Surviving winter on the Rock: how European green crab (Carci-nus maenas) utilizes thermal tolerance and habitat use to tolerate the cold-water temperatures in Newfoundland

Abstract

European green crab, Carcinus maenas, is a successful invasive species and has established populations on every continent, except Antarctica. To date, the coldest winter-time water temperatures experienced by green crab in the northeastern Atlantic are observed in Newfoundland, where temperatures regularly drop below 0 °C during the winter. Previously, thermogeographic models predicted that Newfoundland winter water temperatures are too cold for green crabs to survive and therefore to colonise Newfoundland. Previous research on thermal tolerance of green crabs has primarily focused on their upper thermal tolerance; and any investigation into their lower thermal tolerance has only considered acute responses. In this thesis, I use laboratory experiments to study multiple physiological and behavioural responses of green crab to long-term exposure (ranging from 6 days to 5 months depending on experiment) to water temperatures at the lower end of their thermal tolerance. I also use field experiments to investigate over wintering movement behaviour, habitat use, and feeding behaviour of green crab in Newfoundland. With this combination of lab experiments and in situ field studies, I show that green crab can survive several months exposure to cold water temperatures as low as -1 °C. Temperatures between 4 to 6 °C trigger a torpor-like response, with reduced cardiac and metabolic rates, locomotive activity and feeding behaviour. However, movement and feeding do not completely cease, which indicates that green crab enter a ‘torpor-like’ state rather than true hibernation. Additionally, green crabs are absent from the intertidal zone during the winter but they do not leave the shallow bay for deeper waters, contradicting hypotheses that green crabs either leave the shallow subtidal zone for deeper, more stable water temperatures, or entered a state of hibernation and bury in the mud or hide under a shelter for the duration of winter. These results show green crab can tolerate the extreme cold temperatures during Newfoundland winters without altering their behavioural strategies from that used in their native range, suggesting they may be able to survive long periods at even colder temperatures, indicative of potential further northward geographical range expansion.