Holy crab, shore crabs can really run!

Have you ever been late for the bus? I definitely have. I’ve left my house, hoping I have enough time to get to my stop, only to see my bus half a block away. Then I have to decide if it’s worth running. As if it’s not embarrassing enough that I’ll get there out of breath, if it’s windy and raining and I have my umbrella up, you already know everyone will see me fight the wind as my umbrella leads me around the sidewalk like an over-enthusiastic dog on a walk.

An artistic rendition of how I get to the R4. Image source: https://giphy.com/gifs/mary-poppins-KjSkkxgHBMqI0

Shore crabs sometimes face something similar. They live in the intertidal, feeding on tiny plants and animals, where they’re regularly exposed to danger. To protect themselves, shore crabs hide under rocks where it’s cool and wet. If you flip over a rock on the beach, sometimes you’ll see shore crabs run to the nearest rock. Now, imagine trying to do that while waves crash over you. That might make things challenging, even for the best of us.

A look at how crabs might run for rocks if their parents enrolled them in ballet classes. Image source: https://giphy.com/gifs/crab-gay-nPTXD4lJmDSTe

In the lab, we decided to see how fast shore crabs run to different-sized rocks, with and without turbulence. We set up two tanks, one with big rocks and one with small rocks and gravel.

Small rocks and gravel on the left, large rocks on the right. Image credits: Elliott Evans

Then, we timed how long it took crabs to get to the rocks. To simulate the terrifying turbulence the crabs might experience on a glorious day at the beach, you may think we used some fancy machine that generates waves inside small environments. You know, science-y stuff. In reality, we used plastic rulers to stir the water.

Simulating tidal turbulence using a high-tech, state-of-the-art wave generator plastic ruler. Video credits: Paige Barnes.

We thought since larger rocks offer more protection (and probably have more space, like a bigger bus), crabs would run faster toward larger rocks. We also thought crabs would run faster with turbulence since getting caught in turbulent waters can leave crabs exposed (sort of like me getting ridiculed by my fellow passengers when I run with my umbrella).

Our brave crab comrades are ready to run! Despite their common names, yellow shore crabs (Hemigrapsus oregonensis) and purple shore crabs (Hemigrapsus nudus) come in a variety of colours! Image credits: Elliott Evans.

So, were we right? Sadly, we didn’t have strong enough evidence to find out. But hey, that’s science for you.

And even if we couldn’t conclude anything for certain, we did see some interesting patterns. For one, we noticed small crabs generally ran faster than big crabs, which surprised us. We thought big crabs would run faster because of their size, but the lil’ fellas really took off. Why? We’re not sure, but maybe small crabs feel more vulnerable than big crabs, so they run faster toward safety.

A big crab takes its time. Video credits: Elliott Evans.

We also noticed that when we generated turbulence, the currents sometimes lifted the smaller crabs off the bottom of the tank. The little crabs started floating around and couldn’t gain as much traction as some of their chunkier chums. Even still, we noticed that the crabs generally ran faster when there was turbulence.

A crab floating in the gentle ocean current. Video credit: Paige Barnes.

Can’t get enough of shore crabs? No need to get crabby, there’s plenty more crab content out there. Here’s a publication on shore crab habitat preference and a comparison of heat tolerance in the two species we looked at in our study.

Not a fan of the hard-core science? No worries, here’s another blog post about crabs and their place in our culture.