Survey of Fishes – Classification and life cycle


Debate on taxonomy of fishes is going on for many years. Modern cladistic analysis has revised he taxonomy of this group of vertebrates.

(a) Traditional classification: The fishes are traditionally divided into super classes. This classification is based on the presence or absence of jaws and paired appendages.

(b) Cladistic analysis shows that some members of the agnathans (lampreys and some ostracoderms) are more closely related to jawed fishes (gnathostomes) than to other agnathans.

These traditional groupings are paraphyletic. It has been rejected by most researchers. But terms like agnathans and gnathosomes are common in zoological literature. They are used as convenient non taxonomic grouping in this text book.


They have following characteristics:

1. They lack jaws and paired appendages.

2. They have cartilaginous skeleton.

3. They have persistent notochord.

4. Two semicircular canals are present in them. Hagfishes have one semicircular canal. It is formed by the fusion of two canals.


Ostracoderms are extinct agnathans. They are belonged to several classes. the fossils of predatory water scorpions are found with fossils of ostracoderms. Ostracoderms were sluggish animals. They have bony armor used for defense. Ostracoderms were bottom dwellers. They were about 15 cm long. Most were filter feeders. They filter suspended organic matter from the water Or they extract annelids and other animals from muddysediments. Some ostracoderms used bony plates around the mouth as a jaw. They used these bones to crack gastropod shells or the exoskeletons of arthropods. Agnathans have two classes:

Class Myxini (mysa, slime)

1. Their mouth has four pairs of tentacles.

2. The olfactory sacs open to mouth cavity.

3. Theyhave 5 to 15 pairs of pharyngeal slits

Example: Hagfishes.



Hagfishes are the members of the class Myxini. Hagfishes remain buried in the sand and mud of marine environment.  They feed on soft-bodied invertebrates and eat dead and dying fish. Hagfishes enter the fish through the mouth. It eats the contents of its body. They leave only skin and bones of victim fish. Anglers  must be careful about hagfishes. Hagfishes bite at a baited hook. Hagfishes swallow the hook deeply. The hook is lodged near the anus. Hag fishes have slimy bodies. Therefore, fishermen cannot capture it. Al last he has to cut his lines and tie on a new hook. Most zoologists now consider the hagfishes as the most primitive group of vertebrates.

Class Cephalaspidomorphi (cephak, head + aspidos, shield + morphe, form)

1. They have sucking mouth with teeth and rasping tongue.

2. They have seven pairs of pharyngeal slits.

3. They have blind olfactory sacs.

Example: Lampreys


Habit and habitat

Lampreys are belonged to class Cephalaspidomorphi. They live in marine and freshwater environiments in temperate regions.



Most adult lampreys prey on other fishes. Their larvae are filter feeders. The mouth of an adult of an adult is sucker like. It is surrounded by lips. They lips have sensory and attachment functions. Numerous epidermal teeth line the mouth. They cover a movable tongue like structure. Adult lamprey is attached to prey with their lips and teeth. It uses its tongues to rasp away scales. Lampreys have salivary glands. These glands produce anticoaglant secretions. Lampreys feed mainly on the blood of their prey. Some Lampreys are not predatory.

Members of the genus Lampera are called brook lampreys. The larval stages of brook lamprey  last for three years. They adults do not feed or leave their stream. They reproduce. They die soon after metamorphosis.

Life cycle

1. Reproduction

Adult sea lampreys live in the ocean or the Great lakes. They migrate near the end of their lives to a spawning bed in a freshwater stream. Lamprey makes their nest small depressions in the substrate. A female attaches itself to a stone with her mouth.  A male uses his mouth to attach to the female’s head. It wraps his body around the female. Eggs are shed in small groups over a period of several hours. Fertilization is external. After that the adult died.

2. Development

The sticky eggs are then covered with sand. Eggs are hatched in three weeks and ammocoete larvae are formed. The larvae move down to softer themselves in sand and liter. The larvae feed like amphioxus. Ammocoete larvae grow from 7 cm to about 17 cm in three years. Finally, the larvae are metamorphosed in the adult in several months. Its mouth becomes sucker like. It develops teeth, tongue and feeding musculature. Now lamprey leaves the mud permanently. It goes back to sea and becomes predator.



1. They have hinged  jaws and paired appendages.

2. Vertebral column replaccs the notochord.

3. Their internal ear has three semicircular camds

Two major adaptations take place in the evolution vertebrate. These adaptations were the appearance of  jaws and paired appendages.

(a) Jaws are used in feeding. It is responsible for a transition to more active and predator lifestyles. Pectoral and pelvic fins of fishes are paired appendages in fishes.

(b)     Pectoral fins are present just behind the head. The pelvic tins are located ventrally and posteriorly. Both paired fins are used for steering mechanism. It increases agility in fishes.

There are four classes of gnathostomcs.

1. Class Chondrichthyes: These are cartilaginous fishes.

2. Class Osteichthyes: These are bony fishes. Both these classes have living fishes.

3. Placoderms: They are the armored fishes. They contained the earliest jawed fishes. These are now extinct. They left no descendants.

4. Acanthodians: It is a fourth group of ancient extinct fishes. They are more closely related to the bony fishes.


CLASS CHONDRICHITHYES (chondros, cartilage + ichthyos, fish)

These are cartilaginous fishes. Class Chondrichthyes include the sharks, skates, rays, and rat fishes.

1. Most chondrichthians are carnivores or scavengers. Most are marine.

2. They have biting mouth parts.

3. They have paired appendages.

4. They possess dermal placoid scales and a cartilaginous endoskeleton

A. SUBCLASS ELASMOBRANCHII (elasmos, plate metal + branchia, gills)

1. Their cartilaginous skeleton is partially ossified (bony).

2. Placoid scales are present or they have no scales.

Examples: Sharks, skates, rays


1. Evolution of sharks
Sharks arose from early jawed fishes in the Devonian period, about 375 million years ago. Certain features of bony fishes are absent in elasmobranchs. These features are swim bladder, regulation of buoyancy, a gill cover and a bony skeleton. It is believed that these are primitive features. This is wrong interpretation. These characteristics are lost due to different adaptations in the two groups.

2. Body wall

Sharks have tough skin. Dermal placoid scales over the skin. These scales project posteriorly. They give the skin tough sandpaper  like texture.  Posteriorly pointed scales also reduce friction in water during swimming.

3. Teeth in sharks

The teeth of shark are modified placoid scales. There is a row of teeth on the outer edge.  Another row of teeth is present behind the first row. The teeth of second row, are attached to a band of ligament.

This band covers the jaw cartilage inside the mouth. Sharks shed the old and useless teeth of outer row. New teeth move into their position from inside the jaw and replace them. This replacement is rapid in young sharks. New row of teeth is developed after every seven or eight days in them. Crowns of teeth are adapted for tearing prey or for crushing the shells of molluscs.


4. Size and habits



Sharks range in size from less than 1 m to greater than 10 m (e.g., basking sharks and whale sharks). The largest sharks are not predatory. They are filter feeders. They have modifications in pharyngeal-arch. These arches capture planktons. The great white shark and the mako are most feared sharks. Extinct specimens have reached the lengths of 15m in or more.

Skates and Rays

Skates and rays are specialized for life on the ocean floor. They live in shallow water. They use their blunt teeth to feed on invertebrates. They have lateral expansion of the pectoral tins into wing like appendages. It is a modification for life on the ocean floor. Locomotion takes place by dorsoventral muscular waves. This wave passes posteriorly along the fins. They have elaborate color patterns on the dorsal surface. It provides effective camouflage . The tail of sting ray is modified into a defensive lash. The dorsal fin forms a venomous spine. The electric rays (Narcine and Torpedo) and manta rays (Mania) also include in this group.


B. SUB CLASS HOLOCEPHALI (Or. holos, whole + kephalidos head)

  1. Operculum covers pharyngeal slits
  2. They lack scales.
  3. Their teeth are modified into crushing plates.
  4. Tat rid line receptors are present in an open groove.
  5. It c mains about 30 species.

Example: Chimaeras (Ratfishes)

Evolution of holocephalans

Holocephalans evolved from the Chondrichthyes nearly 300 million years ago. Since then many specializations took place in them. These specializations are: Formation of a gill cover called an opereulum. Teeth are modified in to large plates for crushing the shells of molluscs.


It has a large head with a small mouth. Mouth is surrounded by a large lip. It has a narrow tapering tail. Therefore, they are named as “radish“.


CLASS OSTEICHTFItES (Or. osteon, bone + icthyus, fish)

1. Most fishes have bony skeleton.

2. Operculum covers single gill opening.

3. They have pneumatic sacs which ftmction as lungs or swim bladders.

4. They have 20.000 species. Therefore, it is a successful group of fishes.

Example: Bony fishes.

The First fossils of bony fishes were found from late Silurian deposits (approximately 405 million years old). The two subclasses of Osteichthyes evolved in Devonian period (350 million years ago).

Sub class Sarcopterygii (Lobe finned fishes) (sark, flesh + pteryx, fin)

1. They have paired fins with muscular lobes.

2. The pneumatic sacs function as lungs.

Example: Lungfishes and coelacanths (lobe-finned fishes).

1. Lung Fishes

The air sac in these fish is changed into lungs. Therefore, they are called lung fishes. Only three genera survive today. They all are present in the regions where seasonal droughts are common. These fishes use lungs to breathe in stagnate and dry freshwater lakes and rivers. The species are:

(i) Neoceratodus: It lives in fresh waters of Queensland, Australia. It survives in stagnant water by breathing in air. But they normally use gills. They can not withstand total dry condition.

(ii)  Protopterus: These are found in freshwater rivers and lakes in tropical Africa.

(iii) Lepidosiren: These are found in tropical South America. They can survive when rivers or lakes are dry. They form burrow into the mud. They keep an air pathway. They open this pathway by bubbling air to the surface. Small opening in the earth are produced after the substrate dries. These opening are the only evidence of the sense of lung fish, Lung fishes may remain in aestivation for six months or more. Aestivation is a dormant state that helps an animal to withstand hot, dry periods. The rain again fills the lake or river bed. The lung fishes emerge from their burrows to feed and reproduce.


2. Coelacanths

It is a second group of sarcopterygians. The most recent coelacanth fossils are over 70 million yeaars old. However in 1938, coelacanth was found from the deep water of the coast of South Africa. After that numerous other specimens have been caught in deep water around the Comoro Islands of Madagascar. Then an important fish was discovered. It is called Latimeria chalumnae. It is the closest living fish relative of terrestrial vertebrates.

Latimaria is a large fish. Its weight is up to 80 kg. It has heavy scales on its body wall. Ancient coelacanths lived in freshwater lakes and rivers. Thus the ancestors of Latimaria must have moved from freshwater habitats.

3. Osteolepiforms

A third group of sarcoplerygians is called Osteolepiforms. It became extinct in the Paleozoic period. They are taken as ancestors of ancient amphibians.

Subclass Actinopterygii (Ray tinned fishes) (aktis, ray + pteryx, fin)

1. Their paired fins are supported by dermal rays.

2. The basal portions of paired fins are not muscular.

3. They have homocercal tail. In this case, the tail fin has equal upper and lower lobes.

4. They have blind olfactory sacs.

These fishes are commonly called the ray- finned fishes. Their fins lack muscular lobes. They usually possess swim bladders. Swim bladders are gas-filled sacs. These are present along the dorsal wall of the body cavity. Swim bladder regulates the buoyancy. Zoologists now realize that there are many points of divergence in the evolution of the Actinopterygii. One modern classification system divides the Actinopterygii into two infra classes.

(a) Infra class Chondrosteans

It contains many species. These species lived during the Permian, Triassic, and Jurassic periods. But only 25 species remain today. Ancestral Chondrosteans had a bony skeleton. But living members like sturgeons and paddlefishes have cartilaginous skeletons. Chondrosteans have a tail with a large upper lobe.


Most sturgeons live in the sea. They migrate into rivers to breed. Some sturgeons live in freshwater. But they maintain the migratory habits of their marine relatives. They are large fishes. Bony plates cover the anterior portion of the body. Heavy scales cover the tail. The sturgeon mouth is small and jaws are weak. Sturgeons feed on invertebrates. Sturgeons are important for their caviar. Therefore, they are severely over-fished.


Paddle fishes

Paddle fishes are large, freshwater chondresteans. They have a large paddle like rostrum. Rostrum contains many sensory organs. These sense organs can detect weak electrical fields. They swim through the water with their mouths open. They filter crustaceans and small fishes. They are found mainly in lakes and large rivers of the Mississippi River basin. They are also present in western North America and China.


(b) Actinopterygians

They flourished in the Jurassic period. They succeeded most chondrosteans. Two very primitive genera occur in temperate to warm freshwaters of North America.

1. Lepisosteus (garpike): It has thick scales. It has lone jaws. It uses these jaws to catch fishes.

2. Amia (teleosts or modern bony fishes): These are commonly called dogfish or bowfin. Most living fishes are members of this group. They are called as teleosts or modern bony fishes. They have a symmetrical caudal fin and a swim bladder.The swim bladder has lost its connection to the digestive tract. The teleosts have diverged from ancient marine actinopterygians in the late Triassic period. After that remarkable evolutionary diversification take place in them. Teleosts have adapted to nearly available aquatic habitat. The number of teleosts species are 20,000.

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