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 delta C-13 and delta N-15 values to define life-stage changes in dietary carbon sources and trophic position of 2 co-occurring horse-shoe crab species, Tachypleus tridentatus and Carcinoscorpius rotundicauda, in a mangrove- and sea-grass-vegetated estuary, Pearl Bay, in Guangxi, China. delta C-13 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. rotundicauda reflecting influences of lighter sediment particulate organic matter (POM; -15.1 vs. similar to -15.8%). Overall, seagrass contributed most to dietary carbon of both species (similar to 60%), followed by sediment POM (35%). delta N-15 signatures were similar between species (similar to 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 delta N-15 signatures in horse-shoe 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.