Molluscan shellfish are prolific filter feeding organisms. They can filter great quantities of water through their gills each day and can accumulate constituents of the water column in high concentrations. Occasionally, algae concentrations in surrounding waters increase, which can result in the staining of shellfish gill tissues in unusual colors.
The first photograph above compares the viscera of two oysters. The one on the left displays the normal, cream colored gills commonly seen in oysters. The oyster on the right displays gills that have been stained a blue-green color due to increased levels of naturally occurring blue-green algae in its growing area.
The second picture, above right, is a close-up of the gill tissue stained by blue-green algae. This often occurs in both clams and oysters harvested in North Carolina during the late winter when blue-green algae concentrations often rise. As algae concentrations decline the gill staining will disappear.
This condition routinely results in consumer concerns but has no effect on the health, quality and taste of the shellfish nor does this condition adversely affect the health of consumers. It is interesting to note that in France, a country known for oyster production, oysters with algae-stained gill tissues are a highly-sought after product.