Length-weight relationships and mitochondrial COI variation of orange mud crab (Scylla olivacea) across four mangrove localities in Peninsular Malaysia

Understanding variation in body-size allometry and genetic diversity of the orange mud crab, Scylla olivacea, is important for effective fisheries management and conservation. This study evaluated length-weight relationships (LWRs) and mitochondrial cytochrome c oxidase subunit I (COI) genetic diversity among S. olivacea samples collected once from four mangrove ecosystems (four localities; 28 individuals from each locality) along the east and west coasts of Peninsular Malaysia. Carapace width and body weight data were analyzed to characterize locality-specific growth patterns, while mitochondrial (COI) sequences were used to assess the genetic diversity and population genetic structure. The allometric exponent (b) of the LWRs varied among sampling localities, ranging within 2.160-3.694, with coefficients of determination (R2) between 0.809 and 0.971. Analysis of 112 mitochondrial COI sequences revealed 58 haplotypes, with high haplotype diversity (Hd = 0.955-0.973) and relatively low nucleotide diversity (π = 0.009-0.011). Analysis of molecular variance indicated that most genetic variation was distributed within sampling localities (97.66%), while only 2.34% occurred among sampling locations. The low and non-significant fixation index (FST = 0.023, P = 0.079) suggested weak genetic differentiation based on the mitochondrial marker examined. These findings indicate that spatial variation in growth patterns occurs alongside limited mitochondrial differentiation among sampled localities; however, the observed phenotypic variation cannot be directly attributed to genetic differences without additional environmental and genomic analyses. The findings provide a descriptive baseline for the four sampled locality-dates.