472 Fishes, Biodiversity of Box Why are there so many freshwater fishes? The high global diversity of freshwater fishes (Figure 18) is at first surprising Fresh waters make up only about 0.009% of the earth’s water, which means that almost half of all fish species live in less than 1% of the world’s water This 7,500-fold discrepancy in biodiversity per unit volume is probably best explained by the relative productivity and isolation of freshwater bodies Most freshwater habitats are relatively shallow and receive ample sunlight as well as nutrients running off from adjacent land Hence freshwater habitats are relatively productive and capable of sustaining abundant life Most of the ocean, in contrast, is deep, dark, and nutrient-poor Given that 81% of marine diversity occurs in shallow regions, a relationship between water depth and diversity in fresh waters is not surprising Adding to the influence of available food is the comparative isolation of most freshwater habitats Lakes are often created and affected by climatic and geologic forces (e.g., drought, floods, landslides, earthquakes, uplifts) that separate them from other systems Every lake then can be relatively isolated from other lakes, which means that genetically distinct populations can evolve into new species and that little genetic mixing occurs between lakes Small streams are separated from each other by larger rivers which are barriers to the movement of small fishes, and large rivers are separated by oceans As a result, freshwater habitats are perfect habitats for the speciation process Oceans in contrast are largely continuous habitats that are connected by currents, and ocean fishes typically produce larvae that float for several weeks or months on these currents Hence genetic exchange is common and opportunities for speciation are not as great Oceanic basins have relatively distinct faunas, but connectedness within basins discourages the kind of genetic isolation needed for speciation of the sort seen in lakes and rivers basin to another Among the 19,000 marine fishes, the vast majority (69%) live in shallow, warm areas such as coral reefs The remaining marine species are divided fairly evenly among shallow, cold, deep, open-ocean, and deep-bottom areas (about 10%–15% each) About 2% of marine fishes live in near-surface, open-sea (pelagic) habitats The approximately 160 diadromous species that live in different salinity regions at different times of their lives are divided among three groups Anadromous fishes (54%) live most of their lives in the ocean but then migrate to fresh water to spawn; this group includes lampreys, sturgeons, shad, and salmons Catadromous fishes (25%) spend most of their lives in fresh water and migrate to the sea to spawn; included here are freshwater eels, mullets, and temperate basses Amphidromous fishes (21%) move between fresh and salt water, but migration to the spawning habitat occurs long before the fishes actually spawn; examples include gobies, sleepers, and galaxiids Freshwater Diversity The world’s freshwater habitats occur in six major zoogeographic regions or realms that correspond approximately to continental distributions, with important exceptions Each region has a fairly distinct fish fauna (again with some exceptions and shared elements) (Berra, 2007) The Nearctic Region The Nearctic region consists of subtropical, tropical, temperate, and arctic North America The region stretches from the Mexican Plateau to northern Canada and Alaska The Nearctic contains 14 families of primary freshwater fishes, with about 1000 species The most diverse families are minnows, suckers, North American catfishes, perches (and darters), and sunfishes Other important families include the lampreys, gars, salmons (many of which are anadromous), and whitefishes; sculpins, which are freshwater species in a primarily marine family ( ¼ marine derivatives); pickerels and mudminnows; killifishes; and livebearers The Nearctic is further subdivided into three subregions: the Arctic–Atlantic (with six provinces), the Pacific (with seven provinces), and the Mexican Transition subdivision Eleven major river systems drain the region; major lakes are abundant, the largest being the five Laurentian Great Lakes (Ontario, Erie, Huron, Michigan, and Superior) The Neotropical Region The Neotropical region contains South America and Middle America It is the most speciose region of the world in terms of freshwater fishes, with 32 families and more than 4500 species Particularly diverse groups include the colorful characiforms (1500 species of tetras, piranhas, characins, and freshwater hatchet fishes), 14 families and 2500 species of catfishes, families and 134 species of gymnotiform South American electric knifefishes, and more than 150 species of cichlids Several secondary freshwater and marine derivative groups are included: freshwater stingrays, herrings, silversides, needlefishes, killifishes, and croakers Many species remain to be discovered and described, particularly in South America The Neotropical region has been further divided into eight subdivisions with fairly distinctive faunas Eight major river systems drain the region; major lakes include Lake Titicaca, the world’s highest fish-containing lake The Palearctic Region The Palearctic region encompasses Eurasia, including Europe, northern Africa, and Asia north of the Oriental region Twenty-seven families and about 600 species of temperate freshwater fishes occur in the region, dominated by minnows and loaches but also perches, pickerels, sturgeons, salmons, sculpins (including the Lake Baikal endemics), and 10 species of catfishes in four families Endemism increases to the south, as is also the case in the Nearctic region (Kottelat and Freyhof, 2007) The Palearctic and Nearctic regions share numerous families and genera (sturgeons, paddlefishes, minnows, smelts, salmons, pikes, mudminnows, and perches) but only a few species occur in both (i.e., Northern Pike, Longnose Sucker, Burbot, Threespine Stickleback, and Fourhorn Sculpin) The region is sometimes subdivided into six subregions based on faunal groupings Ten major river systems drain the region; major lakes include the Black and Caspian Seas and Lake Baikal, the world’s oldest and deepest lake The African or Ethiopian Region The African or Ethiopian region is second to the Neotropics in freshwater fish diversity, with 36 families and more than 2000 species of primary and secondary freshwater fishes The African region includes all the African continent south of the Sahara Desert, plus the large island of Madagascar