Stock assessment in wild populations and research involving reproductive biology of crabs rely greatly on the ability to accurately determine sexual maturity status. Although determination of sexual maturity in male mud crabs was commonly based on the presence of spermatophores in the vas deferens, preliminary studies on the mud crab Scylla spp. showed that this method is inconclusive and was reevaluated in this study. In addition, the feasibility of using male abdomen looseness as maturity indicator was explored. Spermatophores were present in the vas deferens of all Scylla spp. regardless of maturity status and body size. Spermatophores also significantly increased in size as the crabs reached sexual maturity. Examination of the vas deferens external morphology (e.g., color and size) consistently differentiated immature from mature males. The size at maturity (CW50) was successfully estimated using three maturity indicators, that is, the looseness of the abdomen, the observation of mating scars, and the vas deferens development. The estimated sizes obtained using all three indicators showed no significant differences, but varied significantly among species. A positive relationship was observed between body size and sexual maturity in all three Scylla species. Determination of sexual maturity using the presence of mating scars as indicator was inconsistent as some crabs that were mature in terms of looseness of abdomen and vas deferens development showed no presence of mating scars. On the other hand, male crabs with loose abdomens were found to be sexually matured with enlarged, milky white vas deferens. This finding suggests that abdomen looseness is an accurate sexual maturity indicator in males. Using looseness of abdomen as sexual maturity indicator in the male Scylla spp., therefore, is feasible and superior to conventional methods, with the advantages of being practical, easy to carry out, and does not require sacrificing of the crabs.
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