Settlement, post-larval growth and gonadal development of Chlamys glabra L. in an off-shore site of the Northern Adriatic Sea (Italy).

A study on the settlement of Pectinidae, mainly of Chlamys glabra, on artificial collectors (“onion-bags”) was carried on in a mussel culture plant, one mile off-shore in the Northern Adriatic Sea, northwards the lagoon of Venice. Samplings were collected every three months, from August 2005 since August 2006. Each sampling consisted of twenty bags suspended at different depths (5m, 7m, 9m, 11m, 13m). The peak of settlement of Pectinidae was in summer at -13m (average 119 individuals m-2 day-1). Very small unidentified Pectinidae were the most abundant component, mainly close to sea bottom; Chlamys varia, C. glabra and Chlamys proteus settled almost uniformly at any depth. The summer-autumn settlement was mostly represented by C. glabra and C. proteus, that preferred to settle at -11m and -13m (average 51 and 87 ind m-2 day-1, respectively). During winter only little quantities of very small unidentified Pectinidae settled. A. opercularis was collected, even if not very abundant (<15 ind m-2 day-1), in the spring sampling; it preferred collectors toward the bottom, particularly those at -11m (average 14 ind m-2 day-1). At any seasons, the multivariate statistical analysis confirmed the preference of different identified species of Pectinidae for settling near the sea bottom. From November to April, in order to assess post-larval growth, bimonthly measures of the length of some specimens of C. glabra, collected in the same site during the previous summer and kept in experimental holders, were carried on. The growth was slow between November and February, showing a mean increase of 2mm between November and December and only 1mm between December and February. In April, the mean size increased up to 2.4cm (maximum 3.6cm). The gonadal development of C. glabra was followed from October to April in specimens from the same holders. The gonadal maturity was reached after about eight months, at length of 4 cm. The scallops did not show presence of any parasites. In the experimental site, C. glabra seemed to be suitable for farming; this practice should reduce the fishing effort, that is currently hazarding toward depletion of the natural beds of scallops, linked to impact on the benthic communities in the Northern Adriatic sea.