Distribution

Warm to tropical seas. Potamotrygonidae only in rivers in South America (Colombia, Guiana, French Guiana, Brazil, Argentina, Uruguay and Paraguay).

Biology

Predominantly marine cartilaginous fishes. Body flattened dorsoventrally, with broad, flat pectoral fins on each side. The body has a flat, discoid shape. The gill slits, mouth and nasal opening are on the bottom of the body. The span of the wing-like pectoral fins can reach up to more than 2 m in some stingrays. The harmless Manta rays can reach a diameter of up to 7 m or more. There is a generally long, whip-like tail that is clearly distinct from the body. On the top of the tail, close to its base or medially, there are one to several flat venomous spines, serrated on the sides and surrounded by an integumentary sheath. The venom glands are located underneath the spine, along two grooves running lengthwise (Fig. 4.20a.).

Fig. 4.23 Dasyatis americana

Myliobatidae and Rhinobatidae are frequently in motion and can enter shallow water in their search for food, often coming close to the surface. The other stingrays, including the freshwater rays in the rivers of South America, prefer to dwell on the sandy or muddy sea or river bottom. Many live in shallow water and thus represent a danger to humans, as the half-buried animals are frequently only noticed when they are startled and dart away. If a stingray is trodden upon, the tail is reflexively whipped forwards. Thus the venomous spine can cause deep lacerations. Remnants of the spine integument often remain in the wound.

Risk

Stingray injuries are not rare and most commonly occur in victims bathing or wading in shallow water. On the Seychelles, there are remarkably frequent incidents of fishermen being stung by Dasyatis sp. (Grainger 1980). Envenoming causes severe, stabbing and radiating pain, which eases after hours or days, as well as oedematisation. The wounds are usually large and generally heal poorly; necroses are common.

Cases of marked systemic envenoming are rare, but the venom spines of several species are extraordinarily long, over 30 cm, and can puncture internal organs. Fatalities due to the spine puncturing the heart have been reported. The most recent fatal accident of this nature involved the Australian television star Steve Irwin.

Literature (biological)

Grainger 1985, Halstead 1988, Russell 1965, Sutherland 1983, Wiliamson et al. 1996