Digestible dietary protein and energy requirements of juvenile Asian horseshoe crabs, Tachypleus tridentatus and Carcinoscorpius rotundicauda

A 84-day feeding experiment of two juvenile horseshoe crab species, including nine formulated diets with three digestible protein (DP) levels (36%, 40% and 44%) and three digestible energy (DE) levels (14, 16 and 18 kJ g^-1) versus the control, fed on frozen brine shrimp (Artemia salina), was conducted. The results showed that the survival rate of Tachypleus tridentatus ranged from 67% to 100%, while that of Carcinoscorpius rotundicauda varied from 44% to 100%. The results also indicated that the formulated diet containing 40% DP with 14 kJ g^-1 DE was the best, in terms of growth and feed utilization, for the two juvenile horseshoe crab species. Final body weight (FBW), thermal-unit growth coefficient (TGC), feed efficiency (FE), nitrogen retention efficiency (NRE) and energy retention efficiency (ERE) of T. tridentatus fed 40% DP and 14 kJ g^-1 DE were significantly higher than that of T. tridentatus fed the control diet. However, for C. rotundicauda, only significantly higher FE, NRE and ERE were found in juveniles fed 40% DP and 14 kJ g^-1 DE than the control diet. The present findings revealed that the optimum DP/DE ratio for the two species was 28 g protein MJ^-1.