The infection of concern is termed Dermo disease, which is lethal to oysters. The disease is characterized by degradation of oyster tissues. The disease is caused by a single-celled animal (a protozoan) called Perkinsus marinus. The infected oyster becomes stressed, its tissues pale in color, its gamete production is retarded, its growth slows, it becomes emaciated, its mantle shrivels and pulls away from the shell.
With the new study a lack of oxygen was shown to reduce the oysters’ ability to fight off the parasite. To show this, researchers suspended hundreds of eastern oysters (Crassostrea virginica) in underwater cages just off Chesapeake Bay in the U.S. After four months, the oysters were sampled and subject to laboratory testing for Dermo infections. It was found that the infection rate ranged from 50 percent to 100 percent. The lower the amount of the available oxygen in the water, then the greater the likelihood that an oyster would become infected. The study also found that young oysters were more likely to become infected compared with older forms of the shellfish.
Oxygen depletion in shallow waters fluctuates, alternating according to daytime and night-time. The term given to this phenomenon is diel-cycling hypoxia. This variation occurs because during the daytime, algae photosynthesize and release oxygen into the water.
To express this oxygen level, the researchers devised a scale, which they called “DOOM” (an acronym for Dissolved Oxygen Oyster Mortality). In practice, levels of normal dissolved oxygen concentrations below 7 milligrams dissolved oxygen/liter was seen to be the point at which oxygen levels fell sufficiently low for an infection to occur.
The findings have been published in the journal PLOS ONE. The research paper is headed “Landscape-Level Variation in Disease Susceptibility Related to Shallow-Water Hypoxia.”
