Life history traits of the bivalve Spisula subtruncata (da Costa) in the Ligurian Sea (northwestern Mediterranean): the contribution of the temporary meiofauna

A population of Spisula subtruncata was studied in four different years (1983-1984; 1990-1993) on a shallow sandy bottom in the Ligurian Sea. Throughout the studied period, the S. subtruncata population was characterised by wide seasonal and interannual fluctuations both in abundance and biomass. Its life-cycle was normally annual, and there was an almost complete disappearance of the bivalve during winter months. The population of Spisula subtruncata was characterised by the dominance of juvenile forms and very few adults (maximum total length observed, 13 mm). Population size varied according to recruitment success. In the 1 mm samples, Spisula was particularly abundant within the community during the spring-summer period of 1991, with peaks of density and biomass of 1017 individuals m-2 (± 30.9 std) and 114.5 mg AFDW m-2 in May. From January 1991 to February 1993 the juvenile density of the bivalve was measured in meiofaunal samples. Juveniles were characterised by wide seasonal fluctuations, with density peaks in May (10,350 individuals m-2 ± 248 and 11,403 individuals m-2 ± 378, in 1991 and 1992, respectively). A mortality rate higher than 90% was estimated (measured as the difference between the density peak of juveniles and the density of the following peak of adults). Production was 0.58 (1983-84), 0.44 (1991-92) and 0.12 g m-2 yr-1 (1992-93). Considering also the contribution of temporary meiofauna, secondary production raised up to 0.52 in 1991-1992; and to 0.18 g m-2 yr-1 in 1992-93. The year 1990 was characterised by very low density so that no production value was estimated. A number of features are discussed: the variability observed in the demographic structure of the bivalve with the latitudinal gradient; the relation between population structure and production values; and the contribution of the smallest size classes to energy flow.