Stingrays hid under sand with eyes on top and mouths below
About 220 stingray species in twenty-nine genera fill eleven myliobatiform families from whiptails and river rays to mantas. Coastal tropics dominate, yet thorntails enter temperate seas and deepwater forms descend the slope—while unregulated fishing left forty-five species Vulnerable or Endangered by 2013.
A stingray's eyes sit on top and its mouth underneath, so once it has caught something it cannot see it; smell and shark-like electroreceptors, the ampullae of Lorenzini, guide it instead. Many species stir up sand and settle beneath it, leaving only eyes and tail exposed. Buried, a ray cannot breathe through its mouth without sucking in sediment, so it draws clean water through spiracles just behind the eyes and passes it straight to the gills. Its loosely hung upper jaw can shoot forward, and flat, constantly replaced teeth crush shellfish.
Rays swim with their broad pectoral fins rather than a tail. Bottom dwellers with short, thick fins ripple them in waves for slow movement, while open-water species with long, thin fins flap them like wings for speed.
Breeding is prolonged. In the Atlantic stingray, mating lasts about seven months before females ovulate in March, one of the longest seasons known in sharks and rays, sustained by high male androgen levels. Male round stingrays can detect the electrical signals of mature females, and a courting male bites at the female's disc before inserting a clasper. Embryos grow inside the mother without a placenta, first on yolk and then on a uterine milk, and litters of five to thirteen are born ready to fend for themselves. At the Sea Life London Aquarium, two females produced seven young after two years without a male, suggesting stored sperm.
Stingrays split from panrays in the Late Jurassic, and the oldest named species is known from Early Cretaceous teeth in England. About 220 species live today, and by 2013 the IUCN listed 45 as vulnerable or endangered, mostly because of unregulated fishing.
Source: Stingray
