There are more than 400 species of sharks in the world (or Shaaaaarks if you are from Boston), and a few spend most of their time in freshwater. Although this may sound terrifying to those hitting rivers, lakes, and swimming holes to cool off in the summer heat, they pose little danger to people.
When most scientists speak of river sharks, they refer to five known species of the Glyphis genus, although biologists think there could be other species lurking in the shadows.
These river sharks live in India, Southeast Asia, and parts of Australia, though they are rarely seen and not well studied. Some are known from only a single specimen.
All five species are threatened thanks to human development, pollution, and fishing.
River sharks tends to have short, broad snouts and tiny eyes. They are thought to reach lengths up to around nine feet (3 meters), although very few adults have been documented. River sharks tend to have broad fins and look a bit more “fish like” than some of the better known sharks.
The Speartooth Shark
The speartooth shark (Glyphis glyphis) is known only from a few immature specimens found in fast-moving, mangrove-lined rivers of northern Australia and New Guinea. The speartooth has a particularly large second dorsal fin. The teeth in its lower jaw are narrow and spear like, hence the moniker.
Speartooth sharks are known to cruise with the changing tides to save energy. They feed on fish and crustaceans, taking advantage of poor visibility in river water. Like many sharks they undergo live births instead of laying eggs.
The International Union for Conservation of Nature (IUCN) lists the animal as endangered, thanks to an estimated population of only 2,500 and pressure from habitat loss and bycatch by fishers.
Cousins to the speartooth include the northern river shark or New Guinea river shark (Glyphis garricki) and Irrawaddy river shark (Glyphis siamensis), a critically endangered species that is known from only a single specimen collected in the 19th century from the Irrawaddy River in Myanmar.
The Borneo river shark (Glyphis fowlerae) is sometimes called “mythical” and is also best known from a few specimens collected in the 19th century. IUCN shark expert Leonard Compagno from the South African Museum said, “We have very little idea of the geographic distribution of these sharks, much less their general biology. They show up like ghosts, few and far between, in a handful of scattered localities. Finding one is cause for celebration.”
In a passage that sounds like something from Heart of Darkness, scientists on an IUCN expedition to find the Borneo river shark said, ”The family led us to the tank of formalin which they had been keeping locked up at the back of their stilt house, insisting that they had a shark for us in there. They looked on in bewilderment; we could barely contain ourselves – could it really be Glyphis? We all crowded round as the tank was opened, oblivious to the formalin fumes. There it was, black beady eyes, blunt snout, fins like we’d never seen before but just like those in the books – there was no doubt about it: this was Glyphis, at last!”
(There is some debate about which exact species they saw, since the Glyphis sharks are closely related and poorly known.)
Another endangered, extremely rare river shark is the Ganges shark (Glyphis gangeticus), which plies freshwater in eastern and northeastern India.
The Ganges shark is often confused with the much more common bull shark (Carcharhinus leucas), which is sometimes called the river shark, cub shark, freshwater whaler, estuary whaler, Zambezi shark, Nicaragua shark, or, confusingly, Ganges shark, though it is not closely related to Glyphis. Bull sharks are much more dangerous to people than the smaller, specialized, extremely rare true river sharks.
Bull sharks are commonly found through much of the world in warm, shallow waters along coasts and in rivers. They often swim quite far into brackish and freshwater, and they are known for their aggression and opportunistic feeding. They have been seen as far inland as Indiana in the Ohio River, and they are thought to be responsible for the majority of near-shore shark attacks on human beings.
Bull sharks evolved a tolerance for freshwater by developing the ability to restrict removal of salt from their bloodstream by the rectal gland. The gills and kidneys also help adjust the amount of salt going in and out.
Bull sharks usually hunt alone but they sometimes work in pairs, taking a wide range of animals. They tend to be territorial, and may attack anything that enters their zone.
Some experts think bull sharks were behind the infamous shark attacks off New Jersey in 1916, which inspired the book and movie Jaws. Too bad the great white gets all the credit.
Brian Clark Howard covers the environment for National Geographic. He previously served as an editor for TheDailyGreen.com and E/The Environmental Magazine, and has written for TheAtlantic.com, FastCompany.com, PopularMechanics.com, Yahoo!, MSN, Miller-McCune and elsewhere. He is the co-author of six books, including Geothermal HVAC, Green Lighting, Build Your Own Small Wind Power System, and Rock Your Ugly Christmas Sweater.