Shore crabs hyped up on caffeine

Hey you coffee lovers. Ever imagined swimming in an ocean of caffeine? This may not be a dream for you anymore, if you are a marine animal.

When you drink that cup of coffee in the morning, your mind is alert and your boss doesn’t have to yell at you. But what does caffeine do to marine animals? We are determined to find out. So we invited our green shore crabs friends (Hemigrapsus oregonensis) into the lab for a caffeine dive.

Green shore crab in caffeinated sea water. Photo credit: Sarah Ho

Green shore crab in caffeinated sea water.
Photo credit: Sarah Ho

We ground up caffeine tablets and mixed it with sea water. No cream, no sugar, not black though, but yellow due to the colour of caffeine tablets. The 4 litres of caffeine solution we prepared is roughly equivalent to 4 cups of coffee, which is quite a large amount for the crab’s small body. Then in go the green shore crabs for a 20 minutes coffee break. For comparison, some crabs were allowed a swim in a tank of just sea water.

During the 20 minutes given for crabs to adjust to their environment, we saw some interesting behaviour. While crabs in plain sea water stood around, even engaged in a wrestling match, the crabs in the caffeine tank were busy doing what seemed like eating undissolved caffeine particles. Their mouthparts moved so fast that someone might guess they’ve been starved for days. Occasionally, crabs would even move their claws in a sweeping motion as if to gather more caffeine particles at the bottom of the tank. Are crabs hungry for caffeine? I can only guess yes.

To find out what caffeine does to crabs, we used travel speed as a way of measurement. If caffeine brings us out of the slow sleepy mode, maybe it will excite crabs as well. It turns out that caffeine crabs travelled faster than sea water crabs, but our data was not sufficient for us to make the claim that this is actually the case.

Picture by: Jasneko

Picture by: Jasneko

Maybe we need to test more crabs to find out. Maybe crabs just need 10 cups of coffee to boost performance. So a tip for green shore crabs, to spend 3 dollars on a morning cup of coffee, the effect is probably not worth it.

But there’s no coffee shop under the sea, you say. Marine animals can’t buy themselves a cup of coffee, so what’s the point of this experiment? In reality, caffeine is making its way into the ocean as a pollutant through our waste disposal systems. Evidence does not point to a nice coffee break for marine life, but instead, a stress.

Find out more about how caffeine is entering the ocean and how mussels take the hit here:


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