Types of Fish: Over half of all known vertebrates are fish. Characterized by scales, fins and gills, the fish class includes 28,000 known species. Most of these — nearly 27,000 — are bony fish, examples of which include salmon, goldfish, eels, seahorses, sunfish, sturgeon, lungfish and coelacanths. There are around 970 species of shark, ray and chimera — animals whose skeletons are made of light, flexible cartilage instead of bone. Lampreys and hagfish, which account for about 108 species, are so primitive they may not be fish at all, but are currently classified as such.

Breathing Water: All fish have gills, instead of lungs, for breathing. To breathe, a fish pulls oxygen-rich water into its mouth, then pushes it over its threadlike gills. Each gill filament is lined with a network of capillaries that exchanges carbon dioxide for oxygen. Some fish can breathe air, especially those that live in shallow waters that are seasonally depleted of oxygen. Some, like the lungfish, can even survive on land for several days.

Getting Around: Fish navigate their environment through the help of fins. The typical setup is two sets of paired fins and several unpaired fins — usually one or two dorsal fins, an anal fin and a tail fin. The tail fin is used for propulsion, while the others are used mostly for navigation and stabilization. To swim, the fish contracts paired sets of muscles on either side of its backbone. These alternating contractions form S-shaped curves that travel down the body. When the curve reaches the tail fin, it pushes water behind the fish and the animal moves forward.

Fish Diversity: With so many species, the fish class is incredibly diverse. Most have small brains, but sharks and elephantfish (freshwater fish native to Africa) have brains as large as birds and mammals relative to body size. A gas-filled sac called a swim bladder allows bony fish to ascend or descend without wasting energy. Fish can be as tiny as the 0.31-inch Paedocypris progenetica — a native of Sumatra and the world's smallest vertebrate — or as large as the 51-foot whale shark, a slow-moving filter-feeder found in oceans worldwide.