Fish species diversity is roughly divided equally between marine (oceanic) and freshwater ecosystems. Coral reefs in the Indo-Pacific constitute the centre of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth. Jawless fishes are the most primitive fish. There is current debate over whether these are really fish at all. They have no jaw, no scales, no paired fins and no bony skeleton. Their skin is smooth and soft to the touch, and they are very flexible. Instead of a jaw, they possess an oral sucker. They use this to fasten on to other fish and then use their rasp-like teeth to grind through their host’s skin into the viscera. Jawless fishes inhabit both fresh and salt water environments. Some are anadromous, moving between both fresh and salt water habitats. Extant jawless fishes are either lamprey or hagfish. Cartilaginous fishes have a cartilaginous skeleton. However, their ancestors were bony animals and were the first fish to develop paired fins. Cartilaginous fishes do not have swim bladders. Their skin is covered in placoid scales (dermal denticles) that are as rough as sandpaper. Bony fishes include the lobe-finned fish and the ray-finned fish. The lobe-finned fish is the class of fleshy-finned fishes, consisting of lungfish and coelacanths. They are bony fish with fleshy, lobed paired fins, which are joined to the body by a single bone. Teleosts are the most advanced or ‘modern’ fishes. They are overwhelmingly the dominant class of fishes (or for that matter, vertebrates) with nearly 30,000 species, covering about 96% of all extant fish species. They are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Included are nearly all the important commercial and recreational fishes.

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