Kleptoparasitism and scavenging by the invasive green crab (Carcinus maenas) have different impacts on native species

Feeding interactions with invaders have powerful impacts on native biota. Recently, a trophic interaction among invasive green crabs (Carcinus maenas) and native dogwhelks (Nucella lapillus) foraging on mussels (Mytilus spp.) in Atlantic Canada was reported, in which a mussel initially attacked by a dogwhelk was then eaten by a crab. This was interpreted as kleptoparasitism, but alternatively could be scavenging of abandoned mussel prey. In this paper, mathematical models of kleptoparasitism and scavenging among these species were developed and used to generate predicted feeding rates, which were then compared to observations from previous work. The available evidence for kleptoparasitism by green crabs on dogwhelks was also considered and compared to hypothesized outcomes of these different foraging modes. The evidence considered does not definitively suggest that scavenging was previously misidentified as kleptoparasitism; in fact, model simulations were only able to match observed foraging when they included kleptoparasitism. Effects of crab kleptoparasitism on whelks were shown to be negative, while crab scavenging had effectively no impact on whelks; therefore, while these two foraging interactions appear superficially similar, they have very different implications and should be carefully distinguished in future studies.