Revisiting the prevalence of Nemertean parasite Pseudocarcinonemertes homari in American Lobster within the Gulf of Maine

The American lobster, Homarus americanus, is an important economic species in the Gulf of Maine (GOM). While the fishery has enjoyed recent increases in production, future temperature projections for the GOM suggest that near shore habitats will become too warm, pushing the lobster further north and offshore. In addition to altering the geographic range of American lobsters, increasing temperatures also present increased risks of exposure to parasites and disease.

The nemertean parasite Pseudocarcinonemertes homari is an ectoparasite known to consume the egg yolks of the American lobster and was first described in the northern GOM in 1981. This early research explored the relationship between nemertean infestation and egg loss in ovigerous lobsters. A range of 3 – 74% of all lobsters from 5 samples sites across the northern GOM were reported to be infected with nemerteans in the northern GOM. While the density of infection was generally considered low, extensive or complete egg mass destruction was observed with nemertean densities greater than 4 worms per 1000 eggs.

To date, no subsequent research on nemertean parasites in American lobsters in the GOM has been conducted. Here we report on the prevalence and intensity of parasitic infection of P. homari in ovigerous American lobsters captured around the Isles of Shoals in the southern part of the GOM. We sampled egg masses from 124 ovigerous lobsters between June-July 2019. Fourteen percent of examined lobsters had nemertean worms within their egg masses. The intensity of infection ranged from 3 to 963 worms per infected lobster. While the majority of lobsters had low mean densities of nemerteans (<2/1000 lobster eggs), 30% of the infected lobsters had 4 or more nemertean worms per 1000 eggs with a maximum observed density of 12 worms/1000 eggs. Nemerteans were most commonly observed in mature embryos (i.e., 80-95% development). Results from this study help to document the potential risk of exposure to P. homari in American lobsters in the southern GOM and serve as an important baseline for future studies of this parasite and its impact on egg loss.