News Article

Shell Disease in Crustaceans

Nicholas Burnett

Growing up in Charleston, South Carolina with a tidal creek in my backyard gave me countless opportunities to explore the endless treasures of the intertidal zone, including the fascinating Atlantic blue crab, Callinectes sapidus. It was not until this past summer while in an internship position at the Hollings Marine Laboratory with the College of Charleston that I began noticing a recurring visible discoloration on the shell of a large number of the blue crabs that were brought into the laboratory. The affected blue crabs showed up in the hottest parts of the summer with splotches of brown deterioration in random locations on their exoskeleton. The lesions that I noticed were comparable to a piece of fruit with rotted sections or holes in its skin. Regardless of the “rotted” spots on the shells of the blue crabs, the animals seemed to function normally. Not surprisingly, this infection is known as shell disease and is a widely known ailment in many crustaceans across the globe in addition to C. sapidus.

Incidences of shell disease first appeared about 10-20 years ago in the New England area and were originally described as “little black spots” (Somers, 2008 and Eldred, 2008). According to recent generalized reports by lobstermen the magnitude of shell disease on individual lobsters has worsened to a point at which it now may encompass whole shells instead of just smaller spots (Somers, 2008). Typically the “rotting” of the exoskeleton by shell disease does not kill the crustacean, but the physical appearance of afflicted crustaceans makes them difficult to sell whole to consumers in the marketplace. Luckily for the fisheries of such crustaceans the meat in the animal is not affected by the disease and the animal can be used for canned meat instead of live sale (Somers, 2008).

The reason lobsters and other crustaceans do not die from shell disease is that crustaceans are capable of molting, or shedding, their exoskeletons and losing any lesions and corrosion that the disease may have caused. Molting is used mainly as a means of growth, but in the cases of shell disease it is used as a stress response by the crustaceans to rid their bodies of potentially harmful elements on the exoskeleton. In some situations the lesions penetrate through the exoskeleton, leaving a complete hole in the shell of the crustacean, and its only defense against having foreign objects and bacteria enter into its internal tissues is to fuse those internal tissues to the surrounding shell, creating a seal. While this protects against harmful entities from entering into the crustacean’s internal space, the animal will not be able to molt because it is attached to the shell from inside its body. This will result in the death of the animal (Somers 2008). Another instance in which a crustacean may not be able to molt as a response to shell disease is when a female is gravid, or carrying eggs. Female lobsters, for example, must carry their eggs to term and keep their exoskeleton for a much longer amount of time than males. However, the females can become too weak to carry the eggs, and in many cases the lobsters prioritize survival above having offspring and resort to abandoning the eggs to molt (Somers, 2008).

Based on the recent increase in incidence of shell disease, crustacean fisheries, like those for lobsters, crabs, and shrimp, do stand some chance of being harmed by shell disease. According to data released by the U.S. National Ocean and Atmospheric Administration (NOAA) and the National Marine Fisheries Service, commercial fishing in the United States brought in about $284,800,000 for American Lobsters, $483,600,000 for crabs, and $424,000,000 for shrimp in the year 2003 alone (NOAA and NMFS, 2004). The financial importance of these crustaceans makes the disease an economic concern as well as a biological issue. Recent estimates of the numbers of lobsters affected by shell disease in Southern New England indicate that about 30% of lobsters have shell disease (Somers, 2008). The percentage of lobsters infected is similar to that of the crab Cancer pagarus with reports of 55% incidence of shell disease in Langland Bay, South Wales (Vogan et al. 1999). Significant shell disease occurrences have also been reported in the brown shrimp Crangon crangon in Poole Harbor, England where 87% of that shrimp species had the disease (Dyrynda, 1998) and in Dunbartonshire, a county in Scotland, where 13.2% of the shrimp population in the Solway Firth exhibit shell disease. Interestingly enough, the latter population of C. crangon shows 1.5 times more infections than any other crustacean species in the surveyed area (Nottage, 1982). With shell disease happening to a broad range of crustaceans all over the world, research is being conducted on several different possibilities for such a large and widespread crustacean disease.

The agent that is suspected to cause shell disease is said to be chitinolytic bacteria, named because the exoskeletons of crustaceans contain chitin, a polysaccharide that is very common in marine ecosystems and is used by many organisms in addition to crustaceans for structural purposes (Flintoft, 2004). Chitinolytic bacteria do occur naturally in the ecosystem. With chitin being such a widely used molecule, the ocean floor would be littered with the molted exoskeletons of crustaceans and microorganisms if it were not for the decomposition performed by the chitinolytic bacteria. Because of these bacteria little chitin is found in the sediments of the marine environments around the globe (Vogan et al. 2008). Although it is generally known what the bacteria do to corrode the exoskeleton of crustaceans, the exact species of bacteria that cause the damage are unknown. Cultures of bacteria found in the shell diseased lesions of the spiny lobster Panulirus argus in the Florida Keys yielded two new types of bacteria belong to the genus Vibrio that were not closely related to any known Vibrio species (Porter et al. 2001). Bacterial decomposition of chitin waste is expected in the marine ecosystem, but the peculiar occurrences of shell disease raise the question of why these bacteria are now decomposing the exoskeletons of living crustaceans. In Massachusetts, the sudden rise of shell disease in lobsters was initially blamed on mosquito spraying and other pesticides being deposited in the ocean (Eldred…). Other pollutants that have been labeled as possible causes are industrial byproducts, surfactants, plastics, paints, and countless other harmful manmade agents. Research has been done to correlate various environmental factors with the increased rate of shell disease in the crustacean population, including water temperature. The findings of that study link the water temperature during the previous winter with the numbers of lobsters affected with shell disease. Warmer winters, with water temperatures above 20 ̊C, resulted in increased numbers of shell disease in the lobster population studied, whereas colder winters yielded fewer affected organisms (Glenn and Pugh, 2006). While the data do show a correlation between water temperature and occurrences of shell disease, more research has been done at a molecular and immunological level, aimed at pinpointing the pollutants and pathogens that are responsible for the disease.

A study of the Atlantic blue crab, C. sapidus, found a decrease in the antibacterial activity of crabs’ hemolymph, the equivalent of blood in mammals, when the crabs had visible signs of shell disease, compared to blue crabs with healthy exoskeletons (Noga, 1994). In other words, the more heavily affected the blue crab is with shell disease, the less capable it is of fighting off bacterial infections within its body. Although the study’s data show a link between crabs’ immune systems activity and shell disease in the animals, the antibacterial activity of an individual in any group of organisms is always highly variable. With such variation among individual animals, it is more difficult to pinpoint a certain defining correlation of any disease or infection to the animal’s immunological capability. There are also some differing opinions and methods on how to accurately assess shell disease in crustaceans. While Noga’s (1994) data do show a link between antibacterial activity and the prevalence of shell disease in the crab, Vogan (2008) attempted to measure the occurrence of shell disease on an individual by classifying the lesions on the shell based on their size and severity. Under this classification system there are two main types of shell disease: the more severe shell disease centering around the chitin degradation, and the less severe epizootic shell disease. Conversely it was found that there is no apparent relationship between the size of the lesions and the extent of the shell damage. The contradicting study suggested that examinations of the rotted spots caused by shell disease are not a reliable method for measuring the damage to the affected crustaceans (Noga et al., 2002). Developing a method for measuring shell disease is important because it will allow scientists to more accurately assess the impact of environmental factors on shell disease in their research.

The presence of bacteria in the hemolymph of crustaceans is expected when the animals are infected, so researchers were more intrigued when the presence of a chemical called alkylphenol in the tissues of the animals was confirmed. Alkylphenol is a chemical that is commonly used in plastics, thus it should not be shocking that about 95% of the human population may show traces of the chemical in their bodies (Eldred, 2008). Alkylphenols have also been shown to accumulate in areas with little or no oxygen, such as sediments where crustaceans and other benthic organisms dwell (Di Corcia et al., 1998). Alkylphenolic compounds are also estrogenic, meaning they act as the hormone estrogen, negatively impacting all the organisms that are exposed to high concentrations of the chemicals over long periods of time. Another common oestrogenic compound that has made recent headlines in the news is Bisphenol A (BPA) (Markey et al. 2001), whose infamy came from its use in baby bottles and can potentially cause birth defects in humans. In the context of the marine environment these chemicals mimic hormones and are capable of causing oysters and zebra fish to change their genders (Nice et al. 2003). The prevalence of alkylphenol in the ecosystem was shown in a study where, in a surveyed area, the internal organs of fish and birds contained concentrations of alkylphenolic compounds at 10 – 1000 times the alkylphenol concentration of the surrounding environment (ENDS, 1999). Several hypotheses as to how the hormonal effects of alkylphenol can alter the exoskeleton structure have been made. One hypothesis is that the crustaceans cannot make their shells hard enough because of the chemicals’ effects and then abrasions on the shell initiate the chitinolytic bacteria’s decomposition of the chitin based shell. Scientists are still working on finding the underlying relationship between pollutants of any kind and bacteria’s new behavior of decomposing crustaceans’ exoskeletons.

Although there remain many possible causes of the appearance of shell disease over the last few decades, the presence of chemicals and other pollutants in the marine environment are a likely source of the disease. Many of the high rates of shell disease occur within heavily polluted bays, harbors, and other stretches of coasts that are bordered by industrialized cities and dense human populations. Manmade pollutants can safely be labeled as the primary cause of the disturbing number of crustaceans affected by shell disease, but researchers must continue investigating the reasoning behind bacteria’s corrosive interaction with crustaceans’ exoskeletons so that humans may know what to do to help the animals. The continuation of shell disease, if it is indeed caused by chemical and hormonal pollutants, will result in increased rates of diseased animals, more-so than there are at this time. It is possible in the future that extremely severe shell disease will begin to kill off members of the crustacean populations, greatly affecting the marine ecosystem and also having negative effects on crustacean fisheries that many communities around the world depend on for survival.

References

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