Bruce J. Barber

Additional Evidence that Juvenile Oyster Disease Is Caused by a Member of the Roseobacter Group and Colonization of Nonaffected Animals by Stappia stellulata-Like Strains

ABSTRACT Juvenile oyster disease (JOD) causes significant annual mortalities of hatchery-produced Eastern oysters, Crassostrea virginica, cultured in the Northeast. We have reported that a novel species of the α-proteobacteria Roseobacter group (designated CVSP) was numerically dominant in JOD-affected animals sampled during the 1997 epizootic on the Damariscotta River, Maine. In this study we report the isolation of CVSP bacteria from JOD-affected oysters during three separate epizootics in 1998. These bacteria were not detected in nonaffected oysters at the enzootic site, nor in animals raised at a JOD-free site. Animals raised at the JOD enzootic site that were unaffected by JOD were stably and persistently colonized by Stappia stellulata-like strains. These isolates (designated M1) inhibited the growth of CVSP bacteria in a disk-diffusion assay and thus may have prevented colonization of these animals by CVSP bacteria in situ. Laboratory-maintained C. virginica injected with CVSP bacteria experienced statistically significant elevated mortalities compared to controls, and CVSP bacteria were recovered from these animals during the mortality events. Together, these results provide additional evidence that CVSP bacteria are the etiological agent of JOD. Further, there are no other descriptions of specific marine α-proteobacteria that have been successfully cultivated from a defined animal host. Thus, this system presents an opportunity to investigate both bacterial and host factors involved in the establishment of such associations and the role of the invertebrate host in the ecology of these marine α-proteobacteria.

Growth and survival of selected lines of eastern oysters, Crassostrea virginica (Gmelin 1791) affected by juvenile oyster disease

Abstract Juvenile oyster disease (JOD) has had severe impacts on eastern oyster, Crassostrea virginica, culture in the northeastern United States in recent years. JOD mortalities in Maine have resulted in juvenile crop losses exceeding 90%. Previous research has demonstrated an inverse relationship between oyster size and JOD-induced mortality; thus the use of genetically selected broodstock along with specific management options may help manage JOD-related mortality. The goals of this project were to determine the size specificity and temporal and spatial variability of JOD outbreaks with respect to several lines selected for fast growth. Two lines (including within line unselected controls) having undergone two generations of selection for faster growth, along with an unselected wild line, were deployed during the 1994–1995 growing seasons at two sites in the Damariscotta River, ME, an estuary historically impacted by JOD. Periodic monitoring of size (shell height and live weight) and mortality of replicate cohorts was conducted to track oyster growth and incidence of JOD-induced mortalities. Due to the asynchrony of spat deployment between the cohorts, differences among lines were not feasible to evaluate, but within two lines (Flowers G 2 and Milford G 2 ), selected sublines both outgrew the within line controls and had lower rates of JOD-induced mortality. The reduced mortality in the selected sublines presumably resulted from their faster growth and hardiness thus enabling the cohort to reach a refuge size/age sooner and reduce its susceptibility to JOD.

Growth and mortality of cultured bay scallops in the Damariscotta River, Maine (USA)

Growth and mortality of hatchery-reared bay scallops, Argopecten irradians, were determined over an 18 month period (July 1994-December 1995) at two locations in the Damariscotta River, Maine, north of its natural distribution. Growth occurred primarily between May and October when mean water temperature exceeded 10 °C. Final mean shell height (56.3 mm) was significantly greater (p ≤ 0.0005) at the upriver site than at the downriver site in association with a higher mean water temperature. Cumulative mortality (43-49%) over this period occurred primarily over the winter months, in conjunction with mean water temperatures below 5 °C. Although it is possible to rear bay scallops to commercial size in Maine, the short growing season, high winter mortality, and costs associated with over-wintering and fouling will likely deter commercial culture.