The family Dasyatidae includes stingrays, or whiprays, and river stingrays, encompassing nine genera and about 70 species. Like other rays, they have enlarged pectoral fins that form a disc. In this family the disc stretches forward to include the head, and ranges from less than 30 cm to over 2 m in diameter. Stingrays can be found in all tropical and subtropical seas. River rays form a freshwater subfamily of Dasyatidae, and live only in fresh water in parts of South America and Africa. Most stingrays are benthic, burying themselves partially under sand or mud in relatively shallow water. This habit makes them easy to accidentally step on, and the sting they deliver in defense has made stingrays famous. They appear in the writings of Pliny, Homer, and Captain John Smith, and aboriginal peoples from various parts of the world have used stingray spines for spear tips and other weapons. Members of Dasyatidae are viviparous (bear live young), and invest a lot of energy in relatively few young over a lifetime. This reproductive strategy renders them potentially vulnerable to human activity.