I’m Not Used to the Cold!!! An Experiment on Shore Crab Cold Tolerance

Animals that live in temperate intertidal zones are subject to daily temperature fluctuations, as well as temperature extremes. This means that the animals that live there year-round must be able to survive in a relatively wide range of temperatures, from freezing temperatures in winter to around 20 and 30 ºC in summer. Many organisms, including the green shore crab Hemigrapsus oregonensis is able to acclimate to the temperature in its environment. However, when it gets down to a certain temperature (usually below 3-4 ºC), the green shore crab starts to lose physical function, and eventually enters what is known as a chill coma. This is where the crab is basically in a state where it cannot move at all, but is otherwise still alive. Even going down to below-zero temperatures, the crab will still be alive, albeit for a limited period of time.

Hemigrapsus_on_Orcas_Island

Hemigrapsus oregonensis: The green shore crab. Isn’t it awesome? Photo from: http://upload.wikimedia.org/wikipedia/commons/c/c8/Hemigrapsus_on_Orcas_Island.JPG

Previous research on the shore crab has shown that acclimatization to colder temperatures increases its tolerance to the cold, but decreases its tolerance to heat. The opposite has also been observed:  acclimatization to warmer temperatures increases its tolerance to the heat, but decreases its tolerance to the cold. What this means is that if the temperatures are usually cool, then the crab will acclimate and be better able to withstand cold temperature extremes, and vice versa. However, this could also put the crabs at risk, especially in the wake of climate change.

Average global temperatures are on the rise, as are world ocean temperatures. This means that animals like the green shore crab could be at risk of having decreased abilities to withstand cold temperature extremes. This is the reason why I wanted to carry out an experiment on the crabs, in order to find whether the crabs from cooler temperatures fare better than crabs of warmer temperatures when faced with cold temperature extremes.

My experimental setup was as follows. I set up three temperature tanks (12 ºC, 16 ºC, and 26 ºC) and placed 15 crabs in each of them and let them acclimatize to the temperature for 24 hours. Unfortunately, a heater malfunction in the 26 ºC tank resulted in the tragic death of all 15 crabs and so I was not able to run any experiments on them. For the crabs that were left, I induced chill-comas in all of them, and recorded the amount of time it took them to recover (by placing them on their backs, and stopping the time when they manage to flip themselves over). The results were clear.

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Just chillin’ in the lab: Crabs in a state of chill coma. Photo credit: Percy Lee

Crabs at acclimated to 12 ºC fare better than crabs acclimated to 16 ºC. The 12 ºC crabs recovered after 161 seconds (on average), whereas the 16 ºC crabs recovered after 350 seconds (on average). Even a relatively small temperature difference of 4 ºC was enough to see recovery time more than double! Thus, it is important to keep researching the effects of rising temperatures on temperate intertidal zones, if we are to save them.

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