Populations of the hermit crab Cestopagurus timidus (Decapoda: Paguridae) associated with the foliar stratum of two Posidonia oceanica meadows of Elba Island (Northern Tyrrhenian Sea – Italy)

Two populations of Cestopagurus timidus were analysed with regard to size, population structure, and occupied shell. The hermit crabs were collected in two differently exposed Posidonia oceanica meadows of the Elba Island: one northwards, the other southwards. The two meadows, growing on a similar bottom and bathymetric gradient, show a similar shoot density. Using the hand-towed net technique, in each site one sample and its replicate were collected at three different depths (-5, -15, -30 m) in three periods (March-April,May-June, and September- October). Almost all the collected hermit crabs were Cestopagurus timidus; remarkable differences have been ascertained between the populations of the two both meadows, regarding mean size and size classes, sex ratio, presence and abundance of ovigerous females, recruitment period, bathymetric distribution. The two populations of C. timidus occupy the same shell species, in approximately 86 % of the cases represented by Bittium latreillii Payraudeau, 1826. This mollusc is very abundant on the leaf stratum at the considered depths (mainly at -5 and -15 m) both with adults and juveniles. The crab occupies B. latreillii‘s shell at all the depths, mainly at -5 and -15 m; also the ovigerous females, especially small and medium-sized ones, inhabit the above shell; at -30 m the crabs live in an assemblage of shells typical of the deep ecotone in the two examined meadows. The crab size is significantly correlated with the height and weight of the occupied shell, demonstrating that the crab uses fit shells. The comparison between the obtained results and the data referred by otherAuthors shows that C. timidus is able to modify different aspects of its biology according to the occupied habitat and the resources it offers.