Trophic relationship of sympatric juvenile Asian horseshoe crabs in Beibu Gulf, Southwestern China

Tri-spine horseshoe crab Tachypleus tridentatus and mangrove horseshoe crab Carcinoscorpius rotundicauda are widely reported to coexist and forage on intertidal flats along the coast of Beibu Gulf, southwestern China. Since partitioning of food resources has been proposed as the principal mechanism explaining the coexistence of competing marine organisms, the possible ecological segregation between these two horseshoe crab species was investigated by analyzing δ¹³C and δ¹⁵N stable isotopes from tissue (previous diets) and fecal (recent food sources) samples of the juveniles. The δ¹³C and δ¹⁵N values in tissues of fourth to seventh instars for both species were significantly lower than that of those over 11th instars. Dual stable isotope and mixing model analyses of juvenile tissues indicated that differences in diet compositions between species were noted at Jinhaiwan (JHW) beach, in which juvenile T. tridentatus primarily fed on bivalves, whereas C. rotundicauda consumed a variety of food sources including particulate organic matter (POM), benthic microalgae (BMA), polychaetes, bivalves, gastropods, and crustaceans. However, the isotopic signatures of tissues in both species overlapped at Xichang (XC) beach, in which POM, BMA, polychaetes, and crustaceans largely contributed to their diet. Isotopic signatures of fecal samples indicated that both species recently foraged on similar food sources at JHW and XC. A lack of resource partitioning in juvenile Asian horseshoe crab populations at XC may be due to the high abundance of prey available in the intertidal areas; otherwise, resource partitioning can occur, as observed at JHW.