Little shore crabs and their “big bad wolf”

You may recall the three little pigs and the big bad wolf as one of your favourite childhood fairy tales but have you heard about the tale of the little shore crabs, Hemigrapsus oregonensis, and their big bad wolf. So, it starts off with, once upon a time, there were little shore crabs who were happily living together with all their crab friends. One day, all the them decided to set out for an adventure in the ocean. They were half way through the journey when suddenly this enormous Metacarcinus magister or Dungeness crab started to use its claw to grab the little shore crabs. Terror and panic spread like wildfire in all the little shore crabs who immediately sought refuge under rocks. It was here that it became clear that the Dungeness crab was the big bad wolf of the ocean. It turns out that these biotic interactions are very important in marine ecosystems. Along with this, fluctuating abiotic factors such as temperature that are to be introduced with climate change could potentially affect the predator-prey interaction. The predator-prey interaction is significant in intertidal food webs and balances needs to be maintained to have a thriving ecosystem.


The Dungeness crab is a predator of the shore crabs. Predators will induce fear in organisms and most will face a trade-off between feeding or seeking refuge. Furthermore, climate change will drastically change the ocean by making it more acidic and warmer. Which factors, abiotic or biotic play a stronger role?



I took matters into my own hands in the name of science and tested both the abiotic and biotic factors. I tested the effects of temperature and presence of Dungeness cue water on the behaviour of shore crabs. I observed both refuge seeking and feeding behaviours of crabs. I would place the crab on one side of the tank and see how likely they were to seek refuge under a rock or begin to eat a mussel. I found that the big bad wolf of the little shore crabs was greatly affecting their behaviours. They were more likely to seek refuge and less likely to eat in the presence of the Dungeness cue water. On the other hand, temperature was not a driving force in predicting their behaviours. Previous research has indicated that higher temperatures would be linked to increased metabolism resulting in the crabs feeding more but that was not observed.


Hemigrapsus oregonensis feeding on a mussel. (Photo Credit: Sukhjit Cheema)


My experiments clearly demonstrate that biotic interactions play vital roles in the marine food webs. If the balance is disrupted, then the functionality will deteriorate as well. We don’t want the tale to end up with the big bad wolf blowing the crabs’ homes away so we must continuously monitor predator-prey interactions to understand the potential implications on future species composition.




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