Ontogenetic changes in dietary carbon sources and trophic position of two co-occurring horseshoe crab species in southwestern China

Understanding which food sources are nutritionally important at nursery grounds is critical in the conservation of the dwindling populations of horseshoe crabs in Asia. We used δ13C and δ15N values to define life-stage changes in dietary carbon sources and trophic position of 2 co-occurring horseshoe crab species, Tachypleus tridentatus and Carci no scorpius rotundicauda, in a mangroveand seagrass-vegetated estuary, Pearl Bay, in Guangxi, China. δ13C signatures in tissues of T. tridentatus and C. rotundicauda varied between species and among life stages within species, with the greatest differences occurring among earlier life stages (instars 5−7; 20−30 mm prosomal width). Life-stage specific diets were consistent with known habitat use by the 2 horseshoe crab species, with T. tridentatus reflecting slightly enriched seagrass carbon sources and C. ro tun di cauda reflecting influences of lighter sediment particulate organic matter (POM; −15.1 vs. ~ −15.8‰). Overall, seagrass contributed most to dietary carbon of both species (~60%), followed by sediment POM (35%). δ15N signatures were similar between species (~11.6‰), with C. rotundicauda adults exhibiting a slightly more enriched signal than juveniles (12.7 vs. 11.6‰), and all horseshoe crabs occupied secondary consumer trophic levels. The δ15N signatures in horseshoe crabs at Pearl Bay were lighter than those from more urbanized waters, likely due to lower anthropogenic nitrogen loading. Our data and those of previous studies suggest that effective international and national strategies for conserving threatened Asian horseshoe crabs would benefit from incorporating efforts to protect seagrass and mangrove habitats. Newly molted juvenile horseshoe crab (left) and its old shell (right) on a tidal flat adjacent to mangroves in the Pearl Bay estuary (Guangxi, China). Photo: Chang-Po Chen