Malayemys subtrijuga

(Schlegel and Müller, 1844)
Malayan snail-eating turtle

This turtle may reach 21 cm in carapace length, but most individuals are much smaller. Its oval carapace is moderately arched, unserrated but notched posteriorly, and contains three discontinuous keels consisting of knobs situated at the posterior border of the scutes on which they lie. The medial keel extends along all five vertebrals, but the lateral keels rarely reach the 4th pleural. Vertebrals are usually broader than long. The underlying neurals are not elongated, and those in the middle of the series are shortest posteriorly. Vertebral 4 covers parts of four neurals. In color, the carapace ranges from light to darker brown, sometimes mahogany, with a yellow or cream-colored border. The knobs of the keels and the marginal seams are darker than the rest of the carapace. The hingeless plastron is narrower than the carapace and notched posteriorly. Its hindlobe is slightly shorter than the bridge. Buttresses are strong; the axillary is attached to the lateroventral surface of the 1st costal and the inguinal extends about a third of the way up along the suture between the 5th and 6th costals. The entoplastron lies anterior to the humero-pectoral seam. The plastral formula is: abd > fem > pect >< an > hum >< gul. The yellow or cream-colored plastron has a pattern consisting of a large dark-brown or black blotch on each scute. Two dark blotches occur on the bridge. The head is relatively large and the snout projects anteriorly. Posterior emargination of the skull is deep but the ventral emargination is very shallow. The posterior emargination separates the supratemporal from the postorbital and extends nearly to the frontal bone. The squamosal is in broad contact with the maxilla. The orbito-nasal foramen is small. The upper jaw is medially notched; its triturating surface is flat and broad with only a very slight medial ridge (an adaptation for crushing snail shells). In the lower jaw, the angular bone is short and the prearticular extends more anteriorly; the coronoid process is very large. The head is black and is adorned with several yellow or cream-colored stripes. The first begins on the snout above the nostrils and extends posteriorly on each side above the orbits to the neck. A second stripe begins on each side of the snout just behind the nostril and curves downward and then backward passing below the orbit to the neck. Two narrow stripes pass from the nostrils to the medial notch of the upper jaw; when the mouth is closed, these almost touch two similar stripes on the lower jaw. Another narrow stripe begins behind the orbit and crosses the tympanum. Other discontinuous stripes may occur on the lower jaw. Dorsally, the head is covered with a single scale, while the back of the head contains numerous scales. Limbs are gray to black with a narrow yellow outer border.
Killebrew (1977a) reported the karyotype to be 2n = 52 (8 metacentric, 3 submetacentric, and 2 telocentric pairs of macrochromosomes and 13 pairs of microchromosomes), but more recently Bickham (1981) found the diploid number to be only 50 chromosomes.
Males have proportionally longer, narrower shells, a V-shaped anal notch (round in females) and a heavier tail base; the plastra of both sexes are flat (Van Dijk and Thirakhupt, in press).

Malayemys subtrijuga occurs in northern Malaysia, Thailand, Cambodia, Laos, and southern Vietnam; it has possibly been introduced to Sumatra and Java (Van Dijk and Thirakhupt, in press).

This turtle is an inhabitant of slow-moving water bodies with soft bottoms and aquatic vegetation. It has been taken from ponds, canals, small streams, marshes, and rice paddies.

Natural History
In Thailand Malayemys subtrijuga nests in the dry season, from December to March. A clutch usually comprises 3-4 (5-6) eggs (Van Dijk and Thirakhupt, in press), but Nutphand (1979) reported clutches up to 10 eggs. The elongated, white, brittle-shelled eggs are 32-45 x 20-25 mm (Smith, 1931; Van Dijk and Thirakhupt, in press). Hatching of eggs collected in the wild (at 28-30°C) took an average of 167 days, but with large variations in hatching time—the last hatchling from the same clutch could emerge up to four months after the first (Srinarumol, 1995). Ewert (1979) reported that 11 hatchlings averaged 35.3 mm in carapace length.
Malayemys subtrijuga is basically a snail eater; larger females also eat freshwater mussels (Van Dijk and Thirakhupt, in press). Nutphand (1979) additionally reported worms, aquatic insects, crustaceans, and small fish as part of its diet.

IUCN Red List Status (1996)
Not listed. This turtle does poorly in captivity and should be strictly left in its natural habitat.