Population structures of the gastropods Gibbula umbilicaris and Gibbula ardens (Trochidae), the two most common species of the genus Gibbula living in shallow beds of the Mediterranean seagrass Posidonia 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 25m). 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 (1-3m depth), where the populations were characterised by higher values of abundance and biomass, confirming that G. umbilicaris and G. ardens are typical species of the shallow community of P. oceanica seagrass beds. Juveniles were exclusively found in 1 month of the whole year: G. umbilicaris in April, G. ardens in July. The survivorship curves of both Gibbula species, derived from the size-frequency distribution of shells, showed a very low mortality rate in the first months after settlement. Demographic analysis of G. umbilicaris suggests 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 ardens has 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.
