The Effect of Oil and Its Dispersants on the Eastern Oyster, Crassostrea virginica

Part of the Young Naturalist Awards Curriculum Collection.

by Alan, Grade 10, Virginia - 2012 YNA Winner


The Deepwater Horizon oil spill of 2010 was devastating to the ecology of the Gulf. Not only did the oil significantly threaten the survival of most aquatic species in the area surrounding the spill, it also spurred potentially destructive relief efforts. These efforts included the use of dispersants with potentially unforeseen effects. Therefore, the experimenter’s objective was to examine the effects of oil and surfactants on the mortality rate of the Eastern oyster.

At the start of this experiment, I hypothesized that if samples of Crassostrea virginica were exposed to oil, they would have a higher mortality rate than oysters exposed to an oil-surfactant blend or to regular Chesapeake Bay water. In previous research by the Environmental Protection Agency and the U.S. Fish and Wildlife Service, it was found that oil affected marine invertebrates in a variety of ways, including asphyxiation, the alteration of metabolic rates, and changes in shell formation. To conduct the experiment, I began with 20 oysters in each 10-gallon tank and proceeded to add various chemical solutions in concentrations of 100 to 500 parts per million. At the end of every day, the 16 tanks were examined for their mortality rates. By the final day of the trial runs, approximately 4 oysters had died in each tank. After statistically examining the data, it was shown that there was no significant difference in mortality between the solutions; however, there was a significant difference in mortality between the oysters with an oil concentration of 500 parts per million and the control group.

From this data, it can be seen that neither the ZI-400 dispersant, the oil-dispersant blend nor the oil slick had any significant effect on the mortality of Crassostrea virginica. This data supported the results of previous experiments in which the Eastern oyster was shown to be extremely resilient to variations in its environment. Given recent discussions about offshore drilling, this data could prove to be valuable in the event of a Chesapeake Bay oil spill.


The purpose of this study was to examine the differences in oyster mortality created by exposure to crude oil and the dispersant ZI-400, the only one obtainable for experimentation. While multiple industrial dispersants have been tested by the EPA, the tests have been primarily conducted on shrimp and fish, with little to no studies of their effects on aquatic invertebrates. After the Deepwater Horizon oil spill in April 2010, the experimenter was intrigued as to how well the overharvested and otherwise weakened Eastern oyster (Crassostrea virginica) would fare under similar conditions.

In the EPA studies, it was shown that the majority of industrial dispersants, as well as crude oil, were moderately to extremely toxic to tested species. The tested species included the Mysid shrimp and the Atlantic silverside fish; however, neither of these species possesses a biological system like that of the Eastern oyster. The Eastern oyster is a bivalve mollusk originally common to most of the Chesapeake Bay; however, due to overharvesting and diseases, the population of Crassostrea virginica has been significantly reduced. In the digestive process of a filter feeder like the oyster, large quantities of water are drawn over the animal’s gills, where plankton and other particles are then transported to the animal’s mouth, followed by digestion and excretion.

It is generally believed that the Chesapeake Bay now contains less than one percent of its peak-level oyster population. At its previous population level, the Eastern oyster was able to completely filter the Chesapeake Bay estuary in three days. The oysters’ filtration removed excess nutrients, algae, plankton and other particulate matter from the water. If such a keystone species is removed from the environment, the overabundance of sedimentary material can lead to the eutrophication of the estuary, overproduction of algal and plant matter, and eventual dead zones after the increased turbidity and more frequent algal blooms deprive submerged aquatic vegetation and organisms of vital light and oxygen. An excellent case study of this would be the Chesapeake Bay, which has been polluted with nitrates and phosphates that fuel the growth of phytoplankton. The phytoplankton are consumed by increasing populations of bacteria, which then consume higher amounts of dissolved oxygen. Such conditions, along with overharvesting, have lead to the reduced filtration capacity of the Eastern oyster population.

With the removal of a keystone species such as the Eastern oyster, conditions in its environment tend to worsen. In a previous study I conducted, I discovered that the Japanese oyster, Crassostrea ariakensis, was significantly larger in weight and body mass than the Eastern oyster, but was unable to filter as effectively as the native oyster. So while foreign oysters can be used in aquaculture for oyster farming, a suitable substitute for the replenishment of the Chesapeake Bay has not yet been found. With the increased mortality of shrimp and fish in the Gulf of Mexico, an offshore oil spill occurring in or near the Chesapeake Bay could prove disastrous to the Eastern oyster, and thus the entire ecosystem.

To combat marine oil spills, various methods can be utilized, including natural dispersion, mechanical dispersion, and even chemical dispersion. Through the use of chemicals known as surfactants or dispersants, an oil spill can be “broken up” for quicker and more efficient dissolution by multiple oil-consuming microorganisms. Essentially, the dispersant attaches to an extremely miniature droplet of oil; the droplet is then coated and contained by the dispersant. The droplet’s coating serves to prevent further contamination of the water, and the droplet is eventually distributed along shorelines or consumed by microorganisms. The dispersant does not remove the oil, but instead contains and decreases the magnitude of the spill.

The nature of these dispersants led me to believe that if various solutions of oil and dispersants were added to experimental marine environments containing the Eastern oyster, then the oysters that experienced an oil solution without a dispersant would have an increased mortality rate compared to the oysters in tanks containing dispersant and oil.

Methods and Materials

Figure 1

This experiment was divided into two parts. The first component was intended to be an examination of the dispersant COREXIT 9527A and crude oil on the mortality of the Eastern oyster. However, due to difficulties in obtaining crude oil and the required dispersant, Motor Oil 6 and the household dispersant Palmolive were utilized. The oysters were donated by the Virginia Institute of Marine Science. Twenty oysters were placed in a 50-liter tank filled with 38 liters of water collected from the Warwick River [Figure 1]. The water temperature was kept at a temperature of approximately 15°C, and the natural salinity of 19.6 parts per thousand was  maintained. 

Figure 2

In order to create solutions of Motor Oil 6, Palmolive dispersant and an oil-dispersant blend, each solution was mixed in a 500 mL container. Each mix sat for 12 hours and was added to the specific test groups in four tanks [Figure 2]. The tanks contained varying concentrations, including two tanks with a concentration of 15 parts per million, one tank with a concentration of 25 parts per million, and another tank with 75 parts per million. For the duration of the experiment, the test oysters were fed with 15 droplets of algae paste every day. The tanks were checked for oyster mortality.

The second component of my experiment began after I was able to obtain the industrial dispersant ZI-400. After visiting the EPA’s website and contacting several dispersant manufacturers, I found one distributer who was willing to donate the dispersant for experimental review. Interestingly, the international freight charge for the dispersant was approximately $300 because of various legal issues focusing up the chemical. To cover the cost, I applied for a Phil Robinson Grant from the Virginia Junior Academy of Science. An additional supply of 400 oysters was needed, and was willingly supplied by the Virginia Institute of Marine Science.

The experiment was conducted for 16 days using a method similar to the previous series of trials with Palmolive. The solutions were mixed up in six containers (two for each solution), as the concentrations of 100 parts per million and 500 parts per million were significantly more concentrated than in the Palmolive trials. Each day the 38-liter tanks, which had 20 oysters each, were checked for mortality. After a trial period of 16 days, all the tanks were drained, and the oysters were returned to a contained aquatic environment. A T-test was run to determine the difference between mortality caused by oil and mortality caused by dispersants with oil.

Experimental Design

Figure 3

Over the course of the study, a consistent experimental design was maintained. The variable being tested was the chemical solution added to the tank, consisting of oil, a surfactant or an oil-surfactant blend. The examined variable was the mortality rate of the samples ofCrassotrea virginica. The concentrations of the solution varied in each component, which were applied in concentrations of 10, 25, 75, 100 and 500 parts per million. The results were compared to a control group of Eastern oysters in Warwick River water with no added chemicals. Experimental constants included the amount of water per tank (38 liters), a water temperature of approximately 15°C, a salinity of 19.6 parts per thousand, a pH of 7, and a dissolved oxygen level of 60% [Figure 3]. Algae paste was used for oyster feeding. Improvements to the design could include testing at different temperatures and salinities, as well as varying the levels of dissolved oxygen.


The Mortality Rate of Crassotrea virginica as Affected by Motor Oil #6 and Dispersant (Palmolive–Pure and Clear)

C-1 C-2 C-3
Day 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0
Day 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Day 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Day 4 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0
Day 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Day 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Day 7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Day 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Day 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Total 0 1 0 0 0 0 1 1 0 1 0 1 0 0 0
Rate 0% 5% 0% 0% 0% 0% 5% 5% 0% 5% 0% 5% 0% 0% 0%

Data Analysis

Before the start of the experiment, the oysters were kept in an environmental test chamber for approximately 21 days. This allowed the oysters to establish consistent filtration cycles and to properly acclimate to their surrounding environment.

The first experiment (the Palmolive trials) served to collect baseline data and provide a reference point for future experimentation. After an examination of the data, it was determined that the maximum mortality rate was 5%. The concentrations used in the experiment were quite similar to those measured in the Gulf of Mexico oil spill. Thus, it can be assumed that the Eastern oyster is extremely resilient to the concentrations of oil spilled in the Gulf. In a future experiment, a significantly higher concentration of oil could be used. It should be noted that in tanks with oil concentrations ranging from 25 to 75 parts per million, a slick of oil could be observed on the water’s surface.

During the second phase of experimentation, a much more defined sheen of oil could be observed on the water’s surface. After approximately 10 days, a significant amount of oil residue could be observed upon the tank’s sides. The T-test showed that only one trial had a significant difference, the trial comparing mortality in the tanks with oil concentrations of 500 parts per million to the control tank. As such, it appears that the Eastern oyster would not be significantly affected by dispersants or crude oil, and thus would be extremely resilient in an oil spill.

Discussion and Conclusion

This study served to reinforce the concept that the Eastern oyster continues to be extremely resilient in environmentally hazardous situations. The chemicals to which the Eastern oyster was exposed have been shown to be moderately to extremely toxic to other aquatic organisms, such as the Mysid shrimp. In the event of an oil spill in Chesapeake Bay, it can be safely assumed that the Eastern oyster would survive relatively well. In future studies, I intend to address the influence of swamp grasses on the spread of oil.


Allen, Stan. Director of the Aquaculture Genetics and Breeding Technology Center (ABC) at the Virginia Institute of Marine Science. Personal interview, Oct 2008.

Booth, Alan. “Oysters Saving the Bay: Crassostrea virginica vs Crassostrea ariakensis.” Unpublished research paper, Jan 2009.

“Corexit EC9500A Technical Product Bulletin.” U.S. Environmental Protection Agency, Office of Emergency Management. Retrieved on 3 Feb 2011 from

“Effects of Oil on Wildlife.” Australian Maritime Safety Authority. Retrieved on 14 Jan 2011 from

“Effects of Oil on Wildlife and Habitat.” U.S. Environmental Protection Agency, U.S. Fish and Wildlife Service. Retrieved from

“Effects of Oil Spills on Aquatic Life and Environments.” Suite101 Online Magazine and Writers’ Network. Retrieved on 12 Jan 2011 from

McMillan, Britt. Hydrogeologist. Personal interview, 23 Nov 2010.