Co-occurrence of juvenile horseshoe crabs Tachypleus tridentatus and Carcinoscorpius rotundicauda in an estuarine bay, southwestern China

Spawning and nursery habitats are critical in the conservation of horseshoe crabs. We examined the abundance and distribution of juveniles of 2 species of horseshoe crabs, Tachypleus tridentatus and Carcinoscorpius rotundicauda, in a mangrove-vegetated estuary (Pearl Bay, Guangxi, China). Intertidal sampling and measurement of environmental variables were carried out in October 2014 at 4 sites spanning similar to 6.3 km from the inner to the outer bay. Both species were found at the 2 innermost mangrove sites but not at the outer 2 sites. Densities were higher than recorded in previous studies, 0.24 ind. m(-2) for C. rotundicauda and 0.05 ind. m(-2) for T. tridentatus. Although only 12 of the latter species were found, they appeared to be more abundant in more seaward, energetic environments in poorly sorted sediments, whereas C. rotundicauda occurred in sediments with higher silt-clay and organic carbon content, and aggregated at the mangrove forest edges. These small-scale distribution differences are consonant with the species' life histories, since only T. tridentatus migrates to the sea while C. rotundicaudata spends its entire life within or near mangrove habitats. International and national conservation strategies for these species should include further efforts to inventory critical habitats, such as Pearl Bay, in southeast Asia.