Growth and production of raft-cultured Mytilus edulis L., in Ria de Aveiro: gonad symbiotic infestation

Abstract

Between 1980 and 1987, 4374 mussels ( Mytilus edulis), from three sites in the two main channels of Ria de Aveiro, a littoral lagoon at the mouth of the river Vouga, western Portugal, were studied in order to detect the presence of symbionts at gonadal level. Following De Bary's (1897) classification, symbionts (in our case Ancistrum mytili, Conchophthirus mytili, Urastoma cyprinae, Proctoeces maculatus, Prosorhynchus crucibulum and Modiolicola bifidus) are grouped under phoresis, commensalism, parasitism and mutualism and are usually found in the mussel mantle cavity. Apart from M. bifidus, all of them had earlier been described as parasites of M. edulis L. In this study only three symbionts ( A. mytili, U. cyprinae and P. maculatus) showed pathogenic characteristics, the action of the others remaining practically unknown. Prevalence of infestation was extremely high from September 1982 to February 1987 for A. mytili and U. cyprinae, whereas mussels revealed high infestation from P. maculatus from September 1982 to November 1983. During the period of 1982–1983, infestation by these three organisms reached a maximum in April/May 1983 which coincided with a peak of daily salinity amplitude. All three organisms showed rhythmic cycles of infestation which were in phase for A. mytili and U. cyprinae but out-of-phase in P. maculatus. This suggests competition for this last species. At CN, the southernmost site, in the largest stream channel, where the study lasted for a longer period of time, infestation appeared to suggest a global and longer cycle, presumably 8–10 years. Parasitism by P. maculatus is referred to by other authors as being lethal. In Ria de Aveiro, however, the condition appears to be restricted to gonadal development, rather than the cause of heavy mortality, probably due to host adaptation of the parasite. Parasitism by this organism with its castrating effect was sex-dependent, affecting especially male hosts. On the other hand, A. mytili showed particularly destructive infestation in female gonad hosts. Defense reactions, by encapsulation, were observed in M. edulis infested by P. maculatus and P. crucibulum. These observations suggest that under adverse environmental conditions such as intensive culture, high infestation levels may occur with negative effects on the cultured populations.