Size affects intraspecific aggression and response to predation threat in juvenile American lobsters

Abstract

Intraspecific aggression and competition can govern access to food and shelter in many marine organisms, but these interactions can be impacted by life stage and presence of predators. Juvenile lobsters (Homarus americanus) have social hierarchies that govern access to valuable resources like food and shelter, but these hierarchies could be impacted by the presence of invasive green crabs (Carcinus maenas). The goal of this study was to determine whether the presence of green crab effluent affected aggressive behavior in different sizes of juvenile lobsters, with the hypothesis that all juvenile lobsters would be more likely to share shelter under threat of predation. Small juveniles (15 mm carapace length) and larger juveniles (45 mm) had substantially different interactions and responses to predator threat cues, with small individuals more likely to both fight over shelter and share shelter when exposed to green crab effluent. There were also several size-based behavioral differences in control treatments, as small juvenile lobsters challenged each other more frequently for shelter and spent vastly more time in the shelter, less time stationary outside the shelter, and less time fighting. Despite being vulnerable to green crab predation, larger juvenile lobsters were generally unaffected by crab chemical cues. This suggests that life history plays a strong role in predator avoidance behavior and that both lethal and nonlethal effects of invasive species on native prey can vary markedly with life stage. Future research should examine these crab-lobster interactions in lobster nursery habitats in the field, as invasive green crabs in these habitats could change behavior, activity patterns and survival of juvenile lobsters.