Ocellate river stingray  (Potamotrygon motoro)

Remarkable for its habitat, the ocellate river stingray is one of a small group of rays that evolved in freshwater from a marine ancestor (3). The first part of the scientific name reflects this lifestyle, potamos means ‘river' in Greek, while the second part, trygon, means ‘three angles' (4), and may refer to its body shape. The body of the ocellate river stingray is an oval disc, with a greyish-brown upper surface patterned with distinct yellow-orange spots, and a white underside (2). While the ocellate river stingray is a beautiful species, it is much feared for the single spine borne at the tip of the robust tail, which is capable of delivering a painful sting (2) (5).

The ocellate river stingray has a widespread distribution, extending across Argentina, Brazil, Paraguay, Uruguay and Venezuela, where is occurs in the Paraná-Paraguay, Orinoco, and Amazon River basins (1).

While most rays are marine, the ocellate river stingray is one of the few species that is restricted entirely to freshwater (1) (5). Like the other river rays, this species favours calm waters, especially the sandy edges of lagoons, brooks and streams (2).

The ocellate river stingray is often found lying still, buried in the sandy sediment at the bottom of a stream, particularly during the warmest part of the day (2) (5). Like all stingrays, the females of this species produce eggs, but these develop inside the female. The young hatch inside the female and are then ‘born' live after a gestation period of no more than three months (5) (6). The litter size of the ocellate river stingray varies massively, from 3 to 21 young (2). Sexual maturity is reached at around three years of age, when the stingray measures between 30 and 35 centimetres across (2).

Initially after birth, the ocellate river stingray feeds on plankton, but as it grows, the diet expands to also include small molluscs, crustaceans and the larvae of aquatic insects, while larger adults also eat certain catfish (those belonging to the family Loricaridae) (2). The ocellate has relatively few predators, except for some larger fish and caiman (3).

Despite being the most abundant and widespread species of Potamotrygon endemic to South America, a lack of information on its life history and the status of populations means that it is not possible to determine the extent to which the ocellate river stingray may be threatened with extinction (1). However, what is known is that a number of factors may be having a detrimental affect on this species.

The ocellate river stingray is commonly hunted, with juveniles being taken for the ornamental fish trade and adults being captured for food (1) (2). When water levels in the streams and lagoons is low, or when the rivers flood and the rays can be found resting over vegetation in shallow water, the ocellate river stingray becomes an easy target for fishermen with harpoons. Some artisanal and commercial fishermen also catch this species on lines (2). Furthermore, habitat degradation in some parts of its range may threaten this species, such as the construction of hydroelectric plants and ports along the Río Paraná system (2).

A legal quota exists for the number of ocellate river stingrays that may be captured and exported, currently standing at 5,000 individuals per year, and it is said that its capture and exportation is monitored (7), although clearly, this does not in any way mitigate the threat of illegal hunting or trade. Further research on this species has been highly recommended, to enable its conservation status to be determined. Given the negative impacts of hunting and habitat degradation that the ocellate river stingray is subject to, it its possible that this freshwater stingray may be threatened (1).

For further information on conservation in the Río Paraná system see:

• The Nature Conservancy:
  www.nature.org/initiatives/freshwater/work/art24061.html

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