STUDY OF SIPHON ACTIVITY IN MYA ARENARIA (L.) IN THE INTERTIDAL ZONE BY MEANS OF AN UNDERWATER VIDEO CAMERA

The siphon activity of Mya arenaria (L.) was filmed at two sites in he intertidal zone characterised by different current speeds. In each ex periment, the behaviour of individuals was studied for two tidal cycle s. Individual behaviour was compared in relation to the sites and to t he orientation of the clams relative to the current. The ability of in dividual clams buried at various depths to extend their siphons above the sediment surface was also tested. The clams kept their siphons ope n without any apparent movement 84.5% of the total submersion time. Th e most frequent movement was simultaneous closing followed, with an ap proximate delay of 10 s, by the simultaneous opening of the two siphon s. These movements are part of a main behavioural sequence that accoun ted for 87.8% of the observed sequences between the 'two siphons open' state. The production of pseudofaeces was the only behaviour correlat ed with stomach content. The clams did not extend their siphons above the sediments. There was no effect of clam orientation on either their behaviour or stomach content. The two groups of individuals observed at the weak current site seemed to have a similar feeding activity, un like the two other groups located where the current velocity was stron ger. The first of these latter groups had a weaker phaeopigment conten t in their stomachs, while the second group had a high frequency of si multaneous closings of the two siphons and a similar stomach content c ompared with the individuals at the weaker current site. However, we c annot state that siphon closure enhances the feeding success of indivi duals or is linked with the flow regime because variations in the beha viour between experiment dates suggest that there is another factor th at may intervene. These inter-date variations could probably be explai ned by differences in the seston availability between tidal cycles. (C ) 1998 Elsevier Science B.V.