With well over 24,000 extant species currently known, the world's fishes comprise by far the largest and most diverse of all vertebrate groups. Occupying almost every conceivable aquatic habitat from high elevation mountain springs more than 5,000 meters above, to the ocean abyss some 7,000 meters below the surface of the sea, ichthyological variety in lifestyle, anatomy, physiology and behavior is unsurpassed among vertebrates.

A diorama of a coral reef, from the American Museum of Natural History's Hall of Ocean Life.
A diorama of a coral reef, from the American Museum of Natural History's Hall of Ocean Life. ©AMNH
Fishes live in water, the original medium for life on Earth. Permanent gills, median fins supported by rays of cartilage or bone, and often paired fins, are all characteristics of fishes that allow them to live successfully in water. No fishes have limbs with digits. The diversity of forms among fishes provide evolutionary biologists with some of the best examples of natural selection, adaptation, divergence, speciation, and historical development of fauna on scales from regional to continental.
Far from being the "dead-end" that we land-dwelling creatures tend to assume, fishes are extraordinarily diverse and their watery habitats provide a vast array of places in which to live and thrive.
Fish species range in size from the smallest known living vertebrate, Trimmatom nanus, a goby, which is mature at a mere 8 millimeters, to the giant whale shark, Rhincodon typus, which can grow as large as 12 meters. There are species of fish living at 5,200 meters above sea level in Tibetan hot springs and fishes that live in a depth of eight thousand meters below the ocean surface — this is an incredible span, over 13 kilometers of vertical distance. No other vertebrate group occupies such a wide band of habitable space.
Marine habitats include the deep sea, the mid-oceans, shrimp burrows, coral heads and sponges, and even the insides of sea cucumbers. Freshwater habitats include streams, rivers, lakes, even 500-meter-deep underground caves, and seasonal pools. African and South American lungfishes can tolerate "suspended animation" by living in dry mud for up to four years.

Certain species of fish can cope with depleted oxygen levels. Air breathing has evolved numerous times in the evolutionary history of fishes. This often involves different parts of the body: i.e. gills, air bladders, “lungs”, skin, the intestines, or the rectum. There are fish species which live in freshwater so pure it resembles distilled, while others live in extremely salty conditions, such as three little North American cyprinodontids: Aphanius dispar, Adinia xenica, and Cyprinodon variegatus, which can tolerate water that ranges from fresh to entirely salty. Other species live in water so cold — -2ºC — that they need a form of "antifreeze" in their blood, while others live in water that is incredibly hot, up to 44ºC in so-called "hot soda" lakes in Africa.
Equally amazing is the breadth of the feeding range within fishes. Fish species eat plankton, plants, snails, coral, wood, and other fishes, and some are so specialized that they eat parts of other fishes, like fish scales, eyes, and eggs. There are even fishes that subsist on hippopotamus droppings! Some fishes are blood-eating parasites which pierce the skin of living fishes, or else enter another fish's gill cavity to feed. Still, other species take parasitism to the extreme, such as certain male ceratoid anglerfish, which permanently attach to their female counterparts to obtain nutrients directly from their blood.
Some species of fishes grow continuously throughout their lifespan—even changing ecology as they grow—for example salmon. There are some fish species that live for less than a year, while others may live for 150 years. When we consider fish reproductive ecology we see tremendous variations: some species breed once and then die, a behavior called "semelparity"—others spawn repeatedly in one season, these are known as "repeat spawners.” Some spawn over repeated seasons—this is called "iteroparity." Some species of cichlids are mouth-brooders, holding their eggs, and later their young, in their mouths for protection.
Some species of cichlids are mouth-brooders, holding their eggs, and later their young, in their mouths for protection. ©AMNH
Variation in egg size and the number of eggs produced per female is immense. The amount of investment by parents outside of gamete production varies from none at all to extensive. This may include internal fertilization, nest-building, mouth-brooding, and brood-feeding. Some fish species begin life as one gender, then change sex, while others are hermaphrodites.
Fish migrations can cover enormous distances. Some Yukon River species of salmon migrate distances of 3,000 kilometers upstream while other fish species live and die within one meter of the place in which they were hatched or born. Such extraordinary diversity makes fishes a model group to study.
Ichthyologists study fishes and only fishes. The total number of living fish species identified to date is about 25,000—this accounts for roughly half of the living species diversity of all vertebrates: The remaining 50 percent—the tetrapods—includes all amphibians, reptiles, birds, and mammals put together. The vast majority of fish species belong to one group: the Actinopterygii, or ray-finned fishes. There are at least 23,700 living Actinopterygian species that have been described by scientists. Over 200 new fish species are discovered annually—ichthyologists are very busy people! The species richness of the Actinopterygii strongly contrasts with the species poverty of the other major fish groups to date.
The large-scale ecological distribution of fishes is strongly bimodal: 58 percent of fish species are marine, 41 percent, or approximately 10,000 species, live principally in freshwater. Roughly 1 percent, or 160 fish species, regularly migrate between salt-water and freshwater. The diversity of freshwater fishes causes some pause: Only about 0.01 percent of Earth's water supply—a tiny amount—is freshwater. Scientists calculate that freshwaters hold a far greater “density” of fish species than the oceans—greater by 7,500 times! However, it must be noted, that most marine species actually live in a relatively small volume of seawater in the productive photic zone, that is the light-penetrating zone, especially around coral reefs.
Our planet’s waters are hot spots of recent and future discoveries. Ongoing studies often suggest and reveal unsuspected relationships among and between fishes, as well as entirely new species. These are times of exciting discovery and advancement of scientific knowledge in ichthyology. New discoveries beckon us to seek the many remaining unknowns in the diversity of life on our planet. These are also times of rapid and destructive change in aquatic habitats around the globe—such threats alert us to the increasing potential for permanent loss and the dangers of remaining ignorant about so much of our planet’s rich aquatic biota.