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).
Disparate localities may have similar geographic conditions, yet fish species composition varies widely across similar regions. In other words, patterns of fish distribution are not simply related to how well a fish is adapted to a particular type of environment, which is why invasive species can be so devastating (see Conservation). The study of zoogeography attempts to answer questions about how and why fish (and other animal) faunas differ across geographic regions. Zoogeography integrates a variety of disciplines within ichthyology (ecology, physiology, systematics , paleontology, geology and biogeography) to explain patterns of fish distribution. While ichthyologists certainly have incomplete knowledge in many of these areas, advances in plate tectonics and phylogenetic systematics have allowed them to define various zoogeographic (or biogeographic) regions (also subregions) and types.
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.
Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Introduced , Native ); ethiopian (Introduced , Native ); neotropical (Introduced , Native ); australian (Introduced , Native ); antarctica (Native ); oceanic islands (Introduced , Native ); arctic ocean (Native ); indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native ); mediterranean sea (Introduced , Native )
Other Geographic Terms: holarctic ; cosmopolitan ; island endemic