Research findings shed new light on shell disease in lobsters

On February 16, 2005, scientists, fisheries managers, fishermen, and the concerned public met to discuss what we know and don't know about shell disease—a disease found in lobsters that has been increasing in intensity and severity since it first started appearing in southern New England in 1996. "We're looking at shell disease as a puzzle and trying to see how the pieces fit together," says Kathleen Castro, Rhode Island Sea Grant Sustainable Fisheries Extension Program director and organizer of the second annual Rhode Island Lobster Health Symposium, sponsored by Rhode Island Sea Grant.

Shell disease is caused by bacteria that invade from the outside of the lobster via pores in its cuticle—the outermost layer of the shell—pores which are usually not visible. Roxanna Smolowitz, a veterinary pathologist at the Marine Biological Laboratory in Woods Hole, Mass., has been studying shell disease in lobsters since its outbreak in Long Island Sound in 1997. In her work, she sees a range in the severity of the disease; from shallow pits that erode the cuticle and cause those unsightly black spots on the shell to ulcerations—holes that fully penetrate the shell, fusing the shell to the underlying membranes and preventing the lobster from releasing its shell, which likely makes it impossible for the lobster to molt and thereby kills the lobster.

"In 1997–98, we saw the incidence of shell disease jump from 5 percent to 20 percent," reported Thomas Angell, R.I. Department of Environmental Management (RIDEM) fisheries biologist. "Then, in 2001–2002, the incidence jumped from 21 percent to 30 percent in inshore lobsters; incidence in offshore lobsters has remained low." Approximately 30 percent of Rhode Island's legal and just sublegal lobsters are showing signs of shell disease. Legal-sized female lobsters are the most severely affected, according to Angell, because they retain their shell for a longer period of time while carrying eggs.

And since no one wants to take a chance on a lobster with a shell that appears to be rotting, shell disease is a big problem for the lobster industry. In 1999, lobster landings were valued at more than $30 million. By 2003, they had dropped by almost half to $16.7 million. In the same four-year time-period, the industry had lost 34 percent of its fishermen—from 420 to 279. "We're seeing a decrease in lobster landings as well as in our trawl surveys. That's leading us to believe that shell disease is increasing natural mortality in affected lobsters," says Mark Gibson, RIDEM deputy chief.

Rhode Island's lobsters are not the only ones affected: Long Island Sound's lobster fishery is all but collapsed. In Massachusetts, outbreaks were reported around the Elizabeth Islands between 1983 and 1989 but were of minimal severity. Then, from 1997 to 2003, the incidence of shell disease migrated northward through the Cape Cod Canal, into Cape Cod Bay, Boston Harbor, and north to Cape Ann. Robert Glenn, Massachusetts Division of Marine Fisheries aquatic biologist, suggests that increasing temperatures may be to blame: "We're seeing severity 'hot spots' in areas like the Elizabeth Islands and not in more polluted areas like Boston Harbor." In addition to temperature, Glenn points to a possible relationship with other environmental factors or the relationship with density-dependent factors as causes for disease proliferation.

Hans Laufer, University of Connecticut professor emeritus, has some thoughts of his own about several of those environmental factors. Alkylphenols, compounds found in everything from detergents to surfactants, paints, and plastics, have been found in higher concentrations in lobsters with shell disease than in unaffected animals. These compounds may be endocrine disrupters, interfering with the lobster's normal hormonal system and stimulating the animal to begin molting. In the laboratory, Laufer found that shell-diseased lobsters produced significantly more ecdysone (molting hormone) than unaffected lobsters. In fact, he found ecdysone concentrations in egg-bearing females that were so high, they molted. (Egg-bearing females do not molt during the 9-month period when they are carrying their eggs. Molting during this time causes the female to lose her entire clutch.) Laufer suggests that molting may be a defense mechanism against shell disease. By molting, the animal can get rid of the disease, if only temporarily, by losing its shell.

Despite what we now know, many questions and conflicting information exist:

• Does shell disease affect molting frequency? How often?
• Does the disease affect natural mortality rates?
  
• Is there an affect on egg production?
  
• What is the cause of higher incidences? Why here, why now?
  
• How can we make it go away?

For the latest information on lobster shell disease, contact the following researchers:

• Kathleen Castro, Sea Grant Sustainable Fisheries Extension Program Director, (401) 874-5063, e-mail: kcastro@uri.edu
• Thomas Angell, R.I. Department of Environmental Management Fisheries Biologist, (401) 423-1931, e-mail: tangell@dem.state.ri.us

Other lobster information: Lobstering in the Ocean State at projo.com

Visit Rhode Island Sea Grant Sustainable Fisheries Extension