Epibiotic community of the horseshoe crab Tachypleus gigas

Horseshoe crabs act as moving substrata for simple to complex communities o f small marine organisms. Amplexed adult pairs migrate for breeding once ev ery 2 weeks from deep waters towards nearshore waters during highest high t ide. Female horseshoe crabs bury themselves to the level of the lateral eye s to deposit eggs while the male crabs fertilize them. Subsequently eggs ar e buried by the female. Tachypleus gigas (Muller) is the most abundant hors eshoe crab species above available along the Orissa coast (India). Adults r each terminal anecdysis once sexually mature and live with their carapace f or 4 to 9 years. In spite of this, epibiosis is limited. In the current inv estigation, differences in the epibiotic community (diatoms and macro-epibi onts) present on horseshoe crabs, according to gender, were evaluated, and the macro-epibiont population from different regions of the carapace was ma pped. In general, female horseshoe crabs harbored fewer epibionts than the males. Among the diatoms, Navicula spp., Nitzschia spp. and Skeletonema sp. were dominant in both sexes. However, the abundance and diversity of diato ms was greater on the carapaces of male crabs. Among the macro-epibionts, t he acorn barnacle (Balanus amphitrite Darwin) and encrusting bryozoan (Memb ranipora sp.) were the most dominant forms. Barnacles and bryozoans were gr eater in abundance in the "rough" zone (cardiopthalmic region and anterior region of the opisthosoma). Mapping of the macro-epibionts from different r egions of the carapace revealed differential distribution in males and fema les. Such differentiated distribution of the macro-epibionts can be related to factors such as changing habitat by the horseshoe crabs during breeding , mechanical abrasion and surface availability during mating and nesting pe riods, requirements of epizootic larvae and surface properties of the carap ace (wettability and roughness). In the case of females, mechanical abrasio n and surface availability played an important role in the epibiotic commun ity structure and distribution patterns. The surface wettability measuremen ts indicated male carapace to be slightly more hydrophobic than the female carapace. Scanning electron microscopy revealed that the male carapace was comparatively rough compared to the smooth carapace of females. A compariso n of surface properties of the carapace indicated that the male carapace is more conducive for epibiosis.