Blue crabs are the basis of an iconic, and important, fishery along the U.S. Atlantic Coast. Georgia fishermen typically land hundreds of tons of blue crab annually. Our family looked forward each summer to netting crabs in the tidal creeks of Sapelo Island salt marshes. But starting around 1996, blue crab landings began to crash. By 2004 the crab catch had nose-dived to a small fraction of the long-term average haul. What was going on?
Photo of a blue crab: "The Childrens Museum of Indianapolis - Atlantic blue crab" by The Children's Museum of Indianapolis. Licensed under CC BY-SA 3.0 via Wikimedia Commons
As I explained in the final chapter of 'Marsh Mud and Mummichogs,' Dick Lee and Marc Frischer, colleagues at the Skidaway Institute of Oceanography in Savannah, decided to find out.
Many of the crabs that were caught during the decline seemed sick. Lee and Frischer knew about a particular disease that had wiped out populations of other crab species in Europe and Alaska. This crab disease is caused, surprisingly, not by a bacterium or virus, but by a protozoan parasite: a dinoflagellate. It turns out that this dinoflagellate, Hematodinium perezi, infects and grows in the blood of marine crabs. Hematodinium (literally, "blood-whirler") is closely related to toxic red-tide-causing dinoflagellates and to a dinoflagellate, Pfiesteria piscicida (a.k.a. The Cell from Hell), that attacks and kills estuarine fish.
Crabs are especially vulnerable to Hematodinium infection because they don't have an immune system like ours. If something strange enters a crab's bloodstream, special cells, hemocytes, engulf the intruder and eliminate it. Hematodinium insidiously feeds on these protective blood cells, eliminating them. The parasitic dinoflagellate also consumes the blood proteins that carry oxygen to cells throughout the crab's body. As the dinoflagellate proliferates in the crab's bloodstream, the crab slowly suffocates from lack of oxygen, becomes lethargic, then comatose, and dies.
Lee and Frischer discovered that the dramatic decrease in blue crab numbers after 1996 was indeed caused in part by a severe Hematodinium infection. They wondered why blue crabs, which had resisted the dinoflagellate parasite in previous years, were succumbing to it now.
Hematodinium forms dinospores, resting stages that hang out in coastal waters until they encounter a crab they can infect by entering the bloodstream through the crab's gills. The dinospores don't survive well when the water temperature is low, so blue crabs rarely have the disease in winter.
Another environmental factor contributing to the infection of the crabs by dinospores is water salinity. Lee and Frischer found that blue crabs living in parts of the estuary that had freshwater sources, such as springs or coastal rivers, were healthy and did not appear to be infected by the dinoflagellate parasite. But, wherever estuarine waters had a salt content higher than 28 parts per thousand, crabs were virtually absent, and those that were caught were full of the blood-eating protist. Lee and Frischer speculated that the severe drought that was widespread in the mid latitudes of the Northern Hemisphere from 1997 to 2002 resulted in reduced river flows to the Georgia coast. Estuaries were flooded with saltier ocean water. Hematodinium flourished, and parasitic infection raged through the blue crab population as the crabs came into the Georgia estuaries in the spring to breed.
Read Lee & Frischers' paper here: Lee, R.F.D. & Frischer, M.E. (2004). The decline of the blue crab. American Scientist 92: 548-553.
Lee and Frischer also suggested that since blue crabs are notorious cannibals, readily attacking and eating sickly crabs, this behavior facilitated the rapid spread of the parasitic disease. A recent study by investigators at Old Dominion University and the Virginia Institute of Marine Science discovered that crabs with only a mild Hematodinium infection become sluggish and more easily caught by predators, including healthy blue crabs, thus transmitting the infection.
After 2004, more normal weather patterns alleviated the severe drought affecting Georgia river flows, and blue crab populations sprang back. But, the abundance of this delectable crustacean along the U.S. Atlantic coast is still threatened by overharvesting. And if drought returns, so might the dinoflagellate disease.