When diners sit down to a lobster dinner, they look forward to seeing a glistening red shell that tempts them to dig underneath - not a speckled, scabby brown crust thin enough to poke a fork through.
That’s why wholesalers pay less for lobsters with shell disease and why many fishermen will just throw a lobster back if it is diseased. By the time the lobster is caught again, hopefully, it will have molted and grown a smooth new shell.
No one knows precisely what causes lobster shell disease, a bacterial infection that does not affect the meat and seems to strike mainly in the warm coastal waters off Rhode Island, Connecticut, and New York. Though it is unlikely to pose a threat to the heart of the industry farther north, researchers are trying to understand why these crustaceans get sick and what, if anything, can be done to protect them.
Scientists at the New England Aquarium are concerned that the combination of warmer waters and human-generated pollution are triggering the disease, and that the same combination of effects might threaten other species.
“As we impact the marine environment, bacteria are the next thing we really need to understand,’’ said Michael F. Tlusty, director of research at the New England Aquarium.
Increasingly, human activity is forcing animals to cope with bacteria that would not otherwise affect them, he said. Two-thirds of right whales, for instance, have been infected by giardia, a bacteria that causes stomach bugs in humans but has an unknown effect on whales. A few years ago, otters in California began dying from toxoplasmosis infections - usually known for infecting cats - after flushable kitty litter became popular, Tlusty said. And he recently heard from fishermen in Mexico that scallops off their coasts seemed to be suffering from some kind of shell disease, too.
Humans keep making changes in the environment, many of which “play into the favor of the bacteria,’’ he said.
During the past decade or so, lobsters in southern New England have been subject to changes such as the loss of coastal eel grass, which acted as a filtering system. They have also had to deal with mosquito-control efforts along Long Island Sound that are meant to disrupt mosquito molting but which may affect lobster molting, and with the use of herring as bait in lobster traps. Lobsters fed herring in the New England Aquarium lab are more susceptible to shell disease, Tlusty said, and herring from traps is a major food source for many lobsters.
It is not clear, he said, whether one or all of these factors or something else has triggered the spread of lobster shell disease, which affected about a third of all lobsters caught along Rhode Island’s shoreline in 2010, though virtually none caught in Maine.
“We don’t have the single smoking gun. We don’t have a ‘This is why it’s happening,’ ’’ Tlusty said.
Rising water temperatures probably play a role in making lobsters more vulnerable to shell disease, said Bob Glenn, senior marine fisheries biologist with the Massachusetts Division of Marine Fisheries.
“The magic number is about 68 degrees Fahrenheit,’’ he said. “Once above that for a prolonged period, [lobsters] undergo increased rates of disease and physical stress. They will try to avoid those temperatures and migrate.’’
The number of days that water temperatures south of Cape Cod top 68 degrees has risen dramatically in recent years, he said. In comparison, the Gulf of Maine is extremely cold, so water temperatures north of the Cape will probably remain resistant to climate change, he said, and the lobsters there are reasonably safe.
Temperatures up north “would have to change dramatically for it to go beyond what a lobster would like,’’ he said. “But people probably said that about southern New England 50 years ago.’’
Lobster shell disease has not had a major effect on the market, as it is present in only a small percentage of lobsters caught in southern New England, which themselves account for only about 10 percent of lobsters sold in the United States, said Dr. Robert Bayer, a veterinarian and director of the Lobster Institute at the University of Maine.
Lobsters with the disease do suffer at the market, though. Lobsters that look good enough to serve on a plate sell for substantially more money than lobsters with shell disease, which must be sold for the meat alone, said Bill Adler, executive director of the Massachusetts Lobstermen’s Association.
“Obviously, we want to have good-looking lobsters as well as good-tasting,’’ he said.
Speaking by cellphone from his fishing boat, the Jeanette T, off the coast of Eastern Long Island last week, Michael Theiler, vice president of the Connecticut Lobstermen’s Association, said he has not seen much lobster shell disease so far this year.
The diseased lobsters sell for about $1 less per pound. “We like to see all nice, premium selects,’’ he said.
The New England Aquarium will hold a lecture about its lobster research on March 29 at 7 p.m. in its Harborside Learning Lab, 1 Central Wharf, Boston. Anita Metzler, assistant scientist and laboratory operations manager at the aquarium, will lead the talk.