Stingray City Limits
Q: I recently returned from a vacation to Grand Cayman, where I did several dives at 'Stingray City'. When I got home, I tried to learn more about stingrays but couldn't find out much. Can you tell me a bit about the natural history of the rays at Stingray City and whether or not they are dangerous?
A: Very little is known about the biology of most skates and rays - including stingrays - because they have been largely overshadowed by their more infamous relatives, the sharks. This is unfortunate, as these 'pancake sharks' are fascinating creatures in their own right. There are at least 96 species of stingrays worldwide (families Dasyatididae and Urolophidae combined), of which 5 are common in the Caribbean. The species predominant at 'Stingray City' is the Southern Stingray (Dasyatis americana). The biology of the Southern Stingray has received scant attention from most of my colleagues. It just so happens, I did some field research on the behavior, ecology, and life history of this species a few years ago in the Bahama Islands. What follows is a synopsis of my findings; much of this information is original and has not been published before - let it not be said I am a closed-fisted teacher!
The Southern Stingray is an inshore species, frequenting shallow, open areas of sand and mud bottom. It is commonly encountered in bays and estuaries, usually buried in sand with only its eyes and spiracles exposed. In addition to sand bottoms, the Southern Stingray has been observed in sea grass beds, turbid estuaries, clear lagoons, and resting on reef faces. Known depth range is from the intertidal zone down to at least 82 feet. Although primarily a tropical to warm temperate Atlantic species, the Southern Stingray is sometimes found as far north as New Jersey, having evidently migrated to higher latitudes as the water warms in summer. The disc of juveniles is greyish-brown above, white below. Adults are usually a uniform dark brown above, cream to beige below; the underside of the pectoral and pelvic fins sometimes have dusky margins. An unusually dark individual, nick-named 'Darth Vader', is a well-known adult female from Stingray City; despite speculation that she may be a separate species, 'Darth Vader' is merely a striking color-variant of the Southern Stingray - jet black on top, and nearly pure white below.
Southern Stingrays have been observed singly, in pairs, and in loose aggregations. Although most stingrays are bottom-dwellers, the Southern Stingray has been reported to leap above the surface, producing a loud clap upon re-entry. Such leaping has been attributed to attempts to free themselves of parasites, but stingrays have overlapping dermal denticles (scales) that preclude most skin parasites; I feel that these leaping behaviors are more likely associated with some kind of social display (mating or maybe even announcing territorial boundaries). This species often swims over or rests on the bottom near cleaning stations, where it is cleaned by Bluehead Wrasse and Spanish Hogfish. Southern Stingrays are typically inactive during daylight hours, often lying buried in sand; individuals become active at night, when they hunt on inshore sand flats and in sea grass beds. The Southern Stingray actively hunts small, bottom-dwelling creatures; buried prey is probably located by electroreception. The Southern Stingray exposes buried prey by 'hydraulic mining' (jetting water forcefully from the mouth into the sediment) or by undulating its pectoral fins vigorously; the ray's body is then raised off the bottom using its pectoral disc and the prey manipulated into its mouth. This species feeds equally on bony fishes, crabs and worms; it occasionally eats clams, shrimps, mantis shrimps, and tunicates. Clams are crushed with modified dental plates and the shell fragments are spit out; bony fish prey includes toadfishes, jawfishes, surgeonfishes, scorpionfishes, and flounders.
Male Southern Stingrays are much smaller than females. Males are sexually mature at a disc width of about 1.5 feet, and rarely get much larger than 2.5 feet across; the onset of maturity is punctuated by rapid clasper growth. Females are sexually mature at a disc width of about 3 feet, and may grow to a width of nearly 6 feet. Mating occurs in shallow water during winter (December to January). Courtship begins with the male following the female at close range for many minutes, possibly responding to a pheromone 'perfume' emitted by sexually receptive females. Prior to mating, the male bites the female's pectoral disc. Copulation occurs on the bottom with the male 'overlapping' the female (his belly to her back) and inserting one of his claspers into her vent; intromission may last from five to twenty minutes. The developing embryos are nourished first by yolk and at later stages by 'uterine milk' secreted from specialized cells lining the mother stingray's uterus; the fetuses either drink this protein-rich bath or absorb it directly via the gills. After a gestation period of about five months, three to five young - each measuring 7 to 8 inches across - are born in shallow estuaries. The young will remain in estuary waters for about three years, often seeking shelter among the proproots of mangroves.
Stingray City has been called "the most exciting 12-foot dive in the world". Thousands of divers have hand fed and handled the resident Southern Stingrays without serious incident. While a few divers have had their hands or arms 'bitten', the flattened dental plates of these rays do minimal damage (often little more than a reddish pucker-mark called a 'stingray hickey'). But there has been at least one serious injury inflicted by a stingray on a diver at Stingray City. Although the rays at Stingray City are pretty much habituated to contact with divers, stingrays are potentially dangerous and one would be wise to bear that in mind when interacting with them. Stingrays, like sharks, are basically 'path of least resistance' types: given an opportunity to flee rather than fight, most will simply swim away. But if persistently harassed, stingrays are quite capable of defending themselves. Some 1,500 stingray-related injuries are recorded in the United States every year, mostly minor wounds around the feet and ankles. But some stingray injuries are deadly serious. A couple of years ago, a 35-year-old Australian bloke on holidays in Fiji was stung in the chest as he swam over a large stingray; the barb punctured his heart and he died a day later as a result of his injuries.
The 'sting' which gives these fishes their common name is a modified dermal denticle mounted near the base of the tail, about one-third along its total length. The sting consists of a blade-like barb with serrations along both edges and a venom gland at the base. The serrae point toward the base of the spine, making removal difficult and very painful. The venom is a fairly powerful nerve toxin which affects the heart in complex and dangerous ways. But like most fish toxins, stingray venom is a large protein that can be broken down by heat. First aid should begin with immersion of the wound in hot but non-scalding water (110 to 113° F) for 30 to 90 minutes. The wound should then be cleaned with soap and water and any broken bits of stingray spine should be removed; no attempt should be made to tape or sew the wound closed, unless necessary to stop excessive bleeding. If the wound shows signs of infection (redness, swelling, pus), administer antibiotics. Treatment by a physician is indicated in any stingray envenomation.
But the best way to treat a stingray spining is not to allow it to happen in the first place. When wading in shallow water, perform the 'Stingray Shuffle': drag your feet rather than step as you would on land; this reduces the likelihood that you accidentally tread on a buried stingray by giving it a chance to escape. When diving over sand flats in deeper water, stay well off the bottom and watch for the rhomboidal outline that could indicate a buried stingray. If a stingray consistently turns to face you or raises its stinger-bearing tail above its back like a scorpion, it would be prudent to back off. Stingrays have enough problems living in the shadow of their infamous sharky cousins and having to go through life looking like a cowflop with eyes.