Kind of fishes

A fish (plural: fish or fishes) is any gill-bearing aquatic vertebrate that lacks limbs with digits. It is a paraphyletic term, a typological, but not a phylogenetic classification.

Fish used to be a class of vertebrates that live in water. Now the term covers several classes of animals:
Jawless fish †Armoured fish Cartilaginous fish Ray-finned fish Lobe-finned fish
Fish are usually covered with scales, and have two sets of paired fins, and several unpaired fins. They are usually cold-blooded. A fish takes in the oxygen from the water using gills. There are many different kinds of fish. They live in fresh water in lakes and rivers, and in salt water in the ocean. Some fish are less than one centimeter long. The largest fish is the whale shark, which can be almost 15 meters long and weigh 15 tons. Most fish live in the water. A group of fish called the Lungfish have developed lungs, because they live in rivers and pools that dry up, certain parts of the year.

Certain animals that have the word fish in their name are not really fish: Crayfish are crustaceans, and jellyfish are Cnidarians. Hagfish and Lampreys do not have a jawbone. Hagfish are craniata, Lampreys are Hyperoartia. Certain animals look like fish, but are not. Whales and dolphins are mammals, for example

Fish or fishes?

Though often used interchangeably, these words have different meanings. Fish is used either as singular noun or to describe a group of specimens from a single species. Fishes describes a group of different species.[4]

[change] Types of fish

Fish, the oldest vertebrate group, includes a huge range of types, from the Middle Ordovician, about 490 million years ago, to the present day. These are the main groups:[4][5][6]
Agnatha: the jawless fish Pteraspids: the head-shields
Anaspids: gills opened as holes. Silurian to end-Devonian. Cephalaspids: early jawless fish
Lampreys: living ectoparasites

Osteostraci: bony-armoured jawless fish.

Gnathostomata: the jawed fish. Includes all types commonly called fish, except the lamprey. Placoderms: heavily armoured fish
Chondrichthyes: cartilaginous fish: sharks, rays and skates.
Acanthodii: extinct spiny sharks

Osteichthyes: bony fish. Actinopterygii: the ray-finned fish. Chondrostei: sturgeons and some other early types.
Neopterygii: first seen in the later Permian, lighter and faster-moving than previous groups. Holostei: the gars and bowfins
Teleostei: the most successful group, Triassic to present day.

Sarcopterygii: the lobe-finned fish Dipnoi: the lungfish; eight genera survive.
Coelacanths: two species survive. Group was probably ancestral to tetrapods.

[change] Anatomy

Bony and cartilagenous fish

Most kinds of fish have bones. Some kinds of fish, such as sharks and rays, do not have real bones (their skeletons are made of cartilage) they are known as cartilaginous fish.

[change] Fish scales

All fish are covered with overlapping scales, and each major group of fish has its own special type of scale. Teleosts (‘modern’ fish) have what are called leptoid scales. These grow in concentric circles and overlap in a head to tail direction like roof tiles. Sharks and other chondrichthyes have placoid scales made of denticles, like small versions of their teeth. These also overlap in a head to tail direction, producing a tough outer layer. Shark skin is available for purchase as shagreen, a leather which as original is smooth in one direction, and rough in the other direction. It may be polished for use, but is always rough in texture and resistant to slipping.

The scales are usually covered with a layer of slime which improves passage through the water, and makes the fish more slippery to a predator.

[change] Swimming

Fish swim by exerting force against the surrounding water. There are exceptions, but this is usually done by the fish contracting muscles on either side of its body in order to generate waves of flexion that travel the length of the body from nose to tail, generally getting larger as they go along. Most fishes generate thrust using lateral movements of their body & tail fin (caudal fin). However, there are also species which move mainly using their median and paired fins. The latter group profits from the gained manoeuvrability that is needed when living in coral reefs for example. But they can not swim as fast as fish using their bodies & caudal fins.[7]

The shape of the body of a fish is important to its swimming. This is because streamlined body shapes makes the water drag less. Here are some common fish shapes.

[change] Streamlining

The picture on the left shows a shark. This shark’s shape is called fusiform, and it is an ovoid shape where both ends of the fish are pointy. This is the best shape for going through water quickly.[8][9] Fishes with fusiform shapes can chase prey and escape predators quickly. Many live in the open ocean and swim constantly, like marlins, swordfish, and tuna. Ichthyosaurs, porpoises, dolphins, killer whales all have similar shapes. This is an example of convergent evolution.

[change] Eel-like

The long, ribbon-like shape of an eel’s body shows another shape. This enables them to hide in cracks, springing out quickly to capture prey, then returning quickly to their hiding spot.

[change] Flatfish

A flounder has both of its eyes on one side of its body.
Flatfish live on the bottom of the ocean or lake. Most use camouflage: they change colors to match the ocean floor.

[change] Compressed

Fish with compressed shapes have flat, vertical bodies, with one eye on each side. They swim upright and can be very thin. They usually live in reefs where their flat bodies can slip in and out among the corals, sponges, and rocks, keeping hidden from predators. Angelfish, surgeonfish, and butterflyfish are all compressed fish.

Swimming in groups

Many fish swim in groups a lot of the time. Schools of fish can swim together for long distances, and may be chased by predators which also swim in schools. Casual groups are called ‘shoals’. This is discussed in shoaling and schooling.

[change] Fish as food

People eat many kinds of fish. The fish that people eat most include carp, cod, herring, perch, sardines, sturgeon, tilapia, trout, tuna, and many others. A person who buys and sells fish for eating is called a fishmonger.

[change] Fish as pets

Some people keep fish as pets. Goldfish and Siamese Fighting Fish are popular types of pet fish. They are often kept by groups of people in public ponds for their beauty and calming nature.

[change] Fishing

See also: Fishing
The word to fish is also used for the activity of catching fish. People catch fish with small nets from the side of the water or from small boats, or with big nets from big boats. People can also catch fish with fishing poles and fishhooks with bait. This is often called fishing. There is also different types of lures that can be used. One is a crank bait. Others are plastic worms and rat-l-traps. These are lots of different ways of catching fish.