Pathogenic impacts on the green sea urchin (Strongylocentrotus droebachiensis) in the Gulf of Maine

The green sea urchin (Strongylocentrotus droebachiensis) supports a large and valuable fishery in the Gulf of Maine (GOM). They are abundant, relatively sessile, small and easily collected, and may readily bioaccumulate metals and community representative microflora populations. As has been seen world wide, cascading environmental stress responses can occur following acute and/or chronic exposures to contaminated environments potentially leading toward mass mortality events. Microbial assemblages have also been implicated in urchin mass mortality events. In order to assess the potential effects of environmental conditions on urchin health via coelomic fluid and tissue histopathologic analysis, 612 urchins collected from various sites in the GOM were examined and sampled while concurrent environmental data (bottom topography, sediment coverage and aquatic plant distribution) were recorded. This report focuses on the pathologic consequences of microbial assemblages and heavy metal accumulation affecting inshore urchin populations. Histopathologic lesions were frequently detected in urchins found in river outflow tracts exposed to heavy metal accumulations. The average scores of tissue necrosis and atrophy, and the sums of mean digestive tract lesions correlated significantly with collection sites. The majority of bacteria isolated were opportunistic pathogens. Vibrio, Sphingomonas and, Brevundimonas, organisms predominated. Within the sample population, 95.5% of all samples contained colonies of Vibrio species. Of the Vibrio species isolated, V. alginolyticus (82.57%), V. vulnificus (25.00%), and V. hollisae (20.04%) were the most common isolates. The prevalence of heavy metal accumulation was associated with the site of collection and bottom topography. Water temperature may play an integral role in defining the urchin disease status of the GOM. In order to assess the potential effects of environmental temperatures on urchin health, a total of 275 S. droebachiensis specimens kept in sand-filtered seawater were subjected to temperatures between 2° and 13°C. Coelomocyte numbers and bacterial microflora increased in direct proportion to temperature from 2 to 10° C. However, coelomocyte numbers dropped substantially between 11 and 13° C. We hypothesize that coelomocyte numbers and bacterial microflora in S. droebachiensis are directly associated with increased water temperatures.