Turtles and Tortoises 5: Black Marsh Turtle

The aquatic chelonians I have been discussing so far all belong to a single family, the Geoemydidae. This is the most diverse of all living groups of turtles, with over 70 different species in 23 genera. The centre of distribution of the family is in south east Asia, with outliers in Europe and North Africa (the European Mauremys species), and a separate group in Central and South America (the Neotropical Wood Turtles Rhinoclemmys). Almost all of them are highly aquatic, and even the more terrestrial forms tend to prefer damp habitats. Despite this, they are generally believed to be close to the fully terrestrial tortoises such as the Aldabra Giant Tortoise and its smaller relatives. The age of the group is not clear, but it probably dates back to the late Cretaceous.

Part of this diversity of aquatic turtles is the species I will right about today, the Black Marsh Turtle Siebenrockiella crassirostris. They are sometimes calling smiling terrapins, as the jaws have an upward curve.As one would expect from their body shape, which is fairly streamlined for a turtle, they are highly aquatic, although they will sometimes emerge to bask. Compared to their relatives, they are more carnivorous, feeding heavily on aquatic insects, snails, and fish. As an animal fairly high up the food chain, they tend to accumulate toxins, and in some areas they have been shown to carry high levels of mercury in their tissues. They prefer shallow water, and spend most of their time prowling over the mud at the bottom, or burying themselves in it.

Black Marsh Turtles have featured heavily in Buddhism, and are often kept in ponds in the grounds of temples, where they are believed to contain the souls of people who died saving others from drowning. One would think that with these connections the species would be in good shape, but unfortunately they share the fate of other Asian turtles in being exported for food and Chinese medicine in vast numbers (135,000 from Malaysia alone in 1999). As a result, it is classed as Endangered in Vietnam and Cambodia, and Vulnerable in the rest of its range, which extends south from Vietnam to Malaysia and the eastern islands of Indonesia.

In common with many of their relatives, they lay small clutches of eggs, one or two usually, but females can produce 3 or 4 clutches in a single breeding season. Unlike almost all other turtles, they have a chromosomal system of sex determination, with males being XY and females XX. In the majority of other turtles the gender of the hatchling is determined by temperature, usually with temperatures below 30 degrees or so producing females and higher temperatures producing males. How widespread this chromosomal system is found is not much studied at present, but I am inclined to wonder if it is an adaptation to very constant environmental temperatures and small egg clutches, which would mean that there was not enough temperature variation within a nest to generate male and female offspring from the same clutch. If so, other turtles with a similar lifestyle may be found to show similar sex determination patterns.

Not all the animals exported for food markets wind up being eaten. In 2001 a customs confiscation in Hong Kong resulted in the founder stock for our Black Marsh Turtles arriving at Bristol. Despite extensive quarantine and veterinary care, the stress and mistreatment they had experienced before arriving resulted in half the rescued animals dying within a year. Despite these setbacks, in 2004 the first baby Black Marsh Turtle to be born in a European zoo hatched at Bristol in 2004. Even so, they are rarely seen in zoos, probably because they are not as colourful or conspicuous as some other species, and the total zoo population in Europe is probably under 50 animals.

Next week, the largest of the Asian Turtles, the Giant Pond Turtle

(images from wikipedia)