Finless sleeper ray
|Finless sleeper ray|
J. E. Gray1831
J. E. Gray, 1831
|Range of the finless sleeper ray|
The finless sleeper ray (Temera hardwickii) is a little-known species of sleeper ray in the family Narkidae, and the sole member of its genus. It is found over the continental shelf off Southeast Asia, from the eastern Andaman Sea to Vietnam and Borneo. Typically growing no more than 15 cm (5.9 in) long, it may be the smallest cartilaginous fish. The finless sleeper ray is the only electric ray that lacks dorsal fins. It has an oval pectoral fin disc that varies from longer than wide to wider than long, depending on age, and a short, robust tail that terminates in a short, deep caudal fin. The trailing margins of its pelvic fins are sexually dimorphic, being more concave in males.
Like other members of its family, the finless sleeper ray can generate an electric shock from paired electric organs in its disc, for purposes of defense. Reproduction is aplacental viviparous: the embryos hatch within the uterus and are nourished by yolk. A litter size of four has been recorded from a single individual. The International Union for Conservation of Nature (IUCN) has assessed the finless sleeper ray as Vulnerable. This slow-reproducing species is caught by intensive bottom trawl and possibly other fisheries throughout its range, which likely cause high mortality regardless of whether it is discarded. Its numbers off Thailand seems to have declined significantly.
The finless sleeper ray was described by British zoologist John Edward Gray, who placed it in its own unique genus, in an 1831 issue of the scientific journal Zoological Miscellany. Gray based his account on two syntypes collected near Penang in Malaysia by General Thomas Hardwicke, and presented to the British Museum.
The pectoral fin disc of the finless sleeper ray is oval in shape; it is slightly wider than long in adults, and circular or longer than wide in juveniles. The eyes are small and bulging, and immediately followed by spiracles of roughly equal size. The rims of the spiracles are smooth. There is a curtain of skin between the small, circular nostrils that reaches the mouth. The nostrils are connected to the corners of the mouth by a pair of grooves. The small mouth is gently arched and protrusible. The teeth are flattened, with hexagonal bases, and are arranged into narrow bands. The five pairs of gill slits are short. The pelvic fins are only slightly overlapped by the disc. Each is large, wide, and roughly triangular, with trailing margins that are more concave in males than in females.
The tail is stout and much shorter than the disc. Unique amongst sleeper rays, there are no dorsal fins. The tail terminates in a fairly large, triangular caudal fin about as long as wide, with rounded corners. The skin entirely lacks dermal denticles. This species is plain light brown in color dorsally, sometimes with darker markings and whitish spots, and pale ventrally, with broad darker margins on the pectoral and pelvic fins. Possibly the smallest cartilaginous fish, the smallest known adult finless sleeper ray measured 8.2 cm (3.2 in) long and weighed 13 g (0.46 oz). It seldom exceeds 15 cm (5.9 in) in length, though there is a dubious historical record of a specimen 46 cm (18 in) long.
Distribution and habitat
The range of the finless sleeper ray extends from the eastern Andaman Sea, near southern border between Thailand and Myanmar, through the Straits of Malacca to Singapore, and northward to Vietnam. Its distribution off Thailand is uncertain. There is one record of this species off Sarawak in Borneo. This bottom-dwelling species inhabits both inshore and offshore waters on the continental shelf, over fine sediment. In the 19th century, it was reportedly abundant year-round in the Straits of Malacca. It remains common in some local areas, although it has become rare off Thailand since 1975.
Biology and ecology
The natural history of the finless sleeper ray is poorly understood. In common with other members of its family, it is capable of generating a moderate electric shock for defense against predators. The shock is generated by a pair of kidney-shaped electric organs, each about one-third as long as the ray, situated on either side of the head and visible through the skin. The organs consist of numerous fluid-filled pentagonal or hexagonal columns, which essentially act as batteries connected in parallel. It presumably feeds on small invertebrates. The robust structure of its jaws suggest that it may prefer hard-shelled prey.
The finless sleeper ray is aplacental viviparous, in which the embryos hatch from the eggs inside the uterus, and are sustained over the remainder of the gestation period by a yolk sac. There is a record of a female 10.5 cm (4.1 in) long that contained four late-term fetuses each 2.9 cm (1.1 in) long; the young were similar in form and coloration to the adult, but had thicker discs. Males reach sexual maturity at around 8.2–10.9 cm (3.2–4.3 in) in length, and females at around 10.5–14.8 cm (4.1–5.8 in) in length. This species is often parasitized by minute worms under the skin, identified as "entozoa" (an obsolete grouping that included acanthocephalans, trematodes, cestodes, and nematodes) and named Cysteocercus temerae by Cantor (1850).
The finless sleeper ray is susceptible to intensive, mixed-species fisheries using bottom trawls and perhaps other demersal fishing gear, operating throughout its range and particularly in the Andaman Sea. It is unlikely to survive capture even if discarded. Catches of this species by Myanmar fisheries are known to be sold frequently in Phuket. Off Thailand, electric ray populations overall appear to be declining. Along with continuing heavy fishing pressure in the region and the finless sleeper ray's low reproductive rate, these factors have led the International Union for Conservation of Nature (IUCN) to assess it as Vulnerable.
- Carvalho, M.R. de, M.E. McCord, and C. Vidthayanon (2004). "Temera hardwickii". IUCN Red List of Threatened Species. Version 2010.2. International Union for Conservation of Nature. Retrieved September 17, 2010.
- Gray, J.E. (1831). "Description of twelve new genera of fish, discovered by Gen. Hardwicke, in India, the greater part in the British Museum". Zoological Miscellany 1831: 7–9.
- Compagno, L.J.V. and P.C. Heemstra (May 2007). "Electrolux addisoni, a new genus and species of electric ray from the east coast of South Africa (Rajiformes: Torpedinoidei: Narkidae), with a review of torpedinoid taxonomy". Smithiana, Publications in Aquatic Biodiversity, Bulletin 7: 15–49.
- Carpenter, K.E. and V.H. Niem, ed. (1999). "Torpedinidae: Narkidae". FAO identification guide for fishery purposes: The living marine resources of the Western Central Pacific. Food and Agricultural Organization of the United Nations. pp. 1443–1446. ISBN 92-5-104302-7.
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- Monkolprasit, S. (1990). "The Electric Rays Found In Thailand". Kasetsart Journal: Natural Science 24: 388–397.
- Cantor, T. (1850). "Catalogue of Malayan Fishes". Journal of the Asiatic Society of Bengal 18 (2): 983–1694.
- Dean, M.N., J.J. Bizzarro, and A.P. Summers. "The evolution of cranial design, diet, and feeding mechanisms in batoid fishes". Integrative and Comparative Biology 47 (1): 70–81. doi:10.1093/icb/icm034.