Predation by two species of crabs on different sizes of juvenile sea urchin Salmacis sphaeroides

The sea urchin species Salmacis sphaeroides holds significant ecological and economic value in the South China Sea. However, natural populations are increasingly at risk due to environmental pollution, overfishing, climate change and other factors. Stock enhancement through breeding and release could mitigate this decline, but crab predation poses a significant threat to released individuals. The predation effects on juvenile S. sphaeroides and the factors influencing these effects are not well understood. This study investigated the predation of two sizes classes of juvenile S. sphaeroides test diameters: large, (36.50±4.11) mm; small, (22.33±1.97) mm by Charybdis japonica and Thalamita crenata, both of which are prevalent in the South China Sea, using both indoor and filed trapping experiments. Results indicated that S. sphaeroides size and predation duration were key factors influencing predation rates, with species-specific differences between the two crab species. C. japonica exhibited high predation rates on small juveniles at both 0-24 and 24-48 h, with rates of (3.75±1.09) ind/d and (3.75±1.30) ind/d, respectively, while predation on large juveniles was significantly lower at (0.50 ± 0.86) ind/d and (0.25±0.50) ind/d, respectively. In 0-24 h, T. crenata showed a high initial predation rate on small juveniles of (4.25±3.27) ind/d at 0-24 h, which decreased to (2.00±1.58) ind/d during 24-48 h, and exerted minimal pressure on large juveniles. Field trapping experiments revealed a strong attraction of juvenile S. sphaeroides to C. japonica, with an average capture rate of (2.40±0.89) ind/(cage·d). This study concludes that crabs are the primary predators of S. sphaeroides, and to enhance restocking efficiency, it is recommended to release large-sized sea urchin seedlings and to control potential predators such as crabs in advance.