Relationship between Temperature and Shell Disease in Laboratory Populations of Juvenile American Lobsters (Homarus americanus)

Abstract

Epizootic shell disease is affecting American lobster (Homarus americanus) populations and the associated fisheries significantly. Historical movement of the disease suggests that temperature has been a critical factor in driving the susceptibility of lobsters to this disease. Here, juvenile lobsters were held in the laboratory at 10°C, 15°C, or 20°C for up to 1,021 days. Survival, growth, and shell disease were tracked throughout, and a separate experiment was conducted to assess the rate at which dietary white lobsters deposited pigment to the cuticle at the different temperatures. In the laboratory, shell disease was tracked as an initial “spot,” the melanization of the cuticle in response to the bacterial ingress, or, as it progressed, a lesion, which was a visible erosion of the shell. The length of the molt cycle (number of days from one molt to the next consecutive molt) was inversely associated with temperature. Average growth (increase in weight and carapace length per molt) at 10°C and 15°C was equivalent and higher than at 20°C, whereas shell disease was greatest at the intermediate temperature. The amount of initial melanization responses to bacteria (spots) was similar across temperatures, but the number of lesions was greatest at 15°C. This is likely a result of the conditions that allow for a combination of longer molt cycles and active bacterial growth, which allowed the disease to progress to the more advanced lesioned state. At the highest temperature, lobsters molted the compromised shell before severe lesion formation could occur. No gross differences in shell structure were observed at the different temperatures. The lack of a linear relationship between shell disease and temperature does not discount temperature as a critical factor in the onset of this disease in lobsters, but signifies that additional factors are important and the disease is complicated and multifactorial in nature. The continued use of a laboratory model system to study this disease further is important to be able to control the multitude of complicating factors appropriately.