Allometric Growth of the Carapace in Trapezia (Brachyura, Xanthidae)

Relative growth of the carapace was studied in three species of xanthid crabs of the genus Trapezia to determine if the species can be separated by their patterns of relative growth. Adult crabs were collected in Panama and Hawaii. Carapace width (cw), carapace length (cl), and the width between the orbits, or ocular width (ow), were measured to the nearest 0.1 mm. Allometric regressions of cw and ow were plotted with cl as the reference dimension, using both raw and log-transformed data. Log-transformed regressions were used to test for allometric growth, and ANCOVA was used to compare species. There were significant differences among the three species in the allometric regressions of cw and ow. Growth in cw was isometric, except that negative allometry was observed in males of Trapezia corallina. In all species females were significantly wider at a given cl than males. The difference in carapace shape between males and females is attributed to the enlargement of the abdomen to allow egg production in females. Growth in ow was negatively allometric in all species, except for females of Trapezia formosa, in which growth was isometric. The differences between sexes in ow were significant, but not in the same direction in all species. Negative allometry of ow may result from a requirement that the distances among the eyes, antennae, and/or mouthparts remain constant. When observed, significant allometry of cw and ow results from a non-zero y-intercept, rather than from a curvilinear relationship between variables, in the untransformed domain. That is, the rates of growth of cw and ow are constant relative to the growth rate of cl throughout adult life, but the ratios of these growth rates differ from carapace proportions at the onset of sexual maturity.