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Determining factors affecting Dermo disease of oysters in Galveston Bay, Texas

Elizabeth H. Silvy, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843
Francis I. Gelwick, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX  77843
George J. Guillen, Environmental Institute of Houston, University of Houston-Clear Lake
Houston, Texas 77058

The Gulf Coast has seen a dramatic decline in oyster take in recent years.  Lack of fresh water flow and other environmental variables have been suggested to cause an increase Dermo disease of oysters, caused by the parasite, Perkinsus marinus, which attacks the tissue of the oyster and is responsible for oyster and reef kills along the Gulf Coast.  As such, a disease has its biggest detrimental effect when conditions create an abrupt increase in density for either the disease or host population.  Therefore, correlating the Dermo parasites distribution and prevalence in the eastern oyster (Crassostrea virginica) to water conditions could be beneficial to the eastern oyster throughout its range.  This study addressed the ecologically conditions, parasite distribution and prevalence within the oyster host populations in Galveston Bay, Texas.  Specific objectives were to determine in Galveston Bay the:  (1) prevalence of Demo in oysters, (2) distribution of Demo infected oysters, (3) concentrations of Perkinsus marinus within infected oysters, and effects of salt concentrations (i.e., fresh water flow) on prevalence of Demo in oysters.  Oysters were collected at 4 study sites (April Fool Reef, Fishers Reef, Frenchys Reef, and Confederate Reef) bimonthly during 2015.  Fishers Reef was a site that was close to the Trinity River and was most affected by fresh water flow.  At each site an oyster dredge was pulled behind a boat for 10 minutes in slow circles and repeated 3-8 times if necessary to collect 20 market-sized oysters.  Oysters were placed on ice until processed.  If a reef was accessible to wading, oysters were collected by hand.  At each site, water quality data (turbidity, depth, salinity, temperature, and wave height) were collected.  Once collected, oysters were taken to the University of Houston-Clear Lake for processing.  Oysters from each site were numbered from 1-20 and kept separated by site.  Each numbered oyster shell was measured with calipers to the nearest millimeter.  Data recoded included date of culture, bill condition of each oyster (i.e., sharp or dull).  Each oyster was shucked using an oyster knife and gloves.  The oyster meat was left in the cupped half shell and meat condition was recorded (i.e., shrunken [small, shrunken, dehydrated appearance] or plump [round, lush, creamy]).  After shucking, 0.05 ml of the Chloromycetin/Nystatin working mixture was added to each Dermo tube (Thio medium) and was mixed by inverting the tube.  For each oyster, a 5 mm2 piece of anterior mantle was placed into a separate Dermo tube and shaken until the tissue is in the fluid.  Tubes were labeled by reef and number of the oyster.  Tubes were stored at room temperature for a week and then the tissue was placed on a slide, masticated with a tweezers, and 1-2 drops of Lugols iodine solution was applied and blended in using the tweezers, then placed under a cover slip, and read under a microscope giving it a rating on the Mackin Dermo Intensity Scale.  Environmental data collected from the oyster reef surveys were used to correlate Dermo prevalence, distribution of Demo infected oysters, and concentrations of Perkinsus marinus within infected oysters with salt concentrations (i.e., fresh water flow).  Above normal precipitation during 2015 appears to have decreased Demo prevalence and distribution in oysters from historic levels in Galveston Bay.

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