Hide and seek: habitat-mediated interactions between European green crabs and native mud crabs in Atlantic Canada

Abstract

The non-indigenous European green crab (Carcinus maenas) has well-documented impacts on many native species. In the Atlantic Canada region, the green crab distribution is increasingly overlapping with the distribution of mud crabs (primarily Dyspanopeus sayi), a prominent native species. Despite the potential for antagonistic interactions, the relationship between the two species has not been examined, particularly in the context of the diversity of habitats available in the region. This study used observational beach seine data collected between 2009 and 2013 from the southern Gulf of St. Lawrence to explore the temporal and spatial relationships between mud crabs and green crabs, and detected a negative relationship between these species. Twenty-four-hour laboratory experiments examined their predator–prey interactions and assessed the influence of habitat complexity on the outcomes of these interactions. Mud crabs and similar-sized green crabs collected during July and August of 2010 and 2011 were used as prey for large green crab. These predators consumed almost twice as many mud crab compared with juvenile green crab in the two less structured habitats (no substrate or sandy substrate), but predation rates were statistically similar in oyster bed habitat. In that particular habitat, mud crab mortality dropped by ~65 %, whereas the generally lower mortality affecting juvenile green crabs was unaffected by habitat. These results suggest that complex habitats mediate predator–prey interactions and dampen the effect of green crab prey preference. As green crab continues to invade areas dominated by mud crabs, they may threaten the sustainability of this native species.