Fish are aquatic vertebrate animals, usually possessing gills in the adult stage and having limbs, when present, in the forms of fins. Fish can be divided into two categories; AGNATHA fish that lack jaws and fins, and GNATHOSTOMATA fish that have jaws and fins. For the purposes of our class will we only focus on members of Gnathostomata.

Gnathostomata can be divided into two subclasses; the cartilaginous fish(Chondrichthyes) and the bony fish(Osteichthyes). Bony fish are often regarded as "true" fish. This group includes most of the fish we are familiar with, such as snapper, groper and gurnard. They have a bony skeleton and a single pair of external gill openings. The mouth is generally at the front of the body and they have a tail fin, with the top and bottom portions nearly the same size. Internally, the intestine of the bony fish is a relatively short tube.

Notice the location of the fins in the diagram above. One or more dorsal fins may be located along the center-line of the back. A caudal fin lies at the end of the tail and is the primary organ for generating thrust in most species. One or more anal fins are situated below the lateral line toward the tail of the fish. The body has two pairs of lateral fins: the pectoral fins, which are usually placed on the sides behind the gill openings, and the ventral fins(which are sometimes called pelvic fins), which lie on the belly between the head and the operculum(gill cover).

Cartilaginous fish include the sharks, skates and rays. Although these look different, they have many features in common. None of them have any true bone - their skeleton is made of cartilage. They have strong jaws and their mouth is on the underside of the body, and their eyes are on top. They cannot see food as it enters their mouth. Some sharks solve this problem by touching their food briefly with their nose first. Some also use a powerful electrosensory system.

Compare the anatomy of a shark with a bony fish. In general, fishes are somewhat torpedo-shaped, with bodies that are moderately compressed at the sides and taper more markedly at the tail than at the head. Typically, the body carries a number of fins, which are membranes supported by rays or spines, that function in propulsion or orientation.

Habitat

Fish adapt to their environment, so different species often develop similar features and adaptations. For example, many deepwater species have common features like big eyes and dark coloring. Most fish are either wholly fresh water or wholly marine, though some spend parts of their lives in both environments, such as the salmon. Freshwater species live in ponds, lakes, rivers and estuaries.

Age

Some fish species are very short-lived and grow quickly. Others are extremely long-lived, enjoying a longer lifespan than we do. Among the shortest-lived species are whitebait, which live only a maximum of three or four years, including their time as adults in fresh water. Several other marine species only live up to about five years, for example, anchovies, pilchards, sprats and flounders. Red cod and hoki also breed and grow quickly. Many species live 10 or 15 years, but a number of very important fish species live to a remarkable age. Tarakihi grow to 20 or 30 years, Trevally to 30 or 40 years, and Snapper to over 60 years. These "middle order" species could be likened to animals like elephants or big cats, in terms of life expectancy. The "tuatara" of the sea is the orange roughy, which is believed to live over 100 years.