A Stacked Deck: The Ecology of the Blue Crab

To put it mildly, the cards are stacked against the blue crab population in the Chesapeake Bay area. In the past 60 years, the human population of the area has jumped from 3.7 million to almost 18 million and, subsequently, farming and industry has exploded (it is often joked that everyone on the eastern shore of Maryland is a chicken farmer), leading to waterways filled with ferrous compounds, nitrates and phosphates.

Essentially, the Chesapeake has become a sink for these pollutants running through 141 streams and five rivers from six states--New York, Delaware, Pennsylvania, Maryland, Virginia and West Virginia. Only one percent of the pollution actually make it into the Atlantic Ocean. There are vast regions of the Chesapeake dubbed "dead zones" for the high levels of oxygen depletion; often these sections are completely devoid of oxygen.

The blue crab, like most decapods, has a multiple larval stages, including the four to seven stage zoea, the megalops, and followed by several stages of juvenile. Once fertilized, the female travels out of the bay, seeking shallower waters with high salinity content. The female dies after depositing her offspring, leaving them to live a largely planktonic lifestyle at the top of the water column, until they have reached the juvenile stage and can return to the waters of the Bay.

As you can see, the life cycle of these creatures is already very delicate. But pollution is only half the problem.

Along the coast of Georgia, scientists have found that blue crabs are being parasitized by a dinoflagellate called Hemotodinium perezi. While not all of them are infectious, dinoflagellates are phylogenetically related to the disease causing apicomplexans and are most well known for toxic blooms like red tides.

Crustacean "blood" is called hemolymph, so named for their lack of separate circulatory systems of mammals. The parasite breaks down these hemolymph cells by consuming hemocyanin, the protein responsible for oxygen transport in crustaceans (analogous to our hemoglobin). H. perezi essentially suffocates its host by limiting the distribution of oxygen through the crab.

This factor, coupled with the oxygen depleted "dead zones" caused by warmer waters and eutrophication could spell disaster for the blue crab population in the Chesapeake Bay. Keep in mind that all of these factors, including the infection, are human induced. Any change in an organism's habitat can cause unforeseen consequences.

The Chesapeake Bay Program has reestablished some of the estuarine ecosystem, reducing phosphates by 27 percent and nitrates by 16 percent, but the human population of the area continues to increase.

Next semester (my last!) I'm hoping to get more info on research being done to conserve the area's wildlife, including the blue crab. I know for sure that UMCES is looking closer at the specific processes that drive the crabs from lower estuarine areas to the bay itself and hoping that this research will present conservation alternatives.

I have a feeling that the area needs more voices defending the Chesapeake. As much as I love seeing "Save the Bay" bumper stickers on SUVs, it takes more than a sticker to get anything done.