Blue beats green: Agonistic interactions between Atlantic blue crabs and European green crabs in the Gulf of Maine

ABSTRACT As ocean temperatures rise, range expansions and biological invasions are increasingly reshaping marine ecosystems. Warming waters are promoting the northward expansion of the Atlantic blue crab ( Callinectes sapidus ) into the Gulf of Maine (GoM) where its arrival has resulted in habitat overlap and novel interactions with the invasive European green crab ( Carcinus maenas ), a pervasive resident of coastal GoM for over a century. Through a series of laboratory trials, we assessed the interactions of paired blue and green crabs sourced from the GoM, specifically investigating the effects of crab size, aggression, and availability of alternative prey on these interactions. Blue crabs were found to be effective predators of green crabs, killing them in half of trials. Blue crabs were more likely to kill smaller green crabs (< 65 mm carapace width), and small to mid-size blue crabs (< 170 mm carapace width) were more likely than large blue crabs to consume green crabs. There was no green crab predation on blue crabs, though larger green crabs (> 65 mm carapace width) displayed more aggressive behavior towards blue crabs. Presence of an alternative prey item had no effect on blue crab predation on green crabs. Blue crab predation on green crabs could shift future ecosystem dynamics, altering community ecology in the GoM. This study highlights how climate-driven range expansions can mediate interactions among introduced species, with implications for ecosystem structure and species management.