Mediterranean mussels Mytilus galloprovincialis were experi~nentally cultured from 5 rafts located in 4 Galician Rias, following the established industrial procedure. Cultures were sampled monthly until mussels exceeded market slze. Observation of histological sections of sampled mussels by light microscopy demonstrated symbionts which could be classified into 3 groups according to their pathogenicity. The first group consisted of symbionts with unnoticeable pathogenic effects including: prokaryotic inclusion bodies (PlB) in digestive gland and gills, an unidentified protistan in digestive primary ducts, a kidney coccidian, intracytoplasmic ciliates in digestive tubules, gill ciliates and a turbellarian in the ~ntestinal lumen. The second group comprised syn~bionts that could damage the host, although unlikely to be lethal, including: the microsporidian Steinhausia mytilovum, the flatworm Urastoma cyprinae, and the copepod Mytilicola intestinahs. The third group included the protistan Marteilia refringens and the trematode Proctoeces rnaculatus, potentially lethal pathogens. In addition, mussels with haemocytic infiltration of tissues and granulocytomas and a few cases of disseminated neoplasia were detected. The qualitative composition of mussel symbiont community was similar at the 5 study sites, except for 3 symbionts which were not detected at some sites Quantitatively, symbiont loads were higher and histological signs of stress more abundant in Moana and Vilagarcia (the most inner sites in the Rias), intermediate in Illa de Arousa and Muros, and lower in Lorbe. Symbionts increased in prevalence as mussels grew. Some of the symbionts were detected in mussel seed at the beginning of the experimental cultures.

The histological location, morphological fedtures and ultrastructural characters of the sporulation of the parasite causing Marteiliasis affecting cultured mussels in Galicia fit the characteristics defined for the type species Marteilia relringens The early stages of the infection are confined to the host stomach epithelium. The spreading of the parasites through the host digestive diverticula involves a n exponential increase in the number of parasites as the infection progresses, suggesting the occurrence of merogony, in addition to the well known sporulation. The infection usually triggers a host hemocytic reaction that may slow down or even stop the infection at times. The study of 5 zones of culture disclosed great spatial variability in the prevalence of Marteiliasis on mussels throughout the bays of Galicia, including a n area in which no infection was detected. The culture zones located within the bays showed higher prevalences than those located outside. There was no significant difference in the intensity of the infections among the affected zones Sporulation occurred throughout the year Several peaks of both prevalence and occurrence of heavy ~nfections took place every year. Considering the areas affected by Marteiliasis, the higher the prevalence of the infect~on the higher the mortality. In some of the studied zones, mortality was significant for the mussel farming industry.

Infection by the protistan Marteilia refringens affects the digestive glands of cultured mussels in Galicia, Spain. Effects of the infection on the reproduction of the mussel were studied at 3 sites. Gonad cycle of mussels involved gametogenesis utilizing stored reserves and leading to spring spawning (conservative strategy). This was followed by various sequences of gonad restoration and spawning utilizing the energy from concurrent feeding directly (opportunistic strategy). M. refringens significantly inhibited gonad development in the periods of opportunistic reproductive strategy, proportionally to infection intensity. However, the inhib~tory effect was not significant in the period of conservatlve strategy, probably because mussels had developed storage tissue before the infection progressed. Development of storage tissue was estimated by the abundance of adipogranular (ADG) cells in the mantle. Infection by M. refringens was clearly associated with inhibition of ADG cell development. The heavier the infection, the lower the abundance of ADG cells. Differences in abundance of ADG cells among categories of mussels with different infection intensity were significant at every site. No significant difference in susceptibility to progression of infection was detected between males and females and a similar mortality rate between sexes can be assumed. Evidence suggests that stress following spawning could enhance the progression of the infection.

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