How Porcelain Crabs live in a World of Ups and Downs

Before deciding on the time and destination for the next year’s beach holiday, one might want to check the climate graphs of the relevant regions, to make sure not to end up in heavy rain or being roasted. But for a myriad of other organisms living at the shore, they experience the climate on a much smaller scale. The so-called microclimate can vary substantially on a small spatial scale, compared to the climate of the region. In the intertidal zone of a particular shore for instance, immersion and emersion periods differ with height. And accordingly, temperatures experienced by the organisms living there can vary extremely.

To investigate the microclimatic variation in such an area, a team of scientists set out to a boulder shore in northern California. Gunderson and colleagues tracked down precise and fine-scale changes in temperature in a common habitat at such a shore: the air spaces under rocks. One of the first findings was that, in areas higher up the shore the temperature under rocks features a greater daily range than that in lower areas in the intertidal zone, especially in summer and spring.

All temperatures recorded by the team of scientists over 18 months under rocks in the high intertidal zone (HIZ, black) and the mid intertidal zone (MIZ, grey), together with the daily mean values in red and blue.

The collection of such a dataset on a particular habitat enables scientists to better understand the effect of microclimatic variation on the beings living there. The porcelain crab Petrolisthes cinctipes, for instance, is one among the large variety of animals flourishing in the intertidal zone of rocky shores. Together with other crustaceans, fish, mollusks, sponges and others, these small crabs live in moist air spaces under rocks. The scientists found that, in average, the porcelain crabs preferred a water temperature of 15.0°C. They tried to escape to colder waters, if the temperature climbed to 20.5°C (mean value). The heat seemed to be particularly unbearable for larger crabs, they retreated earlier. They also found that the crabs could tolerate heat up till 30.5°C in average. And here again, smaller crabs seemed to be more robust. Why is that? Science has not come up with a satisfactory explanation yet. So far it only tells us that, when addressing the question of how the microclimate influences the ecology, one would need to consider how differently the organisms in question are equipped.

Knowing the temperatures under the rocks and the temperature preferences of the crabs of different sizes, the scientists made a simulation of the distribution of crabs on the shore. It predicted proportionally more large crabs and fewer small crabs in the mid intertidal zone than in the high intertidal zone. This is indeed what the scientists found when counting crabs on the actual shore: higher up the shore, where it got hotter, they found a higher proportion of smaller, more heat-tolerant crabs.

But even if they are less susceptible to overheating, how do they manage to survive here? The temperature data showed that the overheating threshold of 30.5°C was regularly reached in the high intertidal zone. Luckily, the data also showed a high degree of spatial thermal variation: the average temperature difference was 5.2°C between the warmest and the coolest daily maximum of under-rock air spaces and on some days the difference was even over 25°C. This means, even if it gets pretty hot in one spot, there are always some other spots that are cool enough where the crabs could seek refuge, all thanks to microclimatic variation.

Frederike Klimm is a PhD student in the Plant Biomechanics group at University of Freiburg. Her research interests are focussed on attachment systems of climbing plants and marine algae. Twitter: @Kelp_Action

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