Aspideretes nigricans

(Anderson, 1875)
Black softshell turtle

The oval to rounded carapace (to 91 cm) is dark brown to olive or black, and, if brown or olive, may bear numerous blackish or rusty brown spots, causing an overall dark appearance. Several longitudinal rows of tubercles occur on the carapace, and a series of enlarged blunt tubercles lies on the anterior rim above the neck. Both a preneural and neural bone separate the 1st of the eight pairs of costals. The 8th costals meet at the midline. The gray plastron is heavily spotted with black, especially over the bones. There are well-developed callosities on the posterior bones and, in some, over the entoplastron. The hyoplastra do not touch medially and are strongly divergent posteriorly; their single, short medial process is thick and blunt, and indistinctly terminally bifurcated. The epiplastra may touch anterior to the entoplastron. The skull is broad, and its bony snout is slightly longer than the diameter of the orbit. Annandale (1912) reported that the triturating surface of the upper jaw bears low median and internal longitudinal ridges, which is unusual for a trionychid. There is also a strong longitudinal ridge on the symphysis of the lower jaw, with a diameter longer than that of the orbit. Head, neck, and limbs are dark olive to black with greenish spots. Some white may occur on the upper lips and sides of the head, and some light banding may be present about the eyes.
Males are distinctly larger than females and have long, thick tails with the vent situated near the terminus.

The original geographic range is unknown; Annandale (1912) suggested that this species occurred from the Brahmaputra drainage to Arakan. However, this is speculation, as the species was already confined to tanks at the Hazrat Bayazid Bostami Shrine at Chittagong, Bangladesh, when first described by Anderson (1875).

Annandale (in Smith, 1931) described the habitat at the shrine:

They live in a large pond attached to the shrine of Sultan Bagu Bastan (a saint who is said to have lived in the eighteenth century), about five miles from the town of Chittagong. The Mohammedans will neither kill them nor permit them to be killed; they believe they are in some way connected with the saint. The tank is surrounded by steps leading down to a platform a few inches under water, and the turtles are so tame that they come to feed when called, placing their forefeet on the edge of the platform or even climbing upon it and stretching their necks out of the water. The largest are tamer than the smaller ones. Some even allowed us to touch them, and ate pieces of chicken from wooden skewers held in our hands. The only sound they emitted was a low hiss. When undisturbed they remained at the bottom of the pond half buried in the mud. A man connected with the shrine told us that they left the water every evening and climbed a small hill, on which they slept. He said that they laid their eggs in the same hill during the rains.

There have been some modifications since Annandale's description. The water depth fluctuates seasonally, increasing up to 5 m during the monsoon season above that of the dry season, as the principal source of water for the tank are rains. At the driest times the first of the steps is exposed and the turtles come upon them to take foods (bread, banana, rice, various meats) offered on skewers by pilgrims. Along the shoreline are abundant algae, moss, ferns, grasses, and some trees. The nesting areas adjacent to the tank and accessible to females are completely enclosed by retaining walls. The most extensive nesting area is a broken and irregular area west of the tank which is covered with a dense canopy at the height of the monsoon. The vegetation hinders much sunlight from reaching the ground, keeping the soil moist and at a relatively constant temperature during the nesting season. The soil is sand to a mixture of clay and sand.

Natural History
The only life history published on this species is based on captives kept at the shrine of Byazid Bostami about five miles from the town of Chittagong. Apparently the total population of the turtle is restricted to the tank at the shrine since the turtle no longer exists in the wild (Ahsan et al., 1991). The population size is estimated to be about 300 individuals, with an adult sex ratio of 1.5 male to one female; juveniles are present.
Oviposition begins in February with the onset of the monsoon and ends in early May with the advent of the dry season. Nesting occurs in the evening and at night (16.00-20.00 h). Females press the snout to the soil and crawl about the nesting area until they find a suitable excavation site. First a shallow bowl-like depression is scooped out with the forelimbs, then the female moves into the depression and begins to dig the egg chamber with the hind feet. The final nest cavity is flask shaped with a narrow entry and an enlarged chamber 14-25 cm deep. Nesting females have carapace lengths of 38-58 cm (Ahsan et al., 1991; Ahsam and Saeed, 1992).
Clutches contain an average 6-38 spherical eggs that average 34.9 (32.4-36.3) mm in diameter (Ahsan et al., 1991; Ahsam and Saeed, 1992). According to Ahsan et al. (1991), the duration of the incubation period varies with the clutch-size; the greater the number of eggs in the clutch, the shorter the incubation period. Incubation periods ranged from 96 to 104 days.
Hatchlings have light olive carapaces (darker near the edges) with dark dashes and spots, especially near the vertebrals; four distinct, five-ringed, orange-yellow and black ocelli, and oval-shaped or irregularly rounded spots along the margins. The outer border of the carapace is a narrow cream-colored band. The plastron is black, with laterally placed yellow spots. The hatchling carapace averages 47 mm (45-51) in length, and they weigh an average of 16.3 g (14.3-19.1) (Ahsan et al., 1991).

Rashid (1990) suggested that A. nigricans might actually represent locally collected A. hurum that, due to prolonged confinement to the shrine and long term inbreeding, may have evolved into a different subspecies rather than a full species.

IUCN Red List Status (1996)
Critically endangered (B1+2a).