Distribution and abundance of horseshoe crabs Tachypleus gigas and Carcinoscorpius rotundicauda around the main island of Singapore

Abstract

A survey and interviews with fishermen to determine the current spatial distribution of coastal Tachypleus gigas and mangrove Carcinoscorpius rotundicauda horseshoe crabs on the main island of Singapore indicated that there are probably no sites that support a breeding population of T. gigas. The only adult T. gigas seen were trapped in nets at 1 site, no juveniles or sub-adults were found at any site, and fishermen see this species infrequently. C. rotundicauda were more abundant, and breeding populations were found on the mudflats, fringed with mangroves. These small areas may be the last sites that support a breeding population of C. rotundicauda. Population density stud- ies of mainly surface crabs on the mudflats at 1 site gave a conservative figure of 0.5 crabs m -2 using non-randomised, longitudinal belt-transects of 5 × 50 m, set from high- to low-tide zones. Smaller ran- domised quadrats, searched for both buried and surface crabs, gave densities of 0.57 to 0.98 individ- uals m -2 , equivalent to a possible abundance ranging from 29 925 to 51 450 individuals in the accessi- ble search area of 52 500 m 2 . Comparisons over different months suggest that density changed little over time. Randomised quadrats and searches to depletion gave higher density figures, but they are labour intensive and difficult to set up in the terrain. Randomised, longitudinal belt- transects of 5 × 50 m are recommended for long-term monitoring of crab density. These findings provide baseline data to monitor the population at the site and to formulate conservation strategies for the 2 crab species.