Distribution of Horseshoe Crabs, Carcinoscorpius rotundicauda, and Tachypleus gigas in Singapore, Including Coney Island

Horseshoe crabs are vital in the marine ecosystem food web, but their populations are decreasing in the face of growing threats. There are four extant species of horseshoe crabs worldwide, but only Carcinoscorpius rotundicauda and Tachypleus gigas are found in Singapore. The distribution of horseshoe crab populations across four locations on mainland Singapore and offshore Coney Island was surveyed. There was a higher abundance of C. rotundicauda than T. gigas due to the larger mangrove coverage at Sungei Buloh Wetland Reserve. The high number of carcasses found at northeastern Singapore may be due to anthropogenic activities. Live T. gigas were found at Coney Island, Punggol Settlement, and Changi Beach, with amplexus pairs reported for Changi Beach and Coney Island during full and new moon periods. Survey results demonstrated that there was little overlap in distribution between both species, with coexistence occurring only on Coney Island, where there was one observed incidence of cross-species amplexus. Results from this survey would help provide additional baseline data to aid horseshoe crab conservation efforts in Singapore, which is important since they are listed as “Data Deficient” in the IUCN Red List, and C. rotundicauda and T. gigas are classified as “Vulnerable” and “Endangered,” respectively, in the Singapore Red Data Book.