fishes, showing the forces developed by an eel-shaped and spindle-shaped fish.

Swimming is the most cost-effective mode of animal locomotion, owing to the fact that aquatic animals are almost perfectly supported by their medium and require little energy to overcome the force of gravity. When we compare the energy cost per kilogramme of body weight of travelling 1 km by different modes of locomotion, we find that swimming costs only 0.39 kcal (salmon), while flying (gull) costs 1.45 kcal and walking costs 5.43. (ground squirrel). However, one unfinished business of biology is figuring out how fish and aquatic mammals can move through water with almost no turbulence. The secret lies in the way aquatic animals bend their bodies and fins (or flukes) to swim and in the friction-reducing properties of the body surface (Adaptations of Fishes).

Neutral Buoyancy and the Swim Bladder

Because their skeletons and other tissues contain heavy elements found only in trace amounts in natural waters, all fishes are slightly heavier than water. Sharks must always move forward in the water to avoid sinking. A shark’s asymmetrical (heterocercal) tail provides necessary tail lift as it sweeps through the water, and the broad head and flat pectoral fins act as angled planes to provide head lift. Sharks’ buoyancy is also aided by their large livers, which contain a special fatty hydrocarbon called squalene, which has a density of only 0.86. As a result, the liver functions as a large sack of buoyant oil, helping to compensate for the shark’s heavy body.

Adaptations of Fishes

Figure 4:  A, Swim bladder of a teleost fish. The swim bladder lies in the coelom just beneath the vertebral column. B, Gas is secreted into the swim bladder by the gas gland. Gas from the blood is moved into the gas gland by the rete mirabile, a complex array of tightly-packed capillaries that act as a countercurrent multiplier to build up the oxygen concentration. The arrangement of venous and arterial capillaries in the rete is shown in C. To release gas during ascent, a muscular valve opens, allowing gas to enter the ovale from which the gas is removed by the circulation. Adaptations of Fishes.

Because their skeletons and other tissues contain heavy elements found only in trace amounts in natural waters, all fishes are slightly heavier than water. Sharks must always move forward in the water to avoid sinking. A shark’s asymmetrical (heterocercal) tail provides necessary tail lift as it sweeps through the water, and the broad head and flat pectoral fins act as angled planes to provide head lift. Sharks’ buoyancy is also aided by their large livers, which contain a special fatty hydrocarbon called squalene, which has a density of only 0.86. As a result, the liver functions as a large sack of buoyant oil, helping to compensate for the shark’s heavy body.

A gas-filled space is by far the most efficient flotation device. In bony fishes, the swim bladder serves this function. It evolved from the paired lungs of Devonian bony fishes. Lungs were most likely a common feature of Devonian freshwater bony fishes, as warm, swampy habitats would have made such an accessory respiratory structure advantageous. Most pelagic bony fishes have swim bladders, but tunas, most abyssal fishes, and most bottom dwellers, such as flounders and sculpins, do not (Figure 4).

A fish can achieve neutral buoyancy and remain suspended indefinitely at any depth by adjusting the volume of gas in the swim bladder with no muscular effort. However, there are significant technical issues. When a fish descends to a greater depth, the gas in its swim bladder compresses, causing the fish to become heavier and sink. To achieve new equilibrium buoyancy, gas must be added to the bladder. The gas in the fish’s bladder expands as it swims upward, making the fish lighter. Unless the gas is removed, the fish will continue to rise at an increasing rate while the bladder expands.

There are two methods for removing gas from the swim bladder. The more primitive phystostomous (Gr., bladder, stoma, mouth) fishes (such as trout) have a pneumatic duct connecting the swim bladder to the oesophagus. These fish could simply expel air from the pneumatic duct. The physoclistous condition, in which the pneumatic duct is lost in adults, is seen in more advanced teleosts. Gas must be secreted into the blood from the ovale, a vascularized area, in physoclistous fishes. Although a few shallow-water phystostomes may gulp air to fill their swim bladder, both types of fish require gas to be secreted into the swim bladder from the blood.

The highly specialised gas gland secretes gas into the swim bladder. The gas gland is fed by a remarkable network of blood capillaries known as the rete mirabile, which acts as a counter current exchange system to trap gases, particularly oxygen, and prevent their loss to the circulation.

A fish living at a depth of 2400 metres exemplifies the device’s incredible effectiveness (8000 feet). To keep the bladder inflated at that depth, the gas inside (mostly oxygen, but also variable amounts of nitrogen, carbon dioxide, argon, and even some carbon monoxide) must have a pressure greater than 240 atmospheres, which is significantly higher than the pressure in a fully charged steel gas cylinder. However, the oxygen pressure in the fish’s blood cannot be higher than 0.2 atmospheres, which is the same as the oxygen pressure at the sea surface.

Physiologists who were initially perplexed by the secretion mechanism now understand how it works. In summary, the gas gland secretes lactic acid, which enters the bloodstream and causes a localised high acidity in the rete mirabile, forcing haemoglobin to release its oxygen load. The rete’s capillaries are arranged in such a way that the released oxygen accumulates in the rete, eventually reaching such a high pressure that the oxygen diffuses into the swim bladder. The final gas pressure achieved in the swim bladder is determined by the length of the rete capillaries, which are relatively short in surface-dwelling fish but extremely long in deep-sea fish.

Respiration

The gills of fish are made up of thin filaments that are covered by a thin epidermal membrane that is folded repeatedly into plate-like lamellae. These are densely packed with blood vessels. The gills are located within the pharyngeal cavity and are protected by a movable flap known as the operculum. This configuration protects the delicate gill filaments, streamlines the body, and allows for a pumping system to move water through the mouth, across the gills, and out the operculum. Rather than opercula flaps like in bony fishes, elasmobranchs have a series of gill slits through which water flows (Figure 5).

Figure 5: Adaptations of fishes Gills of fish. Bony, protective flap covering the gills (operculum) has been removed, A, to reveal branchial chamber containing the gills. There are four gill arches on each side, each bearing numerous filaments. A portion of gill arch (B) shows gill rakers that project forward to strain out food and debris, and gill filaments that project to the rear. A single gill filament (C) is dissected to show the blood capillaries within the plate like lamellae. Direction of water flow (large arrows) is opposite the direction of blood flow. 

The gills of fish are made up of thin filaments that are covered by a thin epidermal membrane that is folded repeatedly into plate-like lamellae. These are densely packed with blood vessels. The gills are located within the pharyngeal cavity and are protected by a movable flap known as the operculum. This configuration protects the delicate gill filaments, streamlines the body, and allows for a pumping system to move water through the mouth, across the gills, and out the operculum. Rather than opercula flaps like in bony fishes, elasmobranchs have a series of gill slits through which water flows.

The bronchial mechanism in both elasmobranchs and bony fishes is designed to pump water continuously and smoothly over the gills, despite the fact that fish breathing appears to be pulsatile to an observer. The flow of water is in the opposite direction of blood flow (counter current flow), which is the best arrangement for extracting the most oxygen from the water. Some bony fishes can remove up to 85 percent of the oxygen from the water that passes through their gills. Only by swimming forward continuously to force water into the open mouth and across the gills can highly active fishes like herring and mackerel obtain enough water to meet their high oxygen demands. This is known as ram ventilation. Even if the water is oxygenated, such fish will be asphyxiated if placed in an aquarium that restricts free swimming movements.

A surprising number of fish can live for varying lengths of time out of water by breathing air. Different fishes use a variety of devices. The lungs of lungfishes, Polypterus, and extinct rhipidistians have already been described. During rainy weather, freshwater eels frequently travel overland, using their skin as a major respiratory surface. Amia, the bowfin, has both gills and a lung-like swim bladder. At low temperatures, it only uses its gills, but as the temperature rises and the fish becomes more active, it breathes mostly air through its swim bladder. Electrophorus, the electric eel, has degenerate gills and must supplement gill respiration by gulping air through its vascular mouth cavity. The Indian climbing perch Anabas is one of the best air breathers of all, spending most of its time on land near the water’s edge, breathing air through special air chambers above much reduced gills.

Osmotic Regulation

Fresh water is a very dilute medium with a salt concentration (0.001 to 0.005 gramme moles per litre [M]) that is much lower than that of freshwater fish blood (0.2 to 0.3 M). As a result, water tends to enter their bodies osmotically, and salt is lost through diffusion outward. Although the scaled and mucous-covered body surface is nearly completely impermeable to water, water gain and salt loss do occur across the gills’ thin membranes. Freshwater fishes are hyperosmotic regulators with multiple defences against these issues. The excess water is first pumped out by the opisthonephric kidney, which can produce very dilute urine. Second, salt-absorbing cells in the gill epithelium actively transport salt ions, primarily sodium and chloride, from the water to the blood. This, along with salt in the fish’s food, compensates for diffusive salt loss. These mechanisms are so effective that a freshwater fish only expends a small portion of its total energy expenditure to maintain osmotic balance.

Figure 6:  Osmotic regulation in freshwater and marine bony fishes. A freshwater fish maintains osmotic and ionic balance in its dilute environment by actively absorbing sodium chloride across the gills (some salt is gained with food). To flush out excess that constantly enters the body, the glomerular kidney produces dilute urine by reabsorbing sodium chloride. A marine fish must drink seawater to replace water lost osmotically to its salty environment. Sodium chloride and water are absorbed from the stomach. Excess sodium chloride is actively transported outward by the gills. Divalent sea salts, mostly magnesium sulfate, are eliminated with feces and secreted by the tubular kidney. Adaptations of Fishes.

Marine bony fishes are hypoosmotic regulators who face a unique set of challenges. They tend to lose water and gain salt because their blood salt concentration is much lower (0.3 to 0.4 M) than the seawater around them (about 1 M). The marine teleost fish is in danger of drying out, much like a desert mammal without water. Again, like their freshwater counterparts, marine bony fishes have evolved an appropriate set of defences (Figure 6). The marine teleost drinks seawater to compensate for water loss. Although this behaviour obviously brings needed water into the body, it also brings a lot of unnecessary salt. Unwanted salt is eliminated in two ways: (1) the major sea salt ions (sodium, chloride, and potassium) are carried by the blood to the gills and secreted outward by special salt-secretory cells; and (2) the remaining ions, mostly divalent ions (magnesium, sulphate, and calcium), are retained in the intestine and voided with the faeces. However, a small but significant proportion of these residual divalent salts in the intestine, ranging from 10% to 40% of the total, penetrates the intestinal mucosa and enters the bloodstream. The kidney excretes these ions. The marine fish kidney excretes divalent ions via tubular secretion, as opposed to the freshwater fish kidney, which forms urine via the usual filtration resorption sequence found in most vertebrate kidneys. Because very little, if any, filtrate is produced, the glomeruli have lost their significance and even disappeared in some marine teleosts.

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