ON THE DRAMATIC INCREASE OF DITRUPA-ARIETINA MULLER,O.F. (ANNELIDA, POLYCHAETA) ALONG BOTH THE FRENCH AND THE SPANISH CATALAN COASTS

The distribution and population structure of the serpulid polychaete D itrupa arietina were assessed along the section of the Mediterranean c oast between Barcelona and Montpellier. The study combined: (1) the an alysis of the existing historical data referring to the period before the 1970s; (2) the results of impact assessment surveys carried out in the late 1980s and the 1990s; and (3) the results of two surveys carr ied out in the Bay of Blanes and along the coast between Cape Cerbere and Port-la-Nouvelle during 1996. The results of the surveys carried o ut in the 1990s show the occurrence of high densities (i.e. >1000 ind m(-2)) of Ditrupa arietina at all the sites sampled. These results can be compared with the few reports of this species before 1970, leading to the conclusion that D. arietina has recently increased all along t he Catalan coast. Ditrupa arietina is preferentially found between 20 and 30 m depth, which mostly corresponds to well-sorted fine sands and muddy sands. Both the results of population monitoring and the hetero geneity of population structure at small spatial scale show that the d ynamics of this species is unstable. The observed distribution pattern is therefore interpreted as resulting from a response to both wide-sc ale environmental parameters (accounting for the dramatic increase of the species at a wide geographical scale), and to local environmental factors (accounting for small-scale heterogeneity in population struct ure). Analysis of the relationship between sedimentary data and densit ies of D. arietina suggests that this species is highly sensitive to t he presence of fine sediments. However, because of the regional import ance of the phenomenon, it is concluded that the dramatic increase of D. arietina along both the Spanish and the French Catalan coast does n ot result from sediment instability. (C) 1998 Academic Press.