Epibiont bryozoans (Bryozoa, Ctenostomatida) of the crab Goneplax rhomboides (Brachyura, Goneplacidae) off the Ebro delta (western Mediterranean)

Abstract

The occurrence of the bryozoan Triticella flava Dalyell, 1848 and Alcyonidium mamillatum Alder, 1857 was detected on the carapaces of the crab Goneplax rhomboides (Linnaeus, 1758) on the continental shelf off the Ebro delta (western Mediterranean). The main characteristics of the morphology of both species are described and illustrated. From the occurrence data of both bryozoans on individuals of the host species, their prevalence is studied as a function of sex, size and maturity stage of the host, as well as their seasonal and bathymetric variability. The host population studied thrives around the river Ebro delta and was sampled using epibenthic trawls. T. flava preferentially colonized the coxae of the pereiopods and the posterior dorsal margin of the carapace, whereas A. mamillatum basically colonized the smooth dorsal surfaces of the carapace and chelipeds. A morphometrical analysis of the individuals of T. flava was performed, showing that those individuals settled on more protected areas of the carapace had longer peduncles than those settled on more exposed areas. The prevalence of T. flava (36.3%) was much higher than that of A. mamillatum (0.7%). A significant increase in prevalence with size was found in males, but not in females, in agreement with the occurrence of different moulting patterns between the sexes. The prevalence was much higher in individuals parasitised by a sacculinid, which seerned to inhibit moulting. Moreover, the prevalence was also much higher in ovigerous than in non-ovigerous females. No significant variation in prevalence was detected with depth. The occurrence of epibionts was found to be fairly regular throughout the year.