Population structure and allometric patterns in mangrove horseshoe crab, Carcinoscorpius rotundicauda from the coastal area of Cox’s Bazar, Bangladesh

A study on population structure, morphological variation and allometric pattern of the mangrove horseshoe crab (Carcinoscorpius rotundicauda) was carried out in four sites (Dorianagar, Khurushkul, Moheshkhali and Rejukhal) in Cox’s Bazar, Bangladesh. Two methods (walk-through and gill netting) were used to collect horseshoe crabs. A total of 202 individuals were sampled, with a male-dominated sex ratio of 1.69:1. The results indicated significant differences in body weight (BW), prosomal width (PW), carapace length (CL), total length (TL), and telson length (TEL) among the sites and between sexes. Females exhibited larger morphometric measurements than males across all locations except Rejukhal, having the highest mean body weight in males (228.2 ± 29.2 g) and females (227.3 ± 44.5 g) of Dorianagar. Males of Rejukhal showed a lower mean body weight (165.42 ± 14.92 g) compared to females (154.20 ± 9.91 g), indicating some variability in trends. Size distribution analysis revealed that most individuals fell within the 13.0–14.9 cm PW range, in case of Khurushkul showing the highest density. Correlation analysis demonstrated significant positive relationships among key morphological traits, particularly between BW, PW, CL, and TL. The Principal Component Analysis (PCA) identified BW, PW, and TL as the most influential traits distinguishing between sexes, with PC1 explaining 73.8 % and 69.1 % of the variation in males and females, respectively. The Discriminant Function Analysis (DFA) further confirmed BW as a key trait for sex differentiation, achieving a 78.7 % accuracy rate in classification. Allometric growth analysis revealed negative allometry (b < 3) in the TL-BW, PW-BW, and CL-BW relationships in males and females across sites, with notable exceptions in males of Khurushkul (PW-BW) and females of Dorianagar (CL-BW) displayed isometric growth (b = 3). However, TL-TEL relationship across all sites showed positive allometry (b > 1) except that of Rejukhal where both males and females showed isometric growth (b = 1). These findings are anticipated to help determine the size variations among the individuals of C. rotundicauda in different sites of Cox’s Bazar and serve as a foundation for future research to compare different populations.