A study of distribution and abundance of Littorina SPP. on Maltese shores

Abstract

Two Littorina species are found in Malta, L. neritoides and L. punctata. Their distribution and abundance on local shores were investigated from May 185 to June '86. L. neritoides was found to reproduce from November to March, when the temperature varied between 13°C and 22°c. Degeneration of the male copulatory organ occurred when the reproductive period was over. L. punctata was found to reproduce when the temperature varied between 10°c and 26°C. The two species are therefore ecologically isolated. Recruitment of L. neritoides juveniles was recorded from September to March, with a peak in January. Juveniles then migrated upshore. Downshore migration of L. neritoides adults occurred in November. Post-reproductive mortality of some adults occurred, as well as loss of male individuals from the population between December and March. Different size classes were not evenly distributed on the shore, the larger individuals being found higher upshore. The sex ratio of the adult L. neritoides population was 1.39 males to 1 female. The sex ratio of the recruit population was 3.02 males to 1 female. Males occupied lower shore levels than females. Mean shell widths of females and males were 3.15 ± 0.01mm and 3.04 ± 0.01mm respectively. L. punctata were found to be relatively rare on Maltese shores. Mean shell widths of L. punctata and L. neritoides were 5.96 ± 0.17mm and 3.09 ± D.D1mm respectively. L. neritoides were found to be less active, being mostly so during part of the reproductive period. Unlike L. neritoides, L. punctata regained activity readily on submergence in sea-water. L. neritoides favoured moist rocks while L. punctata favoured rock pools and hence occupied lower shore levels. Both species were never found on rocks covered with algae. L. neritoides favoured exposed shores of upper and lower coralline limestone. Globigerina limestone shores of similar topology i.e. with pits and crevices, were also inhabited by this species. The more exposed the shore the further upshore the individuals occurred. L. punctata favoured beds of rocky channels and was found on globigerina limestone shores.