River sharks

Feb 24, 2015, 7:00AM EST
River sharks
A small (and rare) shark found exclusively in tropical fresh and brackish water

 River sharks (Glyphis) are five rare species found only in rivers and estuaries of South Asia and Austronesia.  They are related to the larger bull sharks, which are sometimes found in the same waters, but unlike bull sharks, they do not migrate into marine waters.  First documented in the Ganges -Hooghly river systems of West Bengal in the nineteenth century, similar species have since been found in the Irrawaddy River (near Rangoon, now Yangon), and various rivers of Borneo; New Guinea; and Queensland and the Northern Territory of Australia.  Because they are rare, river sharks have not been well-studied.  Adults grow to no more than about six feet in length and are robustly built for their size.  They are gray in color with a short and broad snout, tiny eyes, and a relatively large second dorsal fin.  Teeth in the upper jaw are triangular and serrated, while those in the lower jaw are more spear-like and generally not serrated.  Females give birth to live young.  They prey mostly on fish and crustaceans.  They prefer turbid water over a range of salinities and seem to frequent mangrove swamps.  River sharks are somewhat sluggish, preferring to move with the current or tide.  The presence of ampullae of Lorenzini indicates that they rely on electroreception, as well as scent, to locate prey, rather than using sight.  Not known for posing a threat to humans, river sharks are sometimes blamed for attacks more likely made by larger bull sharks.  Population is threatened by incidental catch by commercial and recreational fishing and by loss of habitat.  It is estimated that there are no more than 2,500 adult river sharks throughout their entire range.  They are assessed as critically endangered by the International Union for Conservation of Nature.  

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