The European green crab isn't too fussy about where it lives. Not content with its native range, which extends from Iceland to northern Africa, the green crab has also set up home in Japan, Australia, South Africa, Argentina and North America. These globe-trotting tendencies make the green crab an excellent model to study how animals cope with temperature changes, explains Carolyn Tepolt of Stanford University, USA. Enlisting the help of George Somero, Tepolt decided to explore the temperature tolerance – and ability to fine-tune that tolerance – of green crab populations around the world (p.1129).
However, Tepolt realised that she wouldn't be able to bring crabs back to her lab – most countries are understandably reluctant to allow highly invasive species to cross their borders – so she would have to take the lab to them. Technician John Lee helped Tepolt transform cooler boxes into portable acclimation tanks by fitting them with aquarium heaters and chillers; but she also needed a simple method to measure the crabs' heart rate. ‘Heart function is a common measure of heat tolerance in cold-blooded animals’, Tepolt explains. ‘As temperatures rise, the animal's heart rate increases. When you reach the animal's maximum tolerated temperature – the critical temperature – the heart rate suddenly plummets.’ To let Tepolt measure crabs' heart rate in the field, Lee constructed a non-invasive portable monitor that could be attached to the crab's shell just over the heart. By measuring infrared light bouncing back from a crab's expanding or contracting heart, Tepolt was able to record heart rate.
Taking her portable lab, Tepolt travelled to seven sites around the globe (Norway, Portugal, three sites along the east coast of North America and two sites along the west coast) to test the thermal limits of their green crab populations. At each site, she trapped green crabs and placed them in acclimation tanks for several weeks, keeping one group at 5°C and another group at 25°C. She then tested the heat tolerance of each group by attaching heart rate monitors to the crabs, ramping up the temperature in the tanks by 5°C an hour, and measuring the critical temperature at which the crabs' heart rate plummeted. To test the crabs' cold tolerance, she decreased the temperature to 0°C and measured their average heart rate. Tepolt expected that green crabs, as an invasive species, would have better heat tolerance than non-invasive species. Sure enough, she found that the average critical temperature for green crabs was 34.5–36.5°C, higher than the 30–35°C critical temperatures previously recorded for other crabs and lobsters living at the same sites. Green crabs were also remarkably cold tolerant; they all survived short-term exposure to 0°C.
But the real key to green crabs' invasive success may lie in their ability to fine-tune their response to shifting temperatures. Tepolt saw that warm-acclimated crabs coped better with higher temperatures than their cold-acclimated counterparts: the critical temperature for 25°C-acclimated crabs was 1.8–2.4°C higher than that seen for 5°C-acclimated crabs. ‘This suggests that green crabs can shift their thermal limits through acclimation’, says Tepolt. However, even after they had acclimated to the same temperature, crabs found in Portuguese waters were better at coping with heat and fared worse at chillier temperatures than their Norwegian cousins. This is exciting, says Tepolt, because it suggests that crab populations are locally adapted to their environments – and that perhaps not all populations are equally suited to invading new regions. In the face of global warming, such variation may mean the difference between life and death.