Population ecology and production of Bittium latreillii (Gastropoda, Cerithidae) in a Posidonia oceanica seagrass bed

Population structure, growth and production of the gastropod Bittium latreillii (Gastropoda, Cerithiidae), were studied in a Posidonia oceanica seagrass meadow off the isle of Ischia (Gulf of Naples, Italy). Sampling was performed ten times at five depths (1, 3, 10, 15, 25 m), during the one year of study. A total of 2780 specimens was counted and measured. The survivorship curve, obtained by plotting the values of abundance of each age class, showed a mortality rate that was low immediately after settlement but markedly increased in individuals exceeding 10 mm shell length. The demographic analysis suggested a life-cycle of about 18 months and a semelparous reproduction. Three cohorts were identified: the first settled in October, a second in late winter (December-February), and a third in June. Winter cohorts showed a very low growth rate until spring when an increase was recorded. Annual secondary production (estimated by the Hynes-Coleman method) ranged between 67.9 (10 m depth) and 733.1 mg m(-2) yr(-1) (25 m), while the P/B value was between 3.5 (15 m) and 5.3 (25 m). Recruitment occurred exclusively at the deepest stands (15-25 m) where the population was characterised by higher values of abundance, biomass, and production, confirming that B. latreillii is a typical species of the deepest stands of P. oceanica seagrass beds.