Fish are cold-blooded vertebrates that typically have fins, gills, and scales. They range in size from a carp as small as a rice grain to a 50-foot whale shark. Fish inhabit the many oceans, lakes, rivers, and streams that cover two-thirds of the Earth’s surface.
Nearly all fish are “oviparous,” meaning their fry (or offspring) hatch from eggs outside the mother’s body. Some, including sharks, are “viviparous” and give live birth to their young.
While most fish use gills to absorb oxygen from the water, a few, like the aptly named lungfish, can breathe air through specialized organs.
Fish eat a vast array of both plant and animal matter. Many species are herbivorous, feeding on microscopic plants called plankton, which forms the base of the aquatic food web. Others eat zooplankton, tiny animals such as krill that are not much larger than plankton.
Some species, like the tropical parrotfish, use their beak-like teeth to scrape algae from rocks and coral. Others prefer a broader menu, eating crabs, squid, sea stars, coral, and other fish. Predatory fish, like the great white shark, are even more ambitious, hunting marine mammals like seals and sea lions.
Fish share many behaviors common to other vertebrates. They socialize, mate, raise their young, use camouflage, groom one another, and even make sounds to attract mates and ward off enemies.
Of the thousand-plus species known to vocalize, most do so by contracting the muscles of their swim bladders, producing an array of thumps, grunts, croaks, and clicks. Seahorses, on the other hand, snap their bony body parts together to communicate.