Relative Growth of the Land Hermit Crab, Coenobita Clypeatus (Anomura, Coenobitidae) from a Coral Reef Island, Southern Gulf of Mexico

Abstract

Information on growth in hermit crabs is very scarce, possibly due to the inherent difficulties of extracting them from their shells. In this work we document the relative growth, sexual size dimorphism, and level of chelae asymmetry of the land hermit crab Coenobita clypeatus (Fabricius, 1787) collected on a coral reef island in the National Park Arrecife Alacranes, southern Gulf of Mexico, during December 2009 and September 2010. Specimens examined were anaesthetized in a clove oil-water solution to avoid unnecessary crab mortality. A total of 387 individuals was measured on six external variables: cephalothorax length and width, and both chelae in length and width. Taking the cephalothorax length as the reference dimension, the remaining measurements were fitted to a power function to examine the degree of allometry in growth and make comparisons between sexes. For each sex, an asymmetry index (AI) was calculated to determine the degree of heterochely, and the significance of its value was evaluated by means of null models. Numerically, males dominated the crab population in the largest size classes (>12.6 mm CL) in both seasons. As in most anomurans, C. clypeatus males attained larger body sizes than females, but their relative growth was most similar. For both sexes, the growth of chela width was positively allometric (t-test, p < 0.05). Also, AI values showed a heterocheleous condition, with the left chela significantly higher (p < 0.001) than the right. Regarding our own results and previous reports, we propose that heterochely could be triggered during the glaucothoal and juvenile stages. Numerical dominance of males in the larger size classes is probably due to the segregation of ovigerous females from the main crab group, searching suitable areas for egg deposition, but also to a better condition of males to carry heavier shells than females, which in their turn have to invest a considerable amount of energy in reproduction.