Size-assortative mate selection and molting increment of Scylla mud crabs in wild coastal mangrove habitats

Size-assortative, nonrandom mate selection and periodic molting for growth, development, and reproduction have been observed in animals, including brachyuran crabs. This study investigated the mate selection of Scylla olivacea from four wild habitats in the Bay of Bengal, Bangladesh, and the molting dynamics of Scylla species in coastal mangroves of Pattani Bay, Thailand. A total of 34 pairs of S. olivacea were collected from four habitats in the Meghna Estuary, where 29 pairs (85%) of males were larger than 5 pairs (15%) of females. A higher abundance of mating pairs was observed in the rainy season (higher temperature and lower salinity) than in the winter season (lower temperature and higher salinity). Significant differences between sexes were found in carapace width (CW), internal carapace width (ICW), carapace length (CL), and body weight (BW). Male size and BW were significantly correlated with female sex. Morphometric traits differed significantly across habitats and seasons. The most frequent CW size class was 8.5–9.0 cm for males (34.3%) and 7.5–8.0 cm for females (26.5%). During molting, S. olivacea and S. paramamosain presented increases by 1.28 ± 0.48 and 1.35 ± 0.18 cm (CW), 0.93 ± 0.33 and 0.83 ± 0.15 cm (CL), and 0.54 ± 0.19 and 0.45 ± 0.17 cm (BH), respectively. No significant differences in molting increment were detected between species or sexes. Premolt size was positively correlated with increment but negatively correlated with percentage increment. These findings offer valuable biological insights for the ecological management and aquaculture practices of Scylla mud crabs.