The term "fish" can be applied to any member of a group of cold-blooded and finned aquatic vertebrates. Most fish, although not all, have scales and respire (breathe) by passing water over organs known as gills.

Over 32,000 species of fish have been identified to date. This means that they make up more than half of all vertebrate species. Fish are the dominant swimming animals of the seas and are found in both fresh and salt water.


CLASSES OF FISH
Modern fish (those that are not extinct) are found within two superclasses of the phylum Chordata. The superclasses are Agnatha, the jawless vertebrates, and Gnathostomata, vertebrates with jaws. Within these superclasses there are four classes and two subclasses:


Superclass Agnatha
The superclass Agnatha, found in both fresh and salt waters, consists of the most primitive types of fish. They have no jaw, but rather have an oral sucker. They also lack a bony skeleton, making them very flexible. Their skin is scaleless and smooth.

The superclass Agnatha consists of two classes:

- Class Myxini - hagfish
- Class Cephalaspidomorphi - lampreys


Superclass Gnathostomata - Vertebrates with jaws

The superclass Gnathostomata consists of two classes:

- Class Chondrichthyes
The class Chondrichthyes contains species of jawed fish with cartilaginous skeletons. This class of fish was the first to exhibit paired fins. They lack swim bladders, have spiral valve intestines, exhibit internal fertilization, and have between five and seven gill arches. Chondrichthyes have cartilaginous upper and lower jaws with arrays of teeth. Additionally, their skin is covered with teethlike "denticles", rather than scales, and their skin has a feeling of abrasiveness, much like sandpaper. There are over 980 species of cartilaginous fish, including sharks, rays, and chimaera (ghost sharks, rat-fish).

- Class Osteichthyes
The class Osteichthyes is the largest class of fish, with over 20,000 known species worldwide. This class is referred to as the "bony fish" due to their possession of calcified skeletons. Bony fishes have a swim bladder with which they control their buoyancy, highly specialized mouths, and great maneuverability and speed. Found in both fresh and salt waters, bony fish have evolved in shape and size to suitably adapt to almost every imaginable habitat.

This class if further divided into lobe-finned fish (Sarcopterygii) and ray-finned fish (Actinopterygii). Lobe finned fish, such as the lungfish and coelacanth, have "fleshy" lobed paired fins that are joined to their bodies by a single bone. In comparison, ray finned fishes have "lepidotrichia," or "fin rays." Their fins consist of webs of skin supported by bony spines.

Ray-finned fishes can be further broken down into three types - the chondrosteans, holosteans, and teleosts. The chondrosteans and holosteans are considered more primitive fishes than the teleosts. The chondrosteans share a mixture of characteristics of teleosts and sharks, while the holosteans are closer to the teleosts and further from sharks. Teleosts are considered to be the more advanced "modern" fishes. Ninety-six percent of all existing fishes are teleosts.


OUTLINE OF MODERN FISH TAXONOMY

Superclass Agnatha - Vertebrates without jaws

- Class Myxini - hagfish

- Class Cephalaspidomorphi - lampreys


Superclass Gnathostomata - Vertebrates with jaws

- Class Chondrichthyes (cartilagineous fish - sharks and rays)

- Class Osteichthyes (bony fish), which has two subclasses:

-- Sarcopterygii (lobe-finned fish) - lungfish and coelacanths
-- Actinopterygii (ray-finned fish) - Chondrosteans, Holosteans, Teleosts


RECOMMENDED READING