The oyster is a sensitive ecological indicator because of its predilection towards population-level catastrophic mortalities. In the Gulf of Mexico the sensitivity is produced by an important disease, Dermo, caused by a small endoparasitic protozoan, Perkinsus marinus. Besides the contaminant analyses, a series of ancillary biological measurements have been conducted. These record long-term changes in Dermo disease intensity and other measures of population health. Dermo disease responds to temperature and salinity, intensifying during periods of high temperature and high salinity.
Mortalities generally occur in the summer months.
Oysters have only a limited ability to control this disease through their own immune system, so they depend on climatic conditions unfavorable for the parasite to control the disease. During periods of low rainfall, catastrophic mortalities can occur as Dermo intensifies with increasing salinity. In the Gulf of Mexico, these cycles of intensification are determined by climatic cycles that raise salinity. One of these resources is the system of oyster reefs that collectively account for over 10,000 hectares of bay substrate. Over the years, concerns have been raised about whether these reefs are declining due to environmental degradation and the activities of the commercial fishery.