Black Softshell Turtle

Nilssonia nigricans, commonly known as the black softshell turtle or Bostami turtle, is a rare species of freshwater softshell turtle native to parts of the Indian subcontinent. It is primarily associated with north-eastern India and Bangladesh and is notable for its unusual conservation history, strong religious and cultural associations, and long-standing taxonomic confusion. Once declared Extinct in the Wild by the International Union for Conservation of Nature (IUCN), the species has since been rediscovered in both semi-wild and wild habitats, making it one of the most significant examples of a “Lazarus species” among reptiles.
Originally described in the nineteenth century, Nilssonia nigricans was long misidentified as an inbred or aberrant form of other Indian softshell turtles. Modern genetic and morphological studies have confirmed its status as a distinct species closely related to Nilssonia hurum and Nilssonia gangeticus, with Nilssonia formosa identified as its sister species.

Taxonomy and nomenclature

The species has undergone several taxonomic reassignments since its first scientific description. Historically, it has been referred to by multiple scientific names, including Amyda nigricans, Aspideretes nigricans, and Trionix nigricans. Advances in molecular phylogenetics led to its placement within the genus Nilssonia, where it is currently recognised as Nilssonia nigricans.
Early zoologists believed the species represented malformed or inbred individuals of the Indian softshell turtle (Nilssonia gangeticus) or the Indian peacock softshell turtle (Nilssonia hurum). However, mitochondrial genome analyses and comparative morphology have demonstrated clear genetic divergence, confirming its independent species status within the family Trionychidae.

Historical and cultural context

Nilssonia nigricans holds exceptional cultural and religious significance, particularly in Bangladesh and north-eastern India. In the nineteenth century, local traditions claimed that the turtles inhabiting the shrine pond of Bayazid Bostami in Chittagong were brought from Iran by the Sufi saint Bayazid Bastami. These turtles were regarded as sacred and were protected by local communities for generations.
Folklore associated with the species includes beliefs that the turtles were once jinns, supernatural beings in Islamic mythology, transformed into turtles as a divine act. As a result, harming the animals was considered taboo, inadvertently providing them with long-term protection. Locally, the turtles are known as mazari, meaning inhabitants of a mausoleum.
In Hindu contexts, particularly in India, the black softshell turtle is associated with Kurma, the tortoise incarnation of the god Vishnu. This belief has led to their protection in temple ponds, where devotees regularly feed them as part of religious practice.

Distribution and habitat

Historically, Nilssonia nigricans is believed to have been native to the lower Brahmaputra River basin, encompassing parts of present-day Assam, Tripura, and Bangladesh. For much of the twentieth century, the only reliably known population existed in the man-made pond of the Bayazid Bostami shrine in Chittagong, where the turtles were entirely dependent on humans for survival.
Subsequent surveys and field studies have expanded the known range of the species. Small wild populations have been confirmed in the Sylhet region of Bangladesh and in Assam, including Kaziranga National Park and the Jia Bhoroli River, a northern tributary of the Brahmaputra. Additional semi-captive populations occur in temple ponds such as the Hayagriva Madhava Temple and Kamakhya Temple in Assam, Tripura Sundari Temple at Kalyan Sagar Lake in Tripura, and the Baneswar Shiva Temple in West Bengal.
Despite these rediscoveries, the species’ overall distribution remains highly fragmented, and most populations exist in artificial or heavily managed environments.

Physical characteristics

The black softshell turtle displays the distinctive traits typical of softshell turtles, including a flattened, leathery carapace that lacks the rigid scutes found in hard-shelled turtles. This flexible shell allows greater agility in water and facilitates burrowing into soft substrates.
A defining feature of the species is its elongated, tubular snout, which functions as a snorkel, enabling the turtle to breathe while remaining submerged. The limbs are strong and paddle-like, with well-developed webbing and hand-like digits that enhance swimming and digging abilities.
Colouration is generally black to dark brown, often appearing plain, though some individuals exhibit pale yellow or white spotting on the head and limbs. The carapace is typically smooth and lacks prominent ornamentation.

Size and growth

Nilssonia nigricans is among the larger softshell turtles of South Asia. Adult individuals commonly measure between 33 and 71 centimetres in carapace length, with exceptional specimens recorded at up to 89 centimetres. Females are known to weigh approximately 54 kilograms, while males are believed to be larger on average, though precise weight data for males remain undocumented.
Growth is slow, consistent with other large freshwater turtles, and individuals may take several decades to reach maximum size.

Lifespan and mortality

The species is reputed to be exceptionally long-lived. Local accounts from shrine custodians and temple authorities suggest that some individuals may be over 100–150 years old, though these claims remain unverified by scientific ageing methods.
Early life stages are characterised by extremely high mortality. Approximately 94 per cent of eggs fail to hatch, and juvenile survival rates are low due to predation, environmental instability, and poor nesting conditions. This life-history strategy makes population recovery particularly slow.

Diet and feeding ecology

In temple and shrine environments, black softshell turtles are heavily dependent on food provided by humans. Visitors commonly offer grains, fruits, vegetables, and meat products. While this practice has ensured the turtles’ survival in captivity, it has also resulted in nutritional imbalances and behavioural dependence.
The natural diet of Nilssonia nigricans is inferred from closely related species and likely includes aquatic vegetation, fish, molluscs, worms, and other small invertebrates. The species is therefore considered omnivorous, with a flexible feeding strategy suited to riverine ecosystems.

Reproductive biology

Nilssonia nigricans is oviparous, reproducing through the laying of eggs. Sexual maturity occurs relatively late; males may become reproductively active at around four years of age, while females typically mature between seven and nine years.
Females lay clutches ranging from 10 to 38 eggs, usually deposited in sandy or soft substrates near water bodies. Incubation lasts approximately 92 to 108 days, depending on temperature and environmental conditions. Recent studies indicate declining hatching success, attributed to habitat degradation, pollution, and disturbance at nesting sites.

Behaviour and hibernation

Like many softshell turtles, Nilssonia nigricans undergoes seasonal hibernation during late autumn and winter. Individuals bury themselves in riverbeds or pond sediments, significantly reducing metabolic activity.
During this period, oxygen uptake becomes more critical than food intake. The species employs pharyngeal respiration, a specialised process in which water is pumped in and out of the throat, allowing oxygen absorption through richly vascularised tissues. This adaptation enables prolonged submergence in low-oxygen environments.

Conservation status and threats

In 2002, the IUCN classified Nilssonia nigricans as Extinct in the Wild, based on the assumption that only captive shrine populations remained. Subsequent discoveries of wild individuals led to a reassessment, though the species remains Critically Endangered.
Major threats include habitat destruction, water pollution, deliberate poisoning by poachers, and unsustainable human interference. A notable incident occurred in 2006 when mass poisoning at the Bayazid Bostami shrine resulted in significant fish mortality, indirectly threatening the turtles’ food supply and altering oxygen levels in the pond.