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A typical fish breathes through gills, has a body covered with scales, maneuvers using fins and is ectothermic (cold-blooded). However, unlike other groups of vertebrate animals, fish are not a single phylogenetic group, but instead a collection of four distinct classes. There are two classes of jawless fish, one of cartilaginous fish, and one of bony fish. Most fish live in either freshwater or the sea for their entire lives, while a few, known as anadromous fish, spend part of their lives in freshwater and part of their lives in salt-water. Fossils suggest the first fish appeared nearly 500 million years ago, with fish being the first animals to evolve moveable jaws nearly 440 million years ago. 

With their streamlined bodies fish are well adapted to travel and live in the water and most species breathe using gills to extract needed oxygen from water. While most fish lay eggs and provide their young with little if any parental care, some do provide extensive parental care and a small number give live birth.

Jawless Fish

First appearing more than 500 million years ago, jawless fish are the oldest group of fish. While history suggests there were once many types of jawless fish today there are just two - hagfish and lampreys.  Both species have elongated bodies, smooth, scaleless skin, and a jawless mouth. There is no bony skeleton in jawless fish, instead a single cartilaginous notochord runs the length of the body.  Gills are almost porelike openings near the head. 

Most lamprey spend their adult lives in marine waters, returning to freshwater streams and lakes to breed.  Young lamprey pass through several larval stages before becoming adults and heading out to sea. Hagfish live and breed only in marine waters and young a miniature versions of the adults.

This sea lamprey illustrates the common features of jawless fish. A circular mouth, small porelike gills, and an elongate body.

Cartilaginous Fish

The most prominent group of cartilaginous fish is the sharks which have been made famous by deadly encounters with surfers and the occasional swimmer. There are a total of three types of cartilaginous fish: sharks; skates and rays; and chimaeras. What these groups have in common is a body filled with structural cartilage instead of bone, teeth that are replaced through-out their lifetimes and skin that's covered with tiny, toothlike scales. All cartilaginous fish are carnivorous - some of the larger sharks are filter feeders with greatly reduced teeth, but their primary food is zooplankton and phytoplankton found floating in the water column. With cartilage instead of bone these fish are generally more flexible than bony fish. While the majority of the cartilaginous fish are marine, a few tropical sharks live entirely in freshwater and some are occasional freshwater visitors.

Despite its size - up to 23 feet across - and menacing appearance, Manta rays are harmless filter feeders that feed primarily on plankton.

Bony Fish

The vast majority of the fish - more than 9 of every 10 species -  observed today are bony fish.  These fish have light, strong, bony skeletons and flexible fins that allow these fish to swim with precision. Most bony fish also have a gas-filled swim bladder that allows them to swim at precise depths by adjusting their buoyancy. Bony fish have adapted to live in every type of aquatic habitat from alpine lakes to deep ocean trenches. 

These fish are widely considered to be "advanced" meaning they have evolved more recently than the other types of fish. Many species are specially adapted for their niche and have wildly divergent features. Bony fish range in size from milimeters in length to as much as 15 feet in the length.  The largest species are swordfish, bluefin tuna, blue marlin, and sunfish.

An oddity of the open ocean, Sunfish are the heaviest of all fish - weighing up to 4000 pounds - and among the most prolific with a single female laying as many as 300 million eggs that drift away with the current.

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