There are more than 33000 different types of fish species. These fish species have been grouped into separate classes based on their analogical and physiological traits.
The 3 classes that the vertebrate fish are grouped into are: Class Agnatha, Class Chondrichthyes, and Class Osteichthyes.
1. Class Agnatha (Jawless Fish)
Class Agnatha represents one of the earliest surviving lineages of vertebrates. This superclass describes fish species that lack jaws and paired fins.
The members of this group were usually small and had no paired fins. They lived at the bottom of the sea, where they preyed on various types of arthropods.
Given below are the two surviving fish species from class Agnatha.
A) Hagfish
Hagfish are considered to be the most disgusting fish species found in the ocean. There are more than 70 species of hagfish around the globe.
These fish have two rows of tooth-like structures that dig deep into their prey and bite off pieces of food. They also help to clean and re-cycle decaying animals from the ocean floor.
Hagfish are known to produce slime to defend their food from other fish or avert predators. The slime helps the hagfish escape when attacked by predators.
B) Lamprey
There are around 38 known surviving species of lampreys and five known extinct species. These fish are primarily found in temperate areas, around coastal and freshwater bodies.
Among lampreys, the parasitic carnivorous lamprey species are more well-known than other species. These carnivores feed by piercing into the flesh of other fish to suck their blood.
In contrast, the adult non-carnivorous lampreys don’t feed. Instead, they live off their fat stores, accumulated during their larval stage.
2. Class Chondrichthyes (Cartilaginous Fish)
Chondrichthyes is a class containing cartilaginous fish with skeletons made up of cartilage. They’re jawed vertebrates with paired fins, placoid scales, and two-chambered hearts.
The fish species in this class are cold-blooded water dwellers with gills and limbs in the form of fins. Members of class Chondrichthyes lack swim bladders and lungs.
Given below are the fish species from class Chondrichthyes.
A) Sharks
A shark is a type of fish characterized by having a cartilaginous body, five to seven gills on each side of its head, and pectoral fins that aren’t fused to the head.
Sharks can be commonly found in deep and shallow waters, with some traveling long distances to breed and feed. Some shark species are solitary, while others live in groups.
A fierce predator, shark jaws are lined with numerous rows of tiny sharp teeth that can tear apart their prey. Their teeth differ in size and shape, from serrated like razors to triangular like spears.
B) Rays
Rays are cartilaginous fish and members of the class Chondrichthyes. They have large pectoral fins attached at the top of their bodies, from the head to their tails.
Rays are predominantly marine species found in every ocean; in tropical, subtropical, and temperate waters. Many ray species are slow-moving and dwell at the bottom of the sea.
These fish have well-developed jaws that are strong enough to break the shells of clams and other invertebrates and devour small fish.
C) Skates
Skates belong to the class Chondrichthyes. They have a round or diamond-shaped body, covered by large pectoral fins that extend from their nose to the base of their tails.
Skates are found in most regions, from warm tropical to cold Arctic waters and from shallow to deep depths. Their population growth is slow due to low reproduction rates.
They’re harmless bottom dwellers, usually found partially submerged. Skates feed on mollusks, crustaceans, and small fish, trapping them by falling on them from above.
D) Chimaeras
Check out what these fish look like over here.
Chimaeras, also known as ghost sharks or ratfish, belong to the class Chondrichthyes. These fish have long, slender bodies with a large head and a single gill opening.
These fish species are found in all the oceans, except the Arctic and Antarctic. They mainly inhabit the deepest parts of the ocean.
They’re opportunists feeders that crush their prey with their three rows of tooth plates. Their diet mainly consists of crustaceans, mollusks, marine worms, octopuses, and sea urchins.
3. Class Osteichthyes (Bony Fish)
Class Osteichthyes is the biggest fish class, with more than 90% of the bony vertebrate fish. Today there are more than 28000 living species belonging to this class.
These fish have distinguishing features like a skeleton made of bone tissue, scales, paired fins, jaws, pair of gill openings, and paired nostrils.
Osteichthyes class is divided into two sub-classes:
- Ray-Finned Fish, and
- Lobe-Finned Fish.
A) Ray-Finned Fish
Around 27000 species from the Osteichthyes class are part of the sub-class Ray-Finned Fish. These fish species have fins webbed together by bones or rigid plates.
Fish of the Ray-Finned Fish sub-class play a vital role in both marine and freshwater ecosystems in the form of prey and predator. They’re also essential for human livelihood, aesthetics, and nutrition.
Given below are a few fish species belonging to the Ray-Finned Fish sub-class.
I) Catfish
Catfish are a diverse group of Ray-Finned Fish. They tend to be negatively buoyant, so they usually sink rather than float because they have a small gas bladder and a heavy head.
They have elongated bodies with dorsal and anal fins that are exceptionally long, almost reaching the caudal fins. They have up to eight barbels on the chin, which helps them detect food.
These bottom-feeders are mainly omnivorous fish that feed at night. Their diet includes aquatic plants, seeds, small fish, insects, mollusks, insect larvae, and crustaceans.
II) Cichlids
Cichlids are Ray-Finned Fish and one of the most diverse groups of colorful fish species native to the freshwater lakes of Africa and river basins of South and Central America.
Cichlids vary widely in their body shapes, ranging from laterally compressed bodies to cylindric and highly elongated ones. These fish usually range from medium to large sizes.
In the wild, a cichlid’s diet varies according to its habitat and dietary group.
Herbivore cichlids eat algae, biofilm, and plants. Carnivore cichlids feed on small wildlife and other fish, while omnivore cichlids feed on both plant matter and meat.
III) Salmon
Salmon belongs to the family Salmonidae, and the name is widely used for several species of Ray-Finned Fish. Salmons are native to the waters of the North Atlantic and Pacific oceans.
Salmon have torpedo-shaped bodies. However, some fish species are flat, and some are string-like or round. The streamlined shape lets the fish navigate effortlessly through the water.
Salmon eat a variety of sea life, including smaller fish such as lanternfish, herring, sand lance, and barracuda. They also eat grasshoppers, krill, squid, and polychaete worms.
IV) Sturgeons
Sturgeon fish belong to the family Acipenseridae, and the name is commonly used for several species of this family. These Ray-Finned Fish inhabit the coastlines of Eurasia and North America.
These fish have long spindle-like bodies with scaleless skin. Their head is covered with scutes that run along the fish’s body from head to tail.
They feed by extending a siphon-like mouth to suck up food from the seafloor. Sturgeons are mainly benthic feeders; they eat shellfish, crustaceans, and small fish.
B) Lobe-Finned Fish
Fish species belonging to this sub-class have fleshy lobed fins. The pectoral and pelvic fins have joint structures similar to tetrapods’ limbs. At present, there are two types of lobe-finned fish.
They also have two dorsal fins with separate base structures, unlike the one dorsal fin of Ray-Finned Fish. These fish species have teeth that are covered with actual enamel.
Given below are two fish species of the Lobe-Finned Fish sub-class.
I) Lungfish
Lungfish are freshwater fish known for their ability to breathe air. There are six known lungfish species that inhabit the waters of Africa, Australia, and South America.
Lungfish have a long body covered by thick skin, paired pectoral fins, pelvic fins, and tail fins. Their lungs are exceptionally well developed so that they can breathe air directly.
Lungfish are voracious eaters, consuming a wide range of aquatic creatures, including members of their own species. They also eat pieces of meat, earthworms, tadpoles, frogs, and small fish.
II) Coelacanths
Coelacanths live primarily in the western Indian Ocean, but they can be found elsewhere along the eastern African coast and in the Indonesian seas.
These fish have large-sized bodies covered in cosmid scales that act as armor. They have eight fins; two dorsal fins, two pectoral fins, two pelvic fins, one anal fin, and one caudal fin.
Coelacanths move slowly and passively near the floor, feeding primarily on cephalopods like cuttlefish, octopus, squid, and fish.
