Physiological mechanisms of survival in the invasive green shore crab

The green crab (Carcinus maenas) is an extremely successful invasive species, thriving in marine environments around the world, and known for its high tolerance to environmental perturbations. The introduction of invasive green crab can have devastating impacts on ecosystems and native fisheries, increasing competition for habitat and resources, with green crab predating upon native organisms, and uprooting vital eel grass beds. Thus, to understand the environmental factors influencing invasive success, the physiological responses to dissolved organic nutrients, temperature, salinity, oxygen availability, and metal contamination were assessed in green crab from the Canadian Pacific estuaries and global capture sites. Recently, it was discovered that green crab could acquire dissolved nutrients across their gill epithelia from the surrounding environment. To evaluate any potential advantage of branchial nutrient acquisition, we compared the capacity for branchial amino acid transport of common crustaceans in Canadian Pacific waters via gill perfusion and whole-body exposure. Branchial active transport of L-leucine was exhibited in C. maenas, Metacarcinus gracilis, Metacarcinus magister, and Cancer productus across their gill epithelia, and is directly influenced by feeding status. This represents the first description of branchial amino acid transport in Canadian native arthropods, suggesting that branchial amino acid transport is a shared trait among arthropods, contrary to existing literature. The invasive success of global green crab populations is largely attributed to a wide tolerance to temperature in variable environments. Less is known about how environmental conditions such as oxygen availability, salinity, and metal contamination could alter the upper thermal limits, and whether the green crab has undergone physiological adaptations to adjust to new conditions across its invasive range. Here, we employ a multi-stressor approach to investigate the effects of oxygen availability, salinity, and metal contamination on the upper thermal limits of green crab invasive to the Canadian Pacific and Atlantic coastlines, and native to Sweden. Oxygen consumption and heart rate were measured during thermal ramping procedures under varying environmental conditions and used to predict temperatures resulting in cardiorespiratory adjustment and strain. Under control conditions, green crab from all capture regions exhibited an increase in oxygen consumption and heart rate from the 16 °C acclimation temperature to a peak around 34 °C followed by an abrupt decline and a subsequent collapse, marking upper thermal limits around 36 – 38 °C. Green crab from different regions also responded similarly to environmental oxygen, exhibiting consistent reductions in upper thermal limits (the point of metabolic collapse) in hypoxia and negligible improvements in hyperoxia. Acute exposure to sublethal copper concentrations caused a significant drop in respiratory breakpoints, with metabolic failure occurring between 36.5 – 36.8 °C, up to 2 °C sooner than breakpoints within control treatments. Here we suggest that acute copper exposure closely resembles the influence of hypoxia on thermal tolerance, with high copper accumulation within the gills and little accumulation within heart tissues. Finally, the Pacific green crab exhibited an upper thermal tolerance irrespective of environmental salinity, despite exposure to a full freshwater environment. Atlantic green crab exhibited cardiorespiratory failure at ~35 °C when salinity was reduced to 20% saltwater, which further decreased to ~32 °C during acute freshwater exposure. We suggest distinct differences in the upper thermal limits between green crab from two Canadian regions with respect to salinity, potentially indicating regional acclimation to exceedingly dilute environments on the Canadian Pacific coastline. Given the suggestion that green crab within the Canadian Pacific estuaries possess a particularly impressive tolerance to extremely dilute and freshwater environments, the survival and physiological responses to freshwater exposure were evaluated. With a measured freshwater LT50 of ~26 h, Pacific green crab exhibited a remarkably high tolerance to extremely dilute environments compared to historical values. Furthermore, Pacific green crab haemolymph osmolarity, branchial cAMP, oxygen consumption, haemolymph protein, and hemocyanin were unaffected within at least the first 6 h of freshwater exposure, indicating a lack of stress response. This could suggest an ability of Pacific green crab to increase their ecological range and access estuarine rivers to forage or avoid competitive interactions and predation. In combination, this thesis offers insights into the physiological response of green crab to a variety of environmental conditions that are expected to influence invasive population survival and range expansion, while drawing comparisons between global green crab at a regional level.

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