Studies of Horseshoe Crabs Around Singapore

Results from students’ research on horseshoe crabs are not often readily available to other workers. A review of these studies has shown that during 6 years since the first published studies, the distribution of horseshoe crabs around Singapore’s main island has remained unchanged, the density of mangrove horseshoe crabs, Carcinoscorpius rotundicauda, at two sites has remained stable, and periods of high and low reproductive activity during the year have been confirmed. The student studies indicated that juvenile C. rotundicauda concentrated at the high tide zones of the mudflats along the Strait of Johor, and smaller juveniles were found on the mud surface and in the small streamlets. Adults were found spread more evenly over the mudflats from the high to low tide zones, on average about 3 m apart, and commonly buried or found in the larger streams among the mangroves. Feeding preference studies suggest mangrove horseshoe crabs are not particularly selective feeders. Sex ratios remained approximately 1:1 all year and at all sites, and mean percentage growth during ecdysis was 34 %, with little difference between the smallest crab of 1.1 cm and the largest of 90.1 cm. A mortality study found death rates of mangrove horseshoe crabs higher at a polluted site (19.8 %) than at a less polluted site (7.5 %). Island wide distribution studies showed that while there is a thriving population of mangrove horseshoe crabs around the north-west, by contrast, Tachypleus gigas, the coastal horseshoe crab, is still very rare around Singapore, but an encouraging finding was the first reported sighting of T. gigas juveniles at one site. These studies, although small and of short duration, are important for the regular monitoring of the current status of horseshoe crab populations around Singapore and offer a major contribution to the understanding of horseshoe crab ecology in the country.