Population Genetics in Natural Bed Oysters in the Damariscotta Estuary

Project Description

MSX is a disease of the Eastern oyster (Crassostrea virginica) caused by the protozoan parasite Haplosporidium nelsoni. It has caused high mortalities and has a considerable economic impact to East Coast oyster fisheries. In 2010, an outbreak of MSX occurred in the commercial oyster operations of the Damariscotta River estuary, resulting in oyster mortalities of up to 90%. In 2011, these operations switched from using the traditional Maine strain of oysters that was known to be cold water tolerant, but susceptible to MSX, to a newer strain (NEH) that was developed to be more disease resistant. In 2012, researchers conducted an MSX prevalence survey of commercial and natural beds in the estuary that sampled two- to three-year-old animals.

This research showed significant presence of the parasite within both commercial and natural bed populations. It was believed that the introduction of the new strain within the commercial operations would not have impacted either the sampled commercial animals or the natural bed animals, as it was too soon for the new strain to have reached two years of age or to have populated the natural beds. A repeat prevalence survey was conducted in 2014 and showed a substantial fall in the prevalence of MSX within all populations. At this time, the commercial samples were NEH animals from the new strain, and it was possible that a significant number of natural bed oysters were also derived from the new strain.

Currently, the natural bed populations are suspected of being under substantial pressure. MSX will have been impacting the old Maine strain animals by reducing fecundity, resulting in mortality. The spat deriving from the commercial operations, suspected to be a primary input of genetics of the natural bed populations, are now originating from the newer NEH strain. This may have resulted in a significant shift in the population genetics away from the Maine strain, toward the NEH strain. However, little is known about the natural bed populations in the Damariscotta River estuary.

This project aims to investigate the genetics of natural bed oysters at a single location in the Damariscotta River estuary and compare observations with the genetics of cultivated stock. The first objective is to discover whether natural bed oyster populations have been impacted by the NEH strain. Secondly, researchers hope to explore the potential of these populations to be used as broodstock to enhance the current commercial stocks.

Results and Accomplishments

Project personnel have completed approximately 95% of genotypes identification using GeneMarker software, and are currently reconciling the data set to ensure that all data are of sufficient quality to generate the required microsatellite sequence size information. Some samples require reprocessing in order to ensure appropriate levels of replication. As a consequence, project personnel are finalizing the PCR and fragment sequencing to ensure the dataset is sufficiently complete to permit the required level of analysis. Personnel have partially completed editing of the fragment analysis data, but are waiting for completion of the PCR analysis and the fragment sequencing to fully complete this work.

Summary of Data Being Collected

Data	Type	Quantity	Location
Animals	Mass, Length	60 animals	Damariscotta
Tissue	Mass	180 animals	Damariscotta
DNA	Fluid	180 animals	Damariscotta
PCR	Gel data	180 animals	Damariscotta
Fragment analysis	Genotypes	180 animals	Damariscotta