Evolutionary Pressures in Fishes


The aquatic environments have physical characteristics. These characteristics are important selective forces for aquatic animals. Fishes have following adaptive characteristics:


Water is dense. It makes movement through it difficult. It makes movement through it difficult. However, fishes uses less energy for swimming than running from a terrestrial organism. The fish have stream lined . Its mucoid secretions lubricate its body surface.


It reduces friction between the fish and the water. The buoyant properties of water also increase he efficiency of movement fish through the water. A fish spends little energy in support against the pull of gravity.

Fishes move through the water with the help of fins. They use the body wall to push against He incompressible surrounding water. The muscle bundles of most fishes are arranged in a special pattern. These muscles extend posteriorly and anteriorly in a zigzag fashion The contraction of each muscle bundle affects a large portion of the body wall. The vertical caudal (tail) fin supplements body movements in very efficient like fast­-swimming fishes like tuna and mackerel. Their caudal fin is tall and forked. The forked shape of the caudal fin reduces surface area.



Evolution of Nutrition

The earliest fishes were filter feeders and scavengers . They move through the mud of ancient sea floors. These eat decaying organic matter, annelids, molluscs or other bottom dwelling invertebrates. The evolution of jaws transformed early fishes into efficient predators. Therefore, fish nutrition has changed now.

Types of food

Most modern fishes are predators. They spend much of their life searching for food. They have different types of preys. Some fishes feed on invertebrate animals. These animals float or swim or live in or on the substrate. Many feed on other vertebrates. Similarly, the fishes eat different kinds of food during different period of life. Fish feed on plankton as a larva. Adult fishes eat large prey like annelids or smaller fish.


Fishes swallow prey as a whole. Teeth capture and hold prey. Some fishes have teeth modified for crushing the shells of molluscs or the exoskeletons of arthropods. Some fishes use the suction to capture prey. The fishes close the opercula and rapidly open the mouth. It creates negative pressure. This pressure sweeps water and prey inside the mouth

Herring, paddlefishes and whale sharks arc filter feeders. They have long gill processes called gill rakers. These gill rakers trap plankton during swimming with Open mouth. A few fishes,  such as carp, feed on different plants and small animals. A few fishes like lamprey are external parasites for some part of lives. A few are herbivores. They feed on plants.

Digestive tract

The digestive tract of fish is similar to other vertebrates. They have a large stomach. It stores large meals. The enzymes are secreted in small intestine. It is the primary site for food digestion. Sharks and other elasmobranchs have a spiral valve in their intestine. The bony fishes possess pyloric coeca. It is pockets of the intestine. It increases absorptive and secretary surfaces.



All vertebrates have a closed circulatory system. Heart pumps blood. It contains red blood cells containing hemoglobin. Blood passes through a series of arteries, capillaries and veins.  The evolution of lungs and circulatory systems take at the same time. These changes are the loss of gills, delivery of blood to the lungs. and separation of oxygenated and deoxygenated blood in the heart.

Blood circulation in fishes

Four embryological enlargements or a ventral aorta take place during development of heart of vertebrates. ‘These enlargements are sinus venosus, atrium, ventricle and conus arteriosus.

1.  Blood flows from the venous system of fishes.

2. It passes through sinus VC110SUS. atrium, ventricle, and colitis arteriosus.

3. Then it enters into the ventral aorta.

4. Five afferent vessels carry blood to the gills. These vessels branch into capillaries in gills.

5. Oxygenation takes place and blood is collected by efferent vessels.

6. It is passed to dorsal aorta. Dorsal aorta distributed it to the body.

Circulation of blood in lung fishes

The lungs have altered the circulatory pattern. Circulation to gills continues. But a branch of aortic arch VI forms pulmonary artery. This artery supply blood to lungs. Blood returns to heart through pulmonary veins from the lungs. It enters into the left side of the heart. The atrium and ventricle of the lung fish heart are partially divided. These partial divisions keep deoxygenated blood separate from the oxygenated blood from the lungs. A spiral valve is present in the coin’s arteriosus. It directs blood from the right side of the heart to the pulmonary artery. It directs the blood from the left side of the heart to the remaining aortic arches. Thus distinction between a pulmonary circuit and a systemic circuit is present in lung fishes.


Water contains 2..5 % of the oxygen present in air. Therefore, less amount of oxygen is available to fishes in water. Fishes must pass large quantities of water across gill surfaces. They extract a small amount of oxygen from this water. It maintains adequate levels of oxygen in their blood stream. There are following mechanisms of inspiration and expiration in fishes:

1. Pumping mechanism: Most fishes have a muscular pumping mechanism. It moves the water into the mouth and pharynx and over the gills. It also moves water out of the fish through gill opening. Muscles of the pharynx and opercular cavity power this ta.am.

2. Ram ventilation: This mechanism is present in some elasmobranchs and open-ocean bony fishes like tuna. These fishes keep their mouths open during swimming. It maintains water flow in the pharynx. This method is called ram ventilation. Elasmobranchi do not  have opercula for pumping water. Therefore, some sharks must keep moving  to survive.

3. Some elasmobranchs have gill bars with external flaps. These flaps are closed and form an opercular cavity like other fishes. Spiracles are modified pharyngeal slits. They open just behind the eyes of elasmobranchs. These spiracles are used as an alternate route for water entering the pharynx.


Gas exchange through gills is very efficient. Gill arctics support gills. Gill filaments extend from each gill arch. Gill filament is composed of pharyngeal lamellae. These


Fig: Gas Exchange at the Pharyngeal Lamellae. (a) The gill arches under the operculum (b)Electron micrograph tip of a trout gill filament showing numerous lamellae. (c,d) A comparison of an aninrercurrent and parallel exchanges. (d).Oxygen diffuse from water

lamellae are vascular folds of epithelium. Branchial arteries carry blood to the gills and into gill filaments. The arteries break into capillary in pharyngeal lamellae. Blood and water move in opposite directions on lamellar epithelium and exchange of gases take place. This opposite flow is called countercurrent mechanism. It maintains a concentration gradient between the blood and the water over the entire length of the capillary. Therefore. exchange of gases takes place efficiently.


Fig: Possible Sequence in the Evolution of Pneumatic Sacs. (a) Pneumatic sacs may have originally developed from ventral outgrowths of the esophagus. (b) Primitive lungs developed further during the evolution of vertebrates. (c) in most bony fishes, pneumatic sacs are called  swimbladders; and these arc modified for buoyancy regulation.


The Indian climbing perch spend its life completely on land. These fishes have gas chambers called pneumatic sacs.

(a) In some fishes, a pneumatic duct connects the pneumatic sacs with the esophagus or another part or the digestive tract. These fishes are nonteleost and some teleosts. Swallowed air enters these sacs. Exchange of gas occurs through its vascular surfaces. Thus, pneumatic sacs function as lungs in the Indian climbing perch. lung fishes, and ancient rhipidistians.

(b) In other bony fishes, pneumatic sacs act as swim bladders.

Most zoologists believe that lungs are more primitive than swim bladder. The evolution of curl bony fishes took place in warm, fresh water lakes and streams during Devonian period. These rivers and streams frequently became stagnant and dried. Only those fishes survive in this condition which had lung. The later evolution of modem bony fishes takes places in marine and freshwater environment. Stagnation was not a problem there. In these environments, they use pneumatic sacs in buoyancy regulation.

Buoyancy Regulation

Fishes maintain their vertical position in a column of water by four adaptations:

1. First adaptation: The fishes incorporate low-density compounds into their tissues. Fish s are saturated with buoyant oils.

2. Second adaptation: The fishes use fins to provide lilt. I he pectoral fins of’ a shark arc ;limning devices. It creates lift as the shark moves through the water. The caudal fins of sharks have large upper lobe. It.provides upward thrust for the posterior end of the body.

3.Third adaptation: There is reduction in the heavy tissues of fishes. The bones of the fishes are less dense than the terrestrial vertebrates. The development of cartilaginous skeleton in elasmobranchs is the adaptive features. The cartilage is only slightly heavier than water.

4. Fourth adaptation: The fourth adaptation is the swim bladder. A fish regulates buoyancy controlling the volume of gas in its swim bladder. There are two adaptations in swim bladders:

(a)  The pneumatic duct connects the swim bladders to esophagus or another part of the digestive tract. It occurs in garpike, sturgeons and other primitive bony fishes. These fishes gulp air and force air into their swim bladders.

(b)The swim bladders of most teleosts have lost a connection to the digestive tract. The blood secretes gases (various mixtures of nitrogen and oxygen) into the swim bladder. There is a countercurrent exchange mechanism in rete mirabile (miraculous net). Pete mirabile is a blood vascular network. Gases are reabsorbed into the blood at the posterior end of the bladder.


The central nervous system of fishes consists of a brain and a spinal cord. Sensory receptors widely distributed over the both . The receptors for touch and temperature are distributed on the body of fishes. Fishes also possess specialized receptors for  olfaction,vision, hearing, equilibrium and balance, and for detecting water movements.

1. Olfaction receptors



The snouts of fishes open out side by external nares. Snout has olfactory receptors. Most fishes have blind-ending olfactory sacs. In a few fishes, the external nares open in to nasal  passages and the mouth cavity. Some fishes rely heavily on their sense of smell. For example. salmon and lampreys return to spawn in the streams in which they hatched years earlier. They cover distances of hundreds of kilometers during their migrations. ‘[he characteristic odors of their spawning stream guide them.

2. Eyes

The eyes of fishes are similar to other vertebrates. However, they are lidless. Their lenses are rounded. They move the lens forward and backward during focusing.

3. Ear

Receptors for equilibrium, balance and hearing are present in the inner ears of fishes. Their functions are similar to other vertebrates.

(a)  Equilibrium: Semicircular canals detect rotational movements. Other sensory patches detect the direction of the gravitational pull for equilibrium and balance. Fishes lack the outer or middle ear. Outer and middle ears conducts sound win es to the inner ear in other vertebrates.

(b)  Hearing: Most fishes can hear. Vibrations pass from the water to the middle ear through the bones of the skull. A few fishes have chains of bony ossicles. These ossicles connect the swim bladder to the hack of the skull. Swim bladders can amplify the vibrations. The ossicles then send them to skull.

4. Lateral line system

Most fishes have lateral-line system. It runs along each side and branching over the head of fishes. The lateral line system consists of sensory pits. These pits are present in the epidermis of the skin. These pits are connected to canals. This canal runs just below the epidermis. The pits have receptors. These receptors are stimulated by water moving against them. Lateral lines are used to detect water currents. They are also used to detect a predator or a prey. Fishes can also detect lo-frequency sounds \kith these receptors.


The activities of nerves and muscles produce weak electrical fields in all organisms. Electroreception is the detection of electrical fields that the fish or another organism generates in the environment. Electroreception and electrogeneration has been discovered in live hundred species of fishes in seven families of chondriehthyes and Osteichthyes. These fishes use their electroreceptive sense for detecting prey. They also used it for orienting towards or away from objects in the environment.

Electroreception in sharks

The sense of prey detection is better developed in rays and sharks. Spin dogfish sharks locate prey by electroreception. A shark can lind and eat a flounder that is buried in sand. It will try to find and eat electrodes that are creating electrical signals similar to those that the flounder. But a shark cannot find a dead flounder buried in the sand or a live flounder covered by an insulating polyvinyl sheet.

Electrorecption and electrogeneration in electric fish

Some fishes are capable of electroreception. These can also generate electrical currents. An electric fish (Gymnarchus niloticus) lives in freshwater in Africa. Muscles near its caudal fin are modified into electrical discharge organ. Its current spreads between the tail and the head. Pore like perforations are present near the head. These pores contain electroreception. The electrical waves circulate between the tail and the head. If any object comes between tail and head, it distorts its electric field. This distortion is detected by changing patterns of receptor stimulation. Gymnarchus live in murky fresh water. Thus it has limited use of eyes. Therefore, it uses electrical sense to locate prey.

Electric eel and electric rays

Electric eel (a bony fish) and electric ray (an elasmobranch) produce strong electrical currents. The electriceel live in river of the Amazon Basin in South America. Electrical currents producing organs is present in the trunk of the electric eel.  It can deliver shocks of 500 volts. The electric ray has electric organs n its fins. It can produce pulses of 50 amperes at about 50 volts. These shocks can stun or kill prey. It discourages large predators to come close to it.



The maintenance of water and salt balance in the body is called osmoregulation. Fishes must maintain a proper balance of electrolytes (ions) and water in their tissues.


The osmoregulation is a major function of the kidneys and gills of fishes. Kidneys are located near the midline of the body. These arc present dorsal to peritoneal membrane. This peritoneal membrane lines the body cavity. The excretory structures in the kidneys are called nephrons. Nephrons filter nitrogenous wastes, ions, water, and small organic compounds through glomeruli. The filtrate then passes through a tubule system. These tubules can reabsorb essential components. The remaining filtrate in the tubule system is then excreted.

Osmoregulation in fresh water fishes

Freshwater contains few dissolved substances. Therefore, osmotic uptake of water across

gill, oral and intestinal surfaces take place. Thus excretion and defecation lose essential ions. These fishes have following adaptations:

1. The freshwater fishes never drink water to control excess water and ion loss. They take water in only during feeding.

2. The nephrons of freshwater fishes possess large glomeruli and short tubule systems. Reabsorption of some ions and organic compounds takes place after filtration. Their nephrons have short tubule system. Thus little water is reabsorbed. Therefore,  freshwater fishes produce large quantity of very dilute urine.

3. Ions can be lost through the urine. These are also lost by diffusion across gill and oral surfaces. The gills of these fishes can absorb ions by active transport. It compensates this ion loss. Fresh water fishes also get some salts through their food.

Osmoregulation in marine fishes

Marine fishes face the opposite problems. Their environment contains 3.5% ions. But their tissues contain 0.65% ion. Therefore, marine fishes face the problem of water loss and accumulation of excess ions. They drink water to compensate the loss of water. They eliminate excess ions by excretion, defecation and active transport through gill. The nephrons of marine fishes possess small glomeruli and long tubule systems. Therefore, less blood is filtered than fresh water fishes. Water is efficiently reabsorbed from the nephron.

Marine Fish

Osmoregulation in Elasmobranchs

Flasmobranchs have a unique osmoregulatory mechanism. They convert some of their nitrogenous wastes into urea in the liver. But most other fishes excrete ammonia. Urea is distributed in tissues all over the body. Thus enough urea is stored in the body. It makes body tissues isosmotic with sea water. Therefore, elasmobranchs do not loss water to their environment. Thus they save the energy used in water conservation. Urea can disrupt important enzyme systems in the tissues. Therefore, this adaptation requires the development of tolerance to high levels of urea.

Despite this unique adaptation, elasmobranchs still regulate the ion concentrations in their tissues. The have ion-absorbing and secreting tissues in their gills and kidneys. The elasmobranchs possess a rectal gland. It removes excess sodium chloride from the blood into the cloaca.

Osmoregulation in Diadromous fishes

The fishes which migrate between freshwater and marine environments are called diadromous. Salmon and marine lampreys migrate from the sea to freshwater to spawn. The freshwater eel (Anguilla) migrates from freshwater to marine environments to spawn. The gills of the diadromous fishes can balance the ions in the body. But osmoregulatory power may not develop during all life-history stages. For example, young salmon cannot enter the sea until certain cells on the gills develop ion-secreting powers.

Exertion in fishes

Fishes do not face much problem in removing the nitrogenous wastes. These nitrogenous wastes are byproducts of protein metabolism. 90% of nitrogenous wastes are eliminated as ammonia through gill by diffusion. Ammonia is a toxic substance. But the aquatic organisms can diffuse ammonia easily in the surrounding water. The remaining 10% of nitrogenous wastes are excreted as urea creatine or creatinine. These wastes are produced in the liver. They are excreted through the kidneys.



Fishes produces a large number of eggs. It increases the chance of fertilization for the survival of the fish. There are other adaptations to increase the chance of fertilization. Some fishes mating behavior. It ensures fertilization. Some fishes show nesting behavior. Nest protects eggs from predation, sedimentation and fouling.

Mating  may occur in large groups. One individual releases eggs or sperm. It often release spawning pheromone. This pheromone induces many other adults to spawn. Huge lasses of eggs and sperm are released into the open ocean. It ensures the fertilization of many eggs.

Some fishes have specialized structures for transfer of sperms. Male elasmobranchs have modified pelvic fins called claspers. The male inserts clasper into the cloaca of a female during copulation. Sperm travel along grooves of the clasper. Fertilization occurs in the female reproductive tract. A large number of eggs are fertilized than in external fertilization. Thus, fishes with internal fertilization produce fewer eggs.


The fishes may be:

1. Oviparous: The majority of fishes are oviparous. Their eggs develop outside the femalefrom stored yolk.

2. Ovoviviparous: Some elasmobranchs are ovoviviparous. Their embryos develop in a modified oviduct of the female. Nutrients are supplied from yolk stored in the egg.

3. Viviparous: Other elasmobranchs like gray reef sharks and hammerheads are viviparous. Their oviduct is modified in to a placenta like outgrowth. Placenta transfers nutrients from the female to the yolk sacs of developing embryos. Internal development of viviparous bony fishes occurs in ovarian follicles. The eggs are retained in the ovary in guppies (Lebistes). Fertilization and early development take place there. Embrvos are then released into a cavity within the ovary. The development starts. Nourishments are supplied by yolk and ovarian secretions.

Parental care in fishes

1. Care before hatching

In many fishes, care of the embryos is limited or absent. However, some fishes construct and tend nests. Some fishes carry embryos during development. Clusters of embryos are brooded in special pouches. These pouches are attached to some part of the body. Embrvo may also he brooded  in the mouth. Some best-known brooders are the seahorses (Hippocampus) and pipefishes (Syngnathus). Males of these fishes carry embryos in ventral pouches. Development takes place there. The male Brazilian catfish broods embryos in an enlarged lower lip.

2. Care after hatching

Most fishes do not care for young after hatching. However,  sunfishes and sticklebacks provide short-term care after hatching of young.

(i)  Male sticklebacks collect fresh plant material into a mass. The youngs live in this mass of leaves. Sometimes, young moves away from the nest. The male bring it in its mouth and spits it back into the nest. Sunfish males also show similar behavior.

(ii) The Cichlidae shows longer-term care. The young are brooded in mouth in some species. In others the species tend young in a nest. After hatching, the young come out from the parent’s mouth or nest. The parent signals danger with a flicking of the pelvic fins. So young return quickly to nest or mouth.

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