Last Updated 11 Jul 2021

Classification of fish

Essay type Classification
Words 790 (3 pages)
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Specific Purpose: To inform my audience about the different classifications of fish. Central Idea: Fish are classified by their different characteristics which sorts them into different classes like Agnatha, Chondrichthyes, and Osteichthyes.


Classification of living things helps people to understand how living organisms may or may not be related to each other. The system of classification we use today was invented 200 years ago by Carl Linnaeus. Carl Linnaeus system of classification groups organisms by their characteristics or features they have in common.

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According to the book Vertebrate Biology by ORR Robert; Fish, mammals, reptiles, birds, and amphibians are classified together as vertebrates because they all have an internal backbone. In researching this particular topic, I learned that the most important characteristics used in fish classification are the type of bone in their skeleton, the gill structure, the number and shape of fins, and the type of scales. Now I will introduce you to the characteristics and the three different classes of fish. “Jawless (Agnatha), Cartilaginous (Chondrichthyes), and Bony Fish (Osteichthyes).


The Agnatha class is the oldest group, similar to fish but with some different characteristics. As explained in The Evolution of Vertebrate Design, certain characteristics like lacking jaws is what classified fish to this class. The Agnatha class lacks an internal bone skeleton, jaws, and paired fins. Instead of jaws, they have circular toothed mouths called cyclostomic. Which they use to suck blood from the side of their prey. They have a smooth, scale less skin which is soft to the touch.

There are two living groups of Agnatha the lampreys and Hagfish.

  • Lamprey have a single dorsal nostril, a pineal eye, three fins (anterior, posterior, and caudal fin), and a long row or circular gill pouch openings. They make a living being a parasite that eventually kills the prey they latch onto.
  • Hagfish have long eel-like bodies, no eyes, no true fins, a single nostril, up to eight barbels (like whiskers/tentacles for fish) and have rows of horny teeth used to grasp food and draw it in.

Chondrichthyes fish today include both fearsome predator characteristics and harmless characteristics. As explained in Vertebrate Biology, all Chondrichthyes lack true bone. The Chondrichthyes fish have full cartilage skeletons.

  • They have teeth that are made of calcium which grow in rows throughout the fish’s life.
  • They have a regular pattern of fins and there external skin is entirely cartilaginous.

Sharks, skates, and rays make up the Chondrichthyes class. Sharks have pointed snouts and crescent shaped mouths with several rows of sharp triangular teeth. They have five gills on each side that have individual gill slits shown externally.

Skates are rounded to diamond shaped. They have large pectoral fins extending from the snout to the base of their tail. The mouth and gills are on the underside of their bodies. Skates are bottom dwellers and trap their prey by dropping down on their prey from above. Rays can be classified into the following groups: electric rays, sawfish, skates, and many families of rays that have slender whip like tails.

The Osteichthyes (bony fish) is the largest class of vertebrates with over 20,000 species. Stephen Savage states in the book Fish that, Osteichthyes have skeletons made of bone, flat scales, and gills.

  • Bony fish have a much stiffer skeleton because it is reinforced by calcium salts.
  • Bony fish also have acute eyesight unlike other classes of fish
  • Bony fish have a special organ called a swim blatter housed under their bony skeleton is a gas filled chamber that allows the fish to remain floating in the water.

Tuna and Seahorses are examples of bony fish (Osteichthyes). The tuna is stout in the middle and tappers to points at both ends. The tuna had two closely spaced dorsal fins on its back. The tuna is generally metallic dark blue color on its sides and silvery on its underside. The seahorse has a heavy bony armor which makes them poor swimmers. They mostly anchor themselves with their tails to things like seaweed or other kinds of plants. They have a transparent dorsal fin on its back that propels them forward.


  1. An animal is not a fish unless it has all the right characteristics
  2. No matter how different fish are, most fish share several basic characteristics: fins, gills, scales, and hatch babies from eggs.
  3. Fish classification is sometimes confusing and difficult, but it is a useful way of learning about different groups of fascinating fish around the world.


  1. Animal Planet. N. p., n. d. Web. 23 Oct. 2012.
  2. http://animals. howstuffworks. com/fish/fish-info5. htm>.
  4. Print. Radinsky, Leonard B. The Evolution of Vertebrate Design: Leonard B. Radinsky. Chicago: University of Chicago, 1987.
  5. Print. Savage, Stephen. Fish. Austin,Texas: Raintree, 2000.
  6. Print. Wallace, Holly. Classification. Chicago: Heinemann Library, 2000. Print.

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