Fish are found throughout the world, from altitudes of more than 5,000 m (3 mi), as in Lake Titicaca, located 3,800 m (2.3 mi) above sea level in the Andes, to depths of about 10 km (6 mi) in the Pacific Ocean. Some, like certain killifishes, Cyprinodon, inhabit hot springs, where the water temperature may reach 45 deg C (113 deg F); others, like the icefishes, Chaenocephalus, are found in Antarctic seas, where water temperature may fall below 0 deg C (32 deg F). About 107 species, including the swordfish, Xiphias, are distributed worldwide in tropical and subtropical waters, but many species have very limited ranges, among the smallest being that of the killifish Cyprinodon diabolis, which is confined to a single spring in Nevada.
About 70% of the Earth's surface is covered by oceans and seas, and about 3.5% of the land surface (1% of the Earth's total surface) is covered by fresh water. Inhabiting these waters are an estimated 20,000 or more fish species, equal to or exceeding the number of all other vertebrate species combined. Bird species number approximately 8,600; reptiles, 6,000; mammals, 4,500; and amphibians, 2,500. About 60% of the fish species live in marine waters; the remaining 40% are found in fresh water.
Most of the world's fishes are continental in orientation, living either as part of the freshwater systems on land or as sea-dwellers staying near and influenced by the coastal environment. High densities of marine fish populations occur near coasts, because the waters there are extremely rich in nutrients. Coastal benefits include chemical and organic enrichment discharged by rivers, upwellings from the ocean depths that recycle previously deposited nitrates and phosphates, aeration caused by surf and tide, and the penetration of sunlight.
Body Temperature - Water Balance - Swimming - Gas Bladder
Lateral Line System - Evolution - Reproduction
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