Abra segmentum (Mollusca: Pelecypoda) of the Lesina Lagoon (southern Adriatic coast, Italy): Observations on variations in the population in relation to the main environmental parameters.

1 - The aim of this work was to evaluate the effects of the main chemical and physical environmental parameters on the dynamics of the population of A. segmentum living in the Lesina Lagoon, as a part of a wider study aimed at characterizing all the phases of the species' reproduction cycle in relation to the specific environmental conditions of the Lagoon. 2 - Sediment samples were collected at monthly intervals by using an Ekman scoop from May 2003 to May 2005; at each sampling site temperature, salinity and pH were recorded by multiparametric probe; in the laboratory the A. segmentum specimens were sorted by a 1 mm mesh sieve and the shell length was measured. 3 - During the two years of samplings, density levels tended to increase, a trend that was inversely correlated with the recorded decrease in salinity (Pearson's r=-0.66). 4 - Differences in the size class patterns were also found: in the 2003-04 samplings, the = 13mm individuals represented the 20%[divided by]50% of the total population, while juveniles disappeared on July 2003 and clearly reappeared only on February 2004. In 2004-05 samplings, juveniles were always recovered, while the oldest specimens, although increased in number, were never higher than 10% of the total population. 5 - The decreasing trend of salinity is supposed to be most important factor involved in the observed increase in population density, and in the consequent differences in the size class patterns.