Ray-finned fishes inhabit a variety of extreme environments. These include high altitude lakes and streams, desert springs (e.g. pupfishes), subterranean caves (e.g. cavefishes), ephemeral pools, polar seas, and the depths of the ocean (e.g. deepsea anglerfishes). Across these habitats water temperatures may range from -1.8˚C to nearly 40˚C, pH levels from 4 to 10+, dissolved oxygen levels from zero to saturation, salinities from 0 to 90 parts per million and depths ranging from 0 to 7,000 m (Davenport and Sayer 1993 in Moyle and Cech 2004:1)! Some fish even spend considerable time outside of water: mudskippers prey on the invertebrates of mudflat habitats, while airbreathing catfishes and gouramies live in stagnant, low oxygen ponds (among other habitats) or migrate over land to colonize new areas. Another extreme example of habitat adaptation is found in hillstream loaches , which live in the steep, torrential watercourses of Asiatic hillstreams. Hillstream loaches have flattened bodies and utilize suckers, permanently clinging to rock faces so they are not swept downstream. Lanternfishes , hatchetfishes , dragonfishes , deep-sea codfishes , halosaurs and spiny eels all have lights (flashing or constant), created by luminescent bacteria or special glandular cells, to find prey, communicate with other individuals, or for defense in the blackness of their deepsea habitats (see Communication, Food Habits, and Predation).
Researchers have long divided freshwater and saltwater habitats. However, habitat boundaries are often crossed by migratory species, some of which are diadromous – meaning they migrate between fresh water and the sea. Depending on the type of migration, they can be anadromous (migrate up rivers to spawn), with a pattern of freshwater-ocean-freshwater (typical of salmon and lampreys), or catadromous (migrate from freshwater to the sea to spawn), which is characteristic of freshwater eels . In the latter family juveniles, carried to river mouths by ocean currents, migrate upstream and live for up to 10 years before returning to spawning grounds in the ocean and dying shortly after (see Behavior as well).
Fresh water covers only a tiny fraction of the earth’s surface (.0093 percent), yet it is home to approximately 41 percent of all fish species. Most of these are concentrated in the tropics (1,500 different species in the Amazon Basin alone), and Southeast Asia probably has the most diverse assemblage of freshwater species. In marine areas, species concentrations are highest around coral reefs, where butterflyfishes and angelfishes , wrasses , parrotfishes and triggerfishes are common. In the arctic seas five notothenoid families dominate: thornfishes , plunderfishes, Antarctic dragonfishes , and notothens.
Habitat Regions: temperate ; tropical ; polar ; saltwater or marine ; freshwater
Terrestrial Biomes: forest ; rainforest
Aquatic Biomes: pelagic ; benthic ; reef ; lakes and ponds; rivers and streams; temporary pools; coastal ; abyssal ; brackish water
Wetlands: marsh ; swamp ; bog
Other Habitat Features: urban ; suburban ; agricultural ; riparian ; estuarine ; intertidal or littoral