Abstract
Non-random mating, especially size-assortative mating, has been observed in animals, including brachyuran crabs. Studies on the mating behaviour and mate selection of mud crabs of the genus ScyllaDe Haan, 1833 (family Portunidae), however, is limited. We investigated the possibility of assortative mating by size in three sympatric species of Scylla in Malaysia, the effect of male sizes on mating success and the difference in the body size of the mating pairs. Larger males mated with larger females and smaller males with smaller females. Males in all guarding pairs (from field populations) and mating pairs (in captivity) were larger compared to their respective female partners. Mating success was not affected by the difference in male-size classes in the three species. The proportion of size difference between mating pairs in captivity was largest in S. olivacea (Herbst, 1796), followed by S. tranquebarica (Fabricius, 1798) and S. paramamosainEstampador, 1949, and increases with male-size class. Strong size-assortative mating was observed in all three species.