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The population structure of Jujubinus striatus and Jujubinus exasperatus (Trochidae), common gastropod grazers of Posidonia oceanica leaves, was investigated in the seagrass bed off Lacco Ameno of Ischia (Gulf of Naples). Sampling was performed monthly, from July 1981 to June 1982, at five stations along a depth gradient (1, 3, 10, 15 and 25 m), using a hand-towed net. Throughout the study period, both populations were characterised by seasonal fluctuations in abundance, with a total of 550 individuals counted and measured. The two populations analysed showed different distributional trends along the depth gradient. Jujubinus striatus settled and occurred mainly at shallower stands (1-3 m depth), while J. exasperatus was found deeper (10-15 m). Both species are semelparous. Juveniles and small individuals (shell height < 4 mm) were mainly found during the late winter and early spring months. The survivorship curves of both Jujubinus species showed a very low mortality rate in the first few months after settlement. Demographic analysis suggests an indeterminate growth rate after sexual maturity, with a lifespan of about 22 months for J. striatus and 17 months for J. exasperatus. For both species the growth rate is fast during the first couple of months after settlement. The findings of this study, compared with earlier work, reveal a certain number of shared traits in the life cycle of the most common gastropod species living on the leaves of P. oceanica seagrass: semelparous reproduction, spring settlement, fast early growth and short lifespan (2 years maximum). [ABSTRACT FROM AUTHOR]

Population structures of the gastropodsGibbula umbilicarisandGibbula ardens(Trochidae), the two most common species of the genusGibbulaliving in shallow beds of the Mediterranean seagrassPosidonia oceanica, were investigated in the seagrass bed off Lacco Ameno of Ischia (Gulf of Naples), at six stations along a depth gradient (1, 3, 5, 10, 15 and 25 m). Sampling was performed on a monthly basis for 1 year. A total of 853 individuals were counted and measured. The two gastropod populations analysed show a rather similar distributional trend in all seasons and depths. Recruitment occurred at the shallowest stations (1–3 m depth), where the populations were characterised by higher values of abundance and biomass, confirming thatG. umbilicarisandG. ardensare typical species of the shallow community ofP. oceanicaseagrass beds. Juveniles were exclusively found in 1 month of the whole year:G. umbilicarisin April,G. ardensin July. The survivorship curves of bothGibbulaspecies, derived from the size-frequency distribution of shells, showed a very low mortality rate in the first months after settlement. Demographic analysis ofG. umbilicarissuggests a rather slow growth rate and a polymorphic maturation strategy with a lifespan of 1 year for most individuals, but lifespan may also reach about 2 years in a few specimens.Gibbula ardenshas a lifespan of about 1 year and a fast growth rate, especially during the first couple of months after settlement. Therefore, on the basis of lifespan and recruitment time, both species seem to be semelparous. [ABSTRACT FROM AUTHOR]