Spatial overlap and interaction between sub-adult American lobsters, Homarus americanus, and the invasive European green crab Carcinus maenas

Invasive species are considered one of the major threats to biological diversity, and they can negatively impact species of particular ecological and/or economic importance. In this study, we investigate the potential threat that the European green crab, Carcinus maenas, represents to sub-adult lobsters, Homarus americanus, in Passamaquoddy Bay, Canada, northwest Atlantic. We conducted transect surveys using SCUBA diving to evaluate spatial overlap between these species inside, and immediately adjacent to, a lobster nursery area, and ran two laboratory experiments to investigate interactions between them in conditions of limited food and shelter availability. We found marked spatial overlap between these species in nature, particularly between adult green crabs and sub-adult lobsters in the shallower parts of the nursery area. It was not uncommon to see green crabs and sub-adult lobsters less than 1-2 m from one another, and we estimated mean nearest-neighbor values as low as 0.5-1 m on certain transects and months, with a grand mean of 5.17 ± 1.62 m (SE). Monte Carlo randomization of positional data revealed that green crabs and lobsters were randomly distributed relative to one another; individuals of one species did not seem to aggregate or segregate relative to individuals of the other species. In the lab, green crabs did not negatively affect survival (one exception), growth, activity, feeding, or shelter use of sub-adult lobsters. In fact, sub-adult lobsters regularly preyed upon green crabs in the first lab experiment, where heterospecific individuals were matched for body mass. Our results suggest that green crabs do not pose a significant threat to sub-adult lobsters in Passamaquoddy Bay, and may in fact represent a food subsidy for them.