Population structure and growth of juvenile horseshoe crabs Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura) in Southern China

Abstract

The abundance, population structure and growth of juvenile Asian horseshoe crabs Tachypleus tridentatus and Carcinoscorpius rotundicauda are rarely reported in the literature. This study investigated the density and growth allometry of these sympatric species on the intertidal areas of three spawning and nursery beaches, Xi Bei Ling (XBL), Jin Hai Wan (JHW) and Xi Chang (XC) along the coast of Beibu Gulf, southern China. While T. tridentatus and C. rotundicauda coexisted on JHW and XC beaches, only T. tridentatus was found at XBL. The density of T. tridentatus (0.9–3.2 individuals/100 m2) was significantly higher than that of C. rotundicauda (0.1–1.7 individuals/100 m2) at JHW, but densities were similar at XC. Juvenile T. tridentatus ranged in size (prosomal width) from 14 to 89 mm and were estimated to represent instars 4–12 and ages 2–8 years based on previous studies. C. rotundicauda juveniles were 16–75 mm in prosomal width and represented instars 5–15. The Hiatt growth model on the relationship between pre- and post-molt size data showed similar growth increments in juvenile T. tridentatus and C. rotundicauda. Growth rates for T. tridentatus varied from 4.4 to 19.7 mm/year, depending on instar stage and site. Growth rates were not determined for C. rotundicauda due to lack of data. These data establish a baseline for density and growth allometry of juvenile T. tridentatus and C. rotundicauda horseshoe crabs in Asian waters and will support future monitoring and assessment of juvenile populations and habitat resources.