SHELL UTILIZATION AND POPULATION ECOLOGY OF LAND HERMIT CRAB Coenobita rugosus AT CAPE PANWA, PHUKET PROVINCE

Abstract

The study on shell utilization and population ecology of the land hermit crab Coenobita rugosus at Cape Panwa, Phuket Province, Andaman Coast of Thailand were investigated by multiple quadrat sampling from April 2011 to March 2012. In natural habitat, C. rugosus was found using 63 molluscan shell species in different percentages. Among these, 62 gastropod shell species from 20 families were used by C. rugosus. Interestingly, the first record of the bivalve shell use by land hermit crab was noted and discussed. The diversity of shells used increased with increases in body size from small to medium sized crabs, but decreased in larger crabs. The most commonly occupied shell species by C. rugosus was Nerita albicilla (19.6 %). However, N. albicilla used by C. rugosus was not the lightest shell species in relation to the ratio between internal volume and weight, which is in contrast to the energy saving hypothesis. Globose shell and ovate aperture shape were the most occupied shell and aperture shapes, respectively. The shell utilization patterns of C. rugosus at the study site were different between sexes and among reproductive stages. Furthermore, strong correlations between internal volume and aperture size of occupied shells and hermit crab characters suggest that the shell internal volume and size of aperture are the main determinants for shell utilization of C. rugosus. As for shell preference experiment in laboratory, both sexes of C. rugosus obviously preferred shells of Thais hippocastanum (84% of males and 92% of females). Both males and females of C. rugosus showed 100% of dissatisfaction rate with previously occupied shells of N. albicilla. The preferred shells had significantly larger internal volume and higher ratio between internal volume and weight than the original shells (p < 0.001), but the weight of preferred shells was significantly higher than that of original shells (p < 0.001). This may suggest that crabs in natural habitat probably try to search and occupy the lighter shells in each shell exchange for their appropriate adequacy. Regarding to population ecology of C. rugosus, tendency of unimodality of annual size frequency distributions was observed for males, non-ovigerous females, and ovigerous females. Major chela length was determined quantitatively as a secondary sexual character (larger in males) for the first time in terrestrial hermit crabs. The average density of C. rugosus during the study period was 6.98 ± 0.36 crabs/m2. Dispersion of males and non-ovigerous females of C. rugosus was clumped, whereas ovigerous females were distributed uniformly in most sampling months. The overall sex ratio was male-biased (1:0.86 male:female). Nevertheless, the monthly and size class sex ratios were close to the expected 1:1 ratio. Individuals of C. rugosus were observed to consume 16 plant species, mostly decomposing leaves and flowers, but three species of animal carcasses including one case of cannibalism were noted. Reproduction of C. rugosus occurred throughout the year with the highest percentage of ovigerous females in April and September (31.2% and 31.6%, respectively). Therefore, the information from this study provides comparative knowledge on shell use and population ecology of C. rugosus and other coenobitid crabs and can be used as fundamental knowledge for the conservation of these animals.