The effect of geographic origin on interactions between adult invasive green crabs carcinus maenas and juvenile American lobsters Homarus americanus in Atlantic Canada

Abstract

The European green crab Carcinus maenas is an invasive species in the northwestern Atlantic that has the potential to compete and prey upon endemics such as juvenile American lobster Homarus americanus. Recent laboratory studies have reached opposite conclusions regarding the dominance relationship between these species, even documenting a reversal of roles as predator and prey. Interestingly, these contradicting results involved crab populations from different regions in Atlantic Canada (St. George's Bay in the Northumberland Strait and Passamaquoddy Bay in the Bay of Fundy), which are genetically different and presumed to be the result of different invasion events. In this study we housed juvenile lobsters and adult green crabs together in 3-m diameter flow-through tanks, and monitored their mortality, growth and behaviour from September through December 2009. The experiment involved five treatments, three experimental treatments with 2.5 lobsters/m2 housed with an equal density of green crabs from one of three regions (Passamaquoddy Bay, St. George's Bay or Chedabucto Bay on the Scotian Shelf) and two control treatments with lobsters alone stocked at either 2.5 or 5 individuals/m2. Chedabucto Bay was chosen as a third region as crabs there are genetically similar to conspecifics in St. George's Bay, but experience environmental conditions more similar to Passamaquoddy Bay. Green crabs had a significant and negative impact on lobster survival during the experiment, and the magnitude of this impact varied in relation to the green crabs' geographic origin, as mortality was significantly higher for lobsters paired with St. George's Bay and Chedabucto Bay crabs than Passamaquoddy Bay crabs. These differences in lobster mortality patterns lead to two important conclusions. First, the greater predatory impact of green crabs from St. George's Bay relative to Passamaquoddy Bay is consistent with the contrasting results recently reported in the literature, suggesting that these are of biological significance and not simply the result of procedural differences between studies. Second, the fact that the predatory impact of Chedabucto Bay green crabs was similar to that of conspecifics from St. Georges Bay, but greater than those from Passamaquoddy Bay, suggests that these differences in interspecific dominance patterns are correlated with genetic differences between green crab populations related to their invasion history in Atlantic Canada. The mechanisms underlying the effect of crab geographic origin on its interaction with juvenile lobster remain elusive, however, as no differences in crab behaviour or morphology were found that correlated with the pattern of lobster mortality.