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Lobster Shell Disease

By Barbara Somers

What is shell disease?
Researchers in the Northeast region first noticed shell disease in the 1980s. At that time, the disease appeared as little black spots affecting lobsters' shells. But in the past several years, shells have sometimes become fully infected by the disease, with the worst cases rotting the shells entirely.

Shell disease is caused by bacteria that invade from the outside of the lobster through pores in its cuticle-or the outermost layer of the shell-that cannot be seen by the naked eye. There is a range in the severity of the disease from shallow pits that eat away at the cuticle and cause those unsightly black spots to ulcerations-holes that fully penetrate the shell, causing the shell and the membranes underneath it to fuse together. This can stop the lobster from releasing its shell and can cause it to become stuck during the molting process and die. A lobster needs to molt in order to grow. It has been suggested 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.

The disease does not taint the lobsters' meat, but makes shells so unattractive that they are too unappetizing to serve whole. The meat may be used for canning. Larger female lobsters are the most severely affected because they retain their shell for a longer period of time while carrying eggs. It can weaken lobsters so much that they are unable to carry their eggs to term and there have been several instances of molted shells found with the eggs still attached.

Some preliminary studies suggest the lobsters may be contracting the disease from alkylphenols, chemicals that are byproducts from industrial sources. These compounds are found in everything from detergents to surfactants (a surface-active substance), paints, and plastics, and have been found in higher concentrations in lobsters with shell disease than in unaffected animals. These compounds may be interfering with the lobster's normal hormonal system and stimulating the animal to begin molting too early. In the laboratory, it has been found that levels of the molting hormone in shell diseased lobsters are much higher than unaffected lobsters. Egg-bearing females do not molt during the nine-month period when they are carrying their eggs. More lobsters are getting shell disease, and the problem has spread from southern New England waters all the way to Maine. Thirty percent of lobsters in coastal areas of southern New England and Long Island Sound are now affected by shell disease. It's still unclear what's causing it to spread. Shell disease is not contagious from lobster to lobster, and biologists suspect that environmental factors such as water temperature or polluted run-off may be weakening the lobster's immune system and allowing the bacteria to grow faster than the lobster can fight it.

See also: The American Lobster fact sheet

Return to Rhode Island Sea Grant Fact Sheets

10/2005