Renewal time in a population of Abra segmentum (Mollusca, Bivalvia): a case of marked r strategy.

1. Abra segmentum is a common and frequently abundant species in the Mediterranean coastal lagoons, playing a dominant role, both in numbers and biomass, in the infauna of these habitats and is an important food for a variety of species. 2 - The main objective of this work is to describe the renewal time of Abra segmentum in Lesina lagoon. The insurgence of hydrogen sulphide which exterminated the benthic macrofauna in an area of Lesina lagoon during July 2008 allowed us to accurately observe the re-colonization time of this bivalve. In order to achieve this aim, the population structure and spatial distribution of A. segmentum before and after the dystrophic crisis were investigated in the entire Lesina lagoon and, in detail, in the area affected by the crisis. 3 - 51 sampling units were collected in Lesina lagoon in May and October 2008, allowing to compare the situation before and after the dystrophic crisis. The length-frequency distribution showed the existence of two size groups in both May and October 2008. The individuals were counted, their individual length measured and their total wet biomass weighed. 4 - The total number of individuals decreased after the crisis in the entire lagoon, but this drop was strongly observed in the perturbed area. The length-frequency distribution showed the existence of two size groups in both May and October 2008. After dystrophic crisis, the two size groups were clearly separated and juveniles group (2-5 mm) was located in the area interested by dystrophic event. 5 - The results obtained in this study highlight the rapidity of re-colonisation of A. segmentum after the strong disturbance caused by the insurgence of dystrophic crisis. The dynamics of the population was particularly clear when considering the area affected by the dystrophic crisis. In May the population was structured as in the rest of the lagoon, but in October there was a marked difference. The insurgence of a dystrophic crisis in Lesina lagoon allowed to clearly observe the resilience of A. segmentum to environmental stress, confirming it as an r strategy species.