Pathology and mass mortality of Pacific oysters, Crassostrea gigas (Thunberg), in 2005 at the East Frisian coast, Germany

Abstract

In 2005, Pacific oysters, Crassostrea gigas, were collected from May to September along the East Frisian coast and processed for histology. Because of mass mortalities in September, additional samples of moribund oysters and apparently healthy blue mussels, Mytilus edulis, were subjected to virological and ultrastructural investigation. The oysters displayed a variety of pathological conditions including viral gametocytic hypertrophy which is reported here for the first time from the German coast. Haemocyte aggregations in the digestive tract, in the intestinal mucosa and submucosa, in the mid-gut gland and in the ventricle of the heart were commonly observed at some stations. In association with mass mortalities, severe gill necrosis occurred which may have contributed to the high mortality rates. Total mortality rates of up to approximately 60% were seen. All size classes and thus age classes of oysters were affected, with highest mortality rates within the youngest age classes which had just reached sexual maturity (shell lengths <40 mm). The smallest dead oysters had shell lengths of 10 mm. The phenomenon was mainly restricted to C. gigas stocks in harbours, probably because of favourable conditions for infection, i.e. limited water exchange, less food availability, reduced oxygen content and higher pollution levels.

Invasion genetics of Pacific oyster Crassostrea gigas shaped by aquaculture stocking practices

“…Since the two invasions now potentially form a secondary contact zone, admixture of the gene pools and directional introgression could also influence the adaptive and invasive potential of Pacific oysters in the Wadden Sea. In this context it is highly interesting that Southern populations encountered strong selection by summer mortality with adult mortalities N 60% (Watermann et al, 2008) from which Northern populations have been spared so far. Northern populations did however suffer from temperature dependent winter mortality during the last years (Büttger et al, in press).…”

Do Temporal and Spatial Parameters or Lifestyle of the Pacific Oyster Crasssostrea gigas Affect Pollutant Bioaccumulation, Offspring Development, and Tolerance to Pollutants?

“…Since 1998, several years have been particularly unfavorable for oyster larval recruitment and several episodes of high mortality of oyster spat have been recorded in the Arcachon Bay. In many countries worldwide, the occurrence of significant mass mortalities in both adult and juvenile C. gigas populations has been reported, generally during the summer months (Mori, 1979;Perdue et al, 1981;Watermann et al, 2008;Cotter et al, 2010;Pernet et al, 2011). Although the mass mortality phenomenon encountered in recent years is often associated with infectious agents, it is also often related to environmental changes and highlights the delicate balance that governs an ecosystem.…”

Hemolymph microbiome of Pacific oysters in response to temperature, temperature stress and infection

“…The outcome in temperate regions can be much less predictable, depending on thermal optima and ranges of hosts and pathogens (Thomas and Blanford, 2003). For example, temperatures over 20 1C are necessary for oyster summer mortalities to occur (Samain et al, 2007;Watermann et al, 2008), but it is low temperatures (o14 1C) that promote development of brown ring disease in clams (Paillard et al, 2004).…”