The effect of simulated warming ocean temperatures on the bacterial communities on the shells of healthy and epizootic shell diseased American lobster (Homarus americanus)

Background/Question/Methods

The American lobster, Homarus americanus, is a vital species for the fishing industry along the North Atlantic coast of North America. However, populations in Southern New England have declined, most likely due to increasing ocean temperatures and prevalence of emerging disease. Our previous work suggested that temperature may not be the sole cause for epizootic shell disease (ESD). Here, we examined the shell bacterial communities and progression of ESD in non-shell diseased and diseased adult female lobsters under three simulated seasonal temperature cycles for a year. Fifty-seven female lobsters were wild-caught from Maine’s management zones F and G, and were assessed for shell disease progression on a scale of 0 (no observable signs) to 3 (visible disease on >50% of the shell surface). ESD-negative lobsters (apparently healthy) and ESD-positive (diseased) lobsters were randomly dispersed into 3 systems, and within each system, healthy and diseased lobsters were placed into separate tanks. These systems were maintained at three temperature ranges comparable to the average seasonal ocean temperatures for Southern New England (SNE), Southern Maine (SME), and Northern Maine (NME) regions. Samples were collected at three timepoints, a baseline “summer” temperature where all tanks were the same temperature, a winter temperature four months later, and a summer temperature 10 months after that. A total of 131 experimental samples, plus 10 controls, passed PCR amplification, amplicon quantification and purification, Illumina MiSeq ver. 4 sequencing, and quality-control filtering. Sequences were processed using the R software platform, using DADA2, phyloseq, vegan, and assorted other packages.

Results/Conclusions

The bacterial richness on lobster shells at the baseline timepoint, when lobsters were wild-caught, was higher than the winter time point, 4 months later, or the summer time point, 10 months later, for the same lobsters after having been kept in tanks, regardless of their temperature or shell disease status. Similarly, the bacterial community membership (unweighted Jaccard similarity) was similar for all samples at baseline, but diverged for later time points. Tank temperature significantly affected microbial community membership (unweighted Jaccard similarity), as well as the abundance of those community members (weighted Bray-Curtis dissimilarity). Contrary to our expectations, ESD shell disease index did not progress over time or in warmer conditions, and we hypothesized that frequent tank water changes and shell moltings may have reduced the microbial load. Preliminary results indicate that shell stage and shell disease index were positively associated with increased bacterial richness on lobster shells.