GROWTH AND POPULATION-STRUCTURE OF PARACENTROTUS-LIVIDUS (ECHINODERMATA, ECHINOIDEA) IN 2 CONTRASTING HABITATS

Two populations of Paracentrotus Lividus (Lamarck) (Echinodermata: Ech inoidea) from the Northwestern Mediterranean were compared to ascertai n the plasticity and adaptive value of certain biological parameters. These populations were found in habitats which differed in terms of st ability of environmental parameters and availability of food. The dens ity was higher and more fluctuating in the unstable community. Sigmoid al growth functions were fitted from plate ring analysis, and reflecte d a higher growth rate in the stable community, resulting in higher me an diameters at equal age. The maximal growth rate was attained in the unstable community at an earlier age than in the stable one. Size-fre quency analyses reflected drastic month-to-month changes in population structure in the unstable community, in which the smaller size classe s were almost absent. In contrast, at the stable site the distribution was much more uniform through time, and featured a polymodal shape co nsistent with the development of several annual cohorts. The smaller s ize classes were the most abundant. It is suggested that different dyn amics underlie both populations, the one at the unstable site being dr iven primarily by episodic storms, which cause high mortalities but ca rry new individuals to the site. The stable community relies instead o n an annual settlement and features a lower and more predictable morta lity which allows for the development of a well-structured population.