BIOMETRY, BATHYMETRIC DISTRIBUTION, AND REPRODUCTIVE-CYCLE OF THE HOLOTHUROID HOLOTHURIA-TUBULOSA (ECHINODERMATA) FROM MEDITERRANEAN SEAGRASS BEDS

Holothuria tubulosa individuals were sampled regularly at three different depths (6, 19, and 33 m) over a 20-month period in a Posidonia oceanica meadow off Ischia Island (Italy). Biometrical analysis demonstrated that holothuroids have a well-marked size-related distribution according to depth, with the smallest individuals in the shallowest part of the meadow and the large ones in its deepest part. The observed distributions reflect the ability of H.tubulosa to grow more efficiently in areas of low water turbulence where Posidonia shoots are scattered and detrital food is more easily accessible. Recruitment takes place in the shallow meadow, and holothuroids with high reproductive potential only occur in the deep meadow. Small holothuroids (shallow individuals of similar sizes) may be either juveniles or sexually mature individuals; environmental constraints apparently restrict the growth of individuals and their gonads. There is a progressive downward migration of growing individuals when the whole population is considered, although at the level of the individual the migration occurs randomly. Accordingly, the size of a sexually mature individual is more a function of its past history in the meadow than of its real age.