Population biology of the cephalaspidean mollusc Haminoea orbygniana in a temperate coastal lagoon (Ria Formosa, Portugal)

The population ecology of Haminoea orbygniana (Férussac, 1822) has been investigated in the Ria Formosa, a temperate coastal lagoon in southern Portugal by monthly sampling between October 2000 and October 2001. The mean annual density was 341 ind m−2 with highest abundance occurring during winter. An aggregated spatial distribution was observed (I = 76.4). The species showed continuous reproduction throughout the year with the main breeding period in April. Two recruitment periods were detected, a subsidiary one in June and a major one in September. Growth is continuous throughout life. Growth patterns and life span were deeply influenced by season and recruitment time. The cohort recruited in June, had a higher growth rate from summer until middle autumn (between July and November), decreasing in the autumn and winter (November-March), and rising again in spring (from March to May). The cohort recruited in September had a higher growth rate over autumn and winter (from October to February), a decrease from late winter to spring (February-May), followed by a new growth rate from middle spring onwards (May/June-September). Life spans were 11 months and 19 months for cohorts recruited in June and September, respectively. Biomass was higher during winter and spring, and secondary production between late winter and early summer.