Shark Ecology 91
have an internal (genetic or learned) map, compass, and clock. They com-
pare where they are and in what direction they are moving with that mental
map. The details of this ability remain one of the great mysteries of shark
biology. Someday, a shark biologist, perhaps a reader of this book, will dis-
cover the answer.
How many shark species live in rivers and lakes?
We think of sharks as marine animals, and this is mostly true. Of the
1,250 or so known species of sharks, skates, and rays, fewer than 50 live all
or most of their lives in fresh water. (Chimaeras live only in the ocean, usu-
ally far from coastlines.) Among the “freshwater” elasmobranchs are one
guitarfish, 6 sawfishes, 8 sharks, and 31 stingrays.
Stingrays make up the largest group of freshwater elasmobranchs. Two
stingray families enter or live full-time in rivers and lakes. The largest fam-
ily, members of which are entirely freshwater inhabitants, consists of the
potamotrygonid river stingrays, with 20 species, 18 of which are in the ge-
nus Potamotrygon. All live in South America, particularly in the Amazon
River. Many are popular aquarium pets, despite the strength of the poisons
in their barbs. The other group consists of 11 species in the large sting-
ray family Dasyatidae. Most freshwater dasyatids occur in Africa, Southeast
Asia, India, Myanmar, northern Australia, and New Guinea. This family
includes what may be the world’s largest stingray, the Giant Freshwater
Stingray of Southeast Asia. These rays can be 4.6 m (15 ft) long and 1.9
m (6.2 ft) across the back and can weigh more than 500 kg (1,100 lb). One
population of Atlantic Stingrays lives in the St. Johns River of Florida,
more than 350 km (217 miles) from the coast. This makes it the only elas-
mobranch in North America that spends its entire life cycle in fresh wa-
ter. These rays are frequently observed by snorkelers in the crystal clear
springs that flow into the St. Johns River.
Lemon Sharks occasionally enter brackish river mouths, but we don’t
normally think of them as “freshwater” sharks. Five other species of car-
charhinids, all in the genus Glyphis and all residents of Asian or Australian
rivers, are permanent freshwater or estuary dwellers. Their names suggest
their habitats: the Ganges Shark of India (Glyphis gangeticus), the Northern
River Shark of New Guinea (G. garricki), and the Irawaddy River Shark of
Myanmar (G. siamensis). Undoubtedly the most infamous freshwater shark is
the Bull Shark. Most Bull Sharks live along coastlines and are perfectly good
marine inhabitants, aside from their well-deserved reputation for attack-
ing humans (see “Are sharks dangerous to people or pets?” in chapter 9).
Bull Sharks, however, commonly swim up rivers, often quite far, and are
known to have attacked people in rivers in Iran, India, and South Africa. In