The black softshell turtle or Bostami turtle (Nilssonia nigricans), previously placed in genus Aspideretes, is a species of freshwater turtle found in India (Assam and Tripura) and Bangladesh (Chittagong and Sylhet). It was long believed to consist of inbred individuals of the Indian softshell turtle (A. gangeticus or N. gangeticus) or the Indian peacock softshell turtle (A. hurum or N. hurum), but while it is a close relative of the latter, it is a distinct species.^[5] In the 1800s it was believed these turtles were brought from Iran to Chittagong shrine pond by Hazrat Bayezid Bostami. His turtles he had brought to this pond were treated as sacred and respected by the public.^[6] Previously declared extinct by the International Union for Conservation of Nature in 2002, these turtles were found still to exist in a temple's pond called the Hayagriva Madhava Temple located in Assam, and in Kalyan Sagar lake in Tripura Sundari Temple in Udaipur, Tripura, India.^[7] Through conservation methods and protection of the species, some of these turtles can be found today throughout the wild, and scientists and environmental biologists are continuing to work hard to preserve this endangered species and their natural habitat. Their mitogenome represents relatedness to 19 other species of the Testudines. When looking at the phylogenetic tree, Nilssonia nigricans is a sister group to Nilssonia formosa.^[8]

Quick Facts Conservation status, Scientific classification ...

Black softshell turtle
Conservation status
Critically Endangered  (IUCN 3.1)[1]
CITES Appendix I (CITES)[2]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Testudines
Suborder:	Cryptodira
Family:	Trionychidae
Genus:	Nilssonia
Species:	N. nigricans
Binomial name
Nilssonia nigricans (Anderson, 1875)[1]
Synonyms[3][4]
Trionyx nigricans Anderson, 1875 Amyda nigricans — Mertens, L. Müller & Rust, 1934 Aspideretes nigricans — Meylan, 1987 Trionix nigricans — Richard, 1999 Nilssonia nigricans — Praschag et al., 2007

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Originally native to the lower Brahmaputra River, the only population ever reliably known consists of a number of the species in a man-made pond which is part of the Bayazid Bostami shrine at Chittagong, where they are dependent on humans for survival. To the locals and worshipers, the black softshell turtle is known as mazari ("Mazar inhabitant"); specimens from this shrine were used in the first scientific description.^[5][9]

In a 2014 survey by Poribesh Banchao Andolon, a Bangladeshi private organization it was found that the amount of oxygen dissolved in the pond water was 2.01 mg/L, whereas the favorable level is 5 mg/L.^[10] In 2012, wildlife biologists of the Center for Advanced Research in Natural Resources and Management found a small population in the wild of Sylhet.^[10] Two tiny wild populations were discovered in Assam, in Kaziranga and in the Jia Bhoroli River which is a northern tributary of the Brahmaputra. Also, another temple population of these turtles was identified in the Kasopukhuri pond on Nilachal Hill, next to the Kamakhya Temple at Guwahati in Assam.^[11] Another temple population is found in the kalyan sagar lake of Tripureshwari temple, Udaipur, Tripura. These turtles are also found in the pond in Baneswar Shiva temple, a shiva temple in Baneswar in the Cooch Behar II CD block in the Cooch Behar Sadar subdivision of the Cooch Behar district in West Bengal, India.

In 2002, the International Union for Conservation of Nature classified the species as Extinct in the Wild. In 2004, 408 turtles were found in the pond of the Bayazid Bostami shrine. According to the shrine committee staff, 90 more turtles were hatched in the pond in 2007, 74 in 2008, 96 in 2009, 28 in 2010, 45 in 2012 and 40 in 2014.^[10]

In 2017, a singular wild turtle was found in the wetlands of Old Akuk Village in Wokha District, Nagaland. DNA from the specimen sent to Bangladesh was tested and ultimately confirmed that it was a member of the black softshell species.^[19]

This extinction has been caused by human intervention, specifically the migration of people into the habitat and the contamination of water and land following this movement.^[20] The land of Bangladesh that N. nigricans have been able to occupy currently had been tested for conditions of survival and mating. The survival rate of juvenile N. nigricans is relatively low due to a low mortality of egg hatching and juvenile survival following hatching.^[21]

Human captivity of black softshell turtles for religious contexts seems to both benefit and harm turtle populations. Shrines keep these turtles in areas protected from outside predators, but these ponds are not suitable for sustained repopulation efforts. In Hinduism, the black softshell turtle represents an incarnation of the deity Kurma. Worshipers come and feed the turtles in the ponds human foods such as biscuits. Such dietary items are harmful to the turtles, often leading to malnourishment.^[22]

Human intervention has also caused a push towards extinction with intentional methods of poisoning. In 2006 there was a mass poisoning of the turtles within the Bostami Shrine pond by poachers in the area. This event occurred within the research period, as the turtles were under heavy observations and were protected from this event, while this not being the first time this method had been used. In this case many of the fish utilized by the N. nigricans for food had been poisoned, making nutrition unsustainable for the turtles. This same poisoning also changed the oxygen levels in the water, making it unsustainable for the turtles. A waiting period had passed to ensure oxygen levels were rich in the water, indicating a sustainable environment, in turn allowing the release of the species.^[21]

Since 2013, TSA India has worked diligently to improve conditions at selected temple ponds in Assam where these turtles were discovered. Many of the ponds were eutrophic and overcrowded and often, turtles were fed human food as religious offerings. The turtles showed signs of poor nutrition and a lack of suitable nesting space resulted in eggs being deposited in areas where there was little chance of hatching. The goal of the project is to eventually rear and release juveniles from these captive colonies to supplement depleted wild populations. As part of that initiative, the TSA India team camped at the Nagshankar temple in April to observe nesting in the 40-45 adult females at that location. At that time, the team transferred ten nests to a hatchery and protected four nests onsite.

With TSA's efforts 44 turtles have hatched so far. The team is currently expanding a headstarting facility at Nagshankar temple to accommodate the turtle's hatchlings of 2016. Neonates and juveniles cannot be released back into the temple pond due to predation by larger turtles and exotic fish. For this reason, the team acquired an earthen pond in the nearest village to provide space for the fast-growing juveniles, improving survival prospects for this extremely rare softshell.

In 2018, Das et al. had collected population sizes of N. nigricans from 2006 to 2012. The population size had shown a gradual increase over time, indicating that the calculated area per turtle had decreased in accordance. He indicated that without an increase in habitat size, the carrying capacity of the species will be reached quicker. This can decrease the overall growth of the species and not allow it to grow past the current conservation status.^[21]

In 2011, Dr. Jayaditya from the Help Earth NGO found a grouping of black softshell turtles in a temple pond in Hajo, Assam. He revitalized contributions aimed towards saving and replenishing the species once again. He and his team embanked the sides of the temple pond and made adjustments to simulate a natural river system and create an ecosystem more conducive to the turtle's natural habitat. Thereafter, they observed a positive egg ratio produced around the temple premises. The State Zoo of Assam is playing an important role in this conservation project, serving as the location where baby turtles are reared for six months. When deemed fit and healthy, and capable of surviving, the young turtles are released into the wild. As of 2016, the State Zoo of Assam has released 300 turtles. Now 16 temples in Assam are helping in this important project. Similar conservation efforts have been undertaken in Tripura in Tripura sundari temple, matabari, udaipur, tripura, to protect the indigenous populations of Bostami turtles.