A dual stable isotope study for diet composition of juvenile Chinese horseshoe crab Tachypleus tridentatus (Xiphosura) on a seagrass-covered intertidal mudflat

To enhance our understanding on the foraging biology of juvenile Chinese horseshoe crab on a seagrass-covered intertidal mudflat, we used dual stable isotopes δ 13 C and δ 15 N to determine the diet composition and trophic position of 6th–11th instar juveniles in summer and winter. The δ 13 C and δ 15 N values in tissues of juveniles and their potential food sources suggested that the juveniles consumed a mixed diet mainly comprised a variety of polychaetes, crustaceans and bivalves, in which these food groups were abundantly available in the study site and largely supported by seagrass biomass in both summer and winter. While δ 15 N values in juvenile tissues were statistically similar between the two seasons, the δ 13 C values in tissues of juveniles had a minimal seasonal difference which was probably due to variations in the source values rather than the changes in diet composition of juveniles. There were no differences in δ 13 C and δ 15 N values among the size groups of juveniles through 6th–11th instars. Although juvenile Chinese horseshoe crabs may consume the prey in response to the seasonal availability on the mudflat, a healthy condition of habitat with high water quality is essential to support the juveniles and a wide range of intertidal invertebrates they consume.