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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]