Backwater Butterfly Ray: Gymnura natalensis

Family: Gymnuridae

Common names

Backwater Butterfly Ray, Diamond Ray.

Binomial

Gymnura natalensis

Synonyms

Pteroplatea natalensis, Urogymnus natalensis.

Identification

A very large butterfly ray with a wide kite-shaped disc. Disc width approximately 1.8-2.2 x length. Pectoral fin apices angular. Snout short; pre-oral length .08-.1 x disc width.
Eyes very small. Thin simple tentacle on posterior margin of each spiracle. Spiracle has a concave inner margin with a convex undulation anterior to tentacle. Mouth arched with a concave symphysis on lower jaw.
Skin generally completely smooth; sometimes with a few scattered denticles in larger adults. Tail very short. 1 or more strong caudal stings usually present.

Colour

Dorsum light grey-brown with large diffuse darker blotches overlaid with a symmetrical pattern of broken white rings (reminiscent of rock lichen). Ventrum white or brownish. Tail usually with 3-5 subtle dark bands.

Size

Maximum disc width 250cm. Disc width at birth 38-47cm.

Backwater Butterfly Ray, Gymnura natalensis. Aka Diamond Ray. Long Beach, Simonstown, False Bay, South Africa, Atlantic Ocean.

Conservation Status

LEAST CONCERN

The backwater butterfly ray is captured by trawl fisheries, line fisheries, beach seines, and in shark control nets. The main fishery impacting the butterfly population is inshore prawn trawling, but fishing activity has tapered off since 2004. Overall the population is thought to be relatively stable.

Habitat

Tropical to warm-temperate water. On sandy or muddy substrates, often in shallow bays and estuaries. Close inshore to a depth of 75m.

Distribution

Southeastern Atlantic to southwestern Indian Ocean. From Namibia to southern Mozambique.

Reproduction

Aplacental yolk sac viviparous. Litter size varies from 2 to 9. Gestation ~12 months.

Diet

The spiny butterfly ray feeds on crabs, squid, small fishes, and polychaete worms.

Behavior

Sedentary. Camouflages its body with sand by flapping its fins while resting on the bottom.

Reaction to divers

Tolerant if not approached too closely. Will sometimes allow divers to slowly waft sand away from its disc but usually bolts.

Diving logistics

Fairly easy to find on night dives off the main beach in Simonstown, South Africa. I saw 1 or 2 at 5m depth on most night dives in January.
Apparently, they are reasonably easy to encounter around Aliwal Shoal during the winter months.