Eastern Oyster

Crassostrea virginica

Eastern Oysters are found throughout the Gulf of Mexico and up the eastern seaboard. Oysters are typically exposed to wide variation in environmental parameters (salinity, temperature, dissolved oxygen, etc.) in their estuarine habitat. The Gulf of Mexico supports the largest commercial harvest of oysters in the United States (Berrigan et al. 1991). Oysters form complex 3-dimensional reefs that support a high diversity of marine species. Oysters are also filter feeders; an individual oyster can filter 3 to 34 L per hour, feeding on phytoplankton and other suspended particles (Newell 1988). The reefs protect shorelines from waves and erosion by reducing wave heights and stabilizing unconsolidated sand or mud. It is estimated that 85% of the oyster reefs worldwide have been lost and it is thought that the same amount has been lost in the Caloosahatchee and San Carlos Bay due to direct use of oyster reefs as road beds and because of interannual variability in flows and salinity (Beck et al. 2011, Kirby 2004). 

Oysters are a biological indicator of the condition of the estuary. They are tolerant of a broad range in salinities typical near the mouth of the Caloosahatchee (Shell Point). While they can tolerate periods of low salinity by closing up and ceasing to feed, they are more vulnerable to long periods of high salinity (above 25) (Shumway 1996). Oysters are also more likely to succumb to disease (Perkensis marina) and predation by gastropods such as crown conchs (Melongena corona) and stone crabs (Mennippe mercinaria) when they become stressed at higher salinities (Ewart and Ford 1993, Garland and Kimbro 2015).


Oysters are ideal for use as indicators due to their attached, filter-feeding life mode. Oysters are tolerant of a wide range of salinities and typically occur at salinities ranging from 10-30 (Gunter 1955). Low salinities (0) can be tolerated for short periods of time (Loosanoff 1965) with optimum adult growth occurring from 14-30. Higher water temperatures generally result in reduced tolerance to salinity (Berrigan et al. 1991).

Oyster High Salinity

At salinities above 30, predation and disease are thought to be significant causes of oyster mortality. Although predation and disease are patchy in the environment the probability of higher mortality of oysters is expected under prolonged high salinity periods. 

Oyster Low Salinities

Low salinities are generally considered not lethal for adult oysters, however, during these periods when salinity is below 10, oysters will stop filter feeding (Pollack et al. 2011). Oysters have the ability to slow down their metabolism until conditions are favorable. At locations such as Shell Point, salinities at high tide are optimal while salinities at low tide are not. Prolonged low salinity periods (weeks to months) can be detrimental to oyster reefs and could result in reef-wide mortality (Volety et al. 2008). 

Adult oysters exist in the range of -2 degrees C in New England to 36 degrees C in the Gulf of Mexico. Oysters in the lower estuary are mostly found in the intertidal, where temperatures above 49 degrees C have been measured. However, normal growth occurs at temperatures ranging from 10-30 degrees C (50-86 degrees F). Gulf of Mexico oysters spawn around 25 degrees C (77 degrees F). 

Oysters are filter feeders that rely on phytoplankton production in the estuary to be delivered to reefs by tidal currents. Chlorophyll a is a measurement of biomass of phytoplankton in the water column. Oysters will stop feeding at low tide if they are exposed to air as the tide goes out. Since most oyster reefs in Tarpon Bay and San Carlos Bay are intertidal, chlorophyll a and depth are important factors in evaluating oyster growth and reef health. Often, higher chl a can be observed at low tide with land-based nutrient runoff promoting phytoplankton growth.