This article is reprinted with permission of Commercial Fisheries News, the Northeast's fishing newspaper for over 30 years, ©2003 Compass Publications Inc. Commercial Fisheries News is published monthly; annual subscriptions are $21.95. To subscribe or request a sample issue: call (877) 263-4496; fax (207) 367-2490; e-mail (cfoster@fish-news.com); or click on the hot link.
by Kari Lavalli, PhD
Vice President, The Lobster Conservancy, www.lobsters.org
This column provides lobster health and handling information.
If you have questions or concerns, contact Lavalli at e-mail klavalli@yahoo.com.
Shell disease has become an increasing concern of lobstermen, especially in the areas off southern Massachusetts and west into Long Island Sound, where its presence is most prevalent. What exactly is shell disease? How is an animal infected? How does it spread? And how can it be prevented?
Those questions will be the focus of a series of "Ask the Lobster Doc" columns that will run in the next few issues of Commercial Fisheries News. Kari Lavalli, PhD, who is vice president of The Lobster Conservancy, will serve as the Lobster Doc for these columns.
Shell disease is not a distinct disease in and of itself. Rather, it is a syndrome, meaning that it is a group of signs and symptoms that characterize a disease. In this case, shell disease manifests itself with the progressive erosion of the shell of a crustacean.
This syndrome is caused by bacteria that are capable of eroding the three innermost chitinous layers of the shell. It is most often seen in stressed animals - those with already damaged shells, bitten off antennae, injured claws, and the like.
Basically, any injury that breaches the outermost layer of the shell, called the epicuticle, exposes the underlying chitinous layers to opportunistic microorganisms like bacteria, fungi, or protozoans.
These microorganisms then can act independently, collectively, or sequentially to destroy the chitin and, in extreme cases, expose underlying tissues to secondary infections.
Molting may eliminate the problem, but recent studies in Rhode Island indicate that about 40% of shedders may not improve or may actually get worse after molting. In all but extreme cases of ulceration, the meat of the animal is not affected and can be safely enjoyed by the public.
The disease has a variety of names- shell disease, box burnt disease, rust disease, brown spot, black spot, or burn spot- and is known to occur in blue crabs, king crabs, shrimps and prawns, crayfish, and lobsters.
What we know
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Side view of carapace of lobster, showing an extreme case of shell disease. White patches show two of the chitinous layers- the remaining calcified exo- and endocuticles. The remainder is the non-calcified third chitinous layer or endocuticular membrane. Specimen from the New England Aquarium.
(Dr. Samuel Tarsitano, Southwest Texas State University, photo)
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Shell disease was first described in 1900 in crabs and reported in 1937 for American lobsters held in pounds in Nova Scotia. In 1937 and again in the 1960s, '80s, and '90s, researchers found that the disease was relatively rare in the wild, infecting only about 5% - 8% of the natural population.
It is less prevalent in cooler, more exposed, deeper waters than in warmer, inshore sites. It is far more common in lobsters held in pounds over the winter, where individual animals are more likely to be stressed from being overcrowded, damaged due to capture or handling, and/or subjected to poor water quality conditions.
Despite its greater prevalence in pounds, shell disease also appears to be more common in wild populations of lobsters living in degraded coastal or estuarine habitats, particularly those with sewage sludge dumpsites where bacterial counts can be particularly high.
