What is a fish?
- They have gills, and breathe water.
- Usually, they are “cold-blooded” – their body temperature will be the same as the surrounding water.
- Usually, they cannot survive in air.
What kinds of fish are there?
Scientists normally group animals into groups of similar creatures. The largest groups are called “phyla” (for example, the vertebrates make up most of the phylum called Chordata. The next smaller groups are called “classes” (for example, the birds make up a class of their own). Fish, however, are somewhere in between. They are only part of the phylum Chordata, but there are actually several classes of fish. The most well-known ones are the Cartilaginous fish (sharks, rays, skates) and the “Bony” fish, which includes two classes – Actinopterygii (“normal” fish) and Sarcopterygii (lungfish and similar creatures). Many of the other classes are completely extinct now. Here’s a list of all the different classes of fish.
- The Acanthodii, sometimes called Spiny Sharks, are an extinct group of fish that had some characteristics of Cartiliginous fish, and others of bony fish.
- The Actinopterygii are the largest class of fish, in fact, the largest class of vertebrates! There are 27000 species of these, living in both sea and fresh water. Scientists call them “ray-finned” fish, you or I might call them “normal” fish. The Leedsichthys was an Actinopterygus.
- The Cephalaspidomorphs are a group of extinct jawless fishes. Actually, scientists are divided as to what animals exactly should belong to this group – some say the lampreys should be included, others disagree.
- The Condrichthyes are the cartilaginous fishes , including sharks and rays. These fish don’t have true bone, instead, their skeleton is made of cartilige. The Whale Shark is a Condrichthys.
- The Conodonts : a group of extinct worm-like animals, known mainly from their fossilized teeth.
- The Hyperoartia is the class of the Lampreys, a group of eel-shaped sea animals, that are so different from fish, that scientists put them in a different group altogether.
- The Myxini : these are the Hagfish – they look a bit like eels, and can tie themselves into knots.
- The Placoderms : another group of extinct fish, which had hard plates to protect their heads.
- The Pteraspidomorphs : a group a extinct fish, which didn’t have jaws.
- The Sarcopterygii, or lobe-finned fish, include the lungfish, and the once-thought-extinct coelacanths.
What is the world’s largest fish?
The world’s largest living fish is a very gentle giant – the Whale Shark. It’s a shark , not a whale, but is completely harmless – the biggest things it eats are krill! It can grow up to 12 metres (about 40 feet) long or more, and weigh around 12000 kg, or 26000 pounds!
The largest fish ever is only known through fossils, but again was a gentle giant – the Leedsichthys Problematicus. Scientists aren’t sure how big it grew – they estimate up to 15 metres (50 feet), or maybe even up to 30 metres (100 feet), the same size as a Blue Whale!