If you've ever had a fever, you'll know how rotten it feels when your temperature rises by even a few degrees. Now imagine yourself in the position of a porcelain crab on the California seashore as the tide withdraws. ‘During low tide, air temperatures in the intertidal zone fluctuate dramatically over short periods of time (e.g. 20°C in 6h)’, says Jonathon Stillman from San Francisco State University, USA. And it could be about to get more uncomfortable as climate change tightens its hold, temperatures rocket and ocean pH falls. Stillman explains that most studies that have assessed the potential impact of climate change have analysed the effects of high temperature and reduced pH over extended periods. However, intertidal species experience massive temperature and pH variations on a daily basis. ‘Scientists who study the effect of climate change on marine organisms have been clamouring for studies that look at multiple stressors that vary in an environmentally realistic sense,’ says Stillman. So he and his student, Adam Paganini, set about building a simulated seashore to find out how porcelain crabs will cope in 2100 (p.).
Constructing the coast simulation from a series of pumps and individual acrylic cylinders, each of which became home to a porcelain crab, the team drained water from the cylinders at 12:00h each day, to simulate low tide, inundated the cylinders at 18:30h to mimic the tide's return and lowered the pH each night before allowing it to return to normal around dawn to recreate the natural nocturnal pH dip that occurs along the California coast. ‘The aquarium system was difficult to design and construct’, admits Paganini, adding, ‘the biggest challenge…was manipulating the pH on a daily high tide cycle and having the pH return to equilibrium before the low tide’. However, after a year of fine tuning, Paganini was able to simulate the modern environment – by allowing the air temperature to rise from 11°C to 25°C – and moderate climate change – by warming the air temperature still further to 30°C. In addition, he lowered the water pH each night from pH 8.1 to 7.6 to recreate the current environment and lowered it more – to 7.1 – for simulations of the future. Then, after allowing the crabs to adapt to the conditions for 17 days, Paganini began assessing the impacts that each environmental scenario had on the animals by measuring their oxygen consumption (respiration rate), and their heart rate while he increased the temperature.
Teaming up with Nathan Miller to analyse the data, Paganini and Stillman could see that the increase in temperature had a more significant impact on the crabs than the drop in pH. They also saw that the crabs that experienced the hottest low tide temperatures and largest nocturnal pH fluctuations were in the most trouble, reducing their respiration rates by 25% – suggesting that they had a reduced energy budget. The team also suspects that these ‘future’ crabs were investing more energy in general body maintenance, as they were able to tolerate slightly higher air temperatures than the modern-day crabs.
Based on the complicated range of responses that the team observed, Stillman says, ‘Our current findings suggest that acclimatization capacity may be beneficial in the short-term yet detrimental in the long-term’. He adds, ‘We think that in the future crabs may have less energy available for growth, reproduction and behaviour’, and he hopes that this approach of simulating the environmental variability in current and future seashores is exactly what the climate change community has been looking for.
