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Reproduction and Disease

Reproductive development

Purple drawing of the cross section of an oyster showing darker purple clusters around the outside edges. About half of the cross section is the inside portion and is in light purple.

Every month biologists collect live oysters from each site to analyze for reproductive analysis and disease monitoring. For reproductive analysis, the soft tissues from individual oysters are removed from the hard shells and are preserved in a fixative for several days. After preservation, the tissues are cut into a thin section that includes the mouth (labial palps), digestive gland and gonadal tissue and is transferred to our histology lab.  The histology lab makes even finer slices of the tissue (about 0.01 mm thick) and places these slices onto histology slides, then stains the tissue with dyes so we can see inside the cells and identify parasites. These slides are then examined under a microscope to assess patterns of reproductive development using a qualitative histological staging technique.

The Eastern Oyster uses water temperature as a cue for reproductive development and to a lesser extent also dependent on salinity and food (phytoplankton) in the water column. Salinities below 5 or 6 ppt can inhibit the development of gametes. During the summer and fall when water temperatures are warm, oysters exhibit their actively spawning reproductive stage. Oysters tend to be reproductively dormant during the winter months when  nutrients and energy are stored for recovering from the previous spawning season and gametes begin to develop for the next season

Dermo disease

Microscopic image with light and dark yellow in the background and multiple black dots covering almost the entire image.

Oysters are also monitored for the presence of the debilitating disease, Perkinsus marinus (dermo disease). P. marinus is a microscopic protozoan that infects oyster tissue and can be lethal to the oyster at high levels. While dissecting oyster samples each month, our biologists take small portions of the gill and mantle to estimate the severity of the dermo disease.  The tissue analysis that is placed in a test tube filled with culture media called Rays thioglycolate media and set to incubate in a dark area for 7-10 days. After the incubation period, the tissue is taken out and finely macerated on a slide. Once macerated, an iodine dye is used on the tissue samples which enables the dermo disease spores to be seen under a microscope.

Higher temperatures and salinities increases the likelihood of an oyster becoming infected with this disease. This disease can inhibit gametogenesis, cause mortality, and lowers physiological condition. While it is the most common parasite, it isn’t harmful to humans.  Short pulses of freshwater from heavy rains can benefit oyster populations by reducing predators and parasites, such as P. marinus, however, mass oyster die-offs can occur from excessive freshwater input. The dermo disease can also persist in low salinity areas once already established. Transmission of the dermo disease is direct from oyster to oyster. It could spread through the water column in warmer months in highly infectious areas. Typically, it is observed in its singe cell stage, called a trophont, and these cells can divide into the multicellular stage.