A lot of people these days have trouble relating to the trials and tribulations of life in the intertidal. Living in constant fear of being nibbled on by a bird or gobbled up by a fish is the stuff of Hitchcock and Spielberg, right? But being scared of what’s around you is a big part of living in the crab-eat-crab world that is the great big blue. Especially for the green shore crab!
Scientists talk a lot about the “ecology of fear”, and what that means in the natural world. Essentially, the ecology of fear is the study of how being scared can change the impact you make on your surroundings. As a crab, maybe you eat at different times of day to avoid that nasty seagull afternoon rush, or you spend your time at high tide hanging out in your burrow for fear of being carted off to a fish buffet. What you choose to eat and where you spend your time changes when you’re scared, and this can have a big impact on ocean ecosystems.
But what about the impact your surroundings make on you when you’re scared? Does your strategy to run or to hide (or perhaps to stay veeeerrrrrryyyy still!) change depending on where you are?
For my independent project, I wanted to find out how green shore crabs change their behaviour when they’re scared depending on their surroundings. Do they react differently to threats when they’re above water at low tide, or below water at high tide? And if so, why?
I started by seeing how my crabs behaved normally, without any threat of predators, both submerged underwater and on dry sand. If you want to see how crabs act when we’re not around, you have to check them out at night! So that’s just what I did…
Crabs don’t see red light, so I could watch their regular night time behaviour without them seeing me! As you might expect, the crabs just hang out. Pretty standard stuff. No running away. No burrowing. No nothing. This happened both in the crabs that were underwater and in the crabs that were on dry sand. So what about when they’re scared?
I set up two different “predator” threats. The first one was just under standard lab lighting so I could see how the wet crabs and dry crabs reacted to the normal sort of slow threat, like rock flipping they’d encounter on a busy beach. The second one was a GIANT PENDULUM OF DEATH.
Just kidding, no death! But it was a large pendulum under a bright lamp that cast harsh shadows over the crabs to simulate a bird swooping in for the kill. And what did I find?
I found that the crabs underwater and the crabs on dry sand reacted differently out of fear! The underwater crabs chose to run away and hide under some rocks, and the dry sand crabs chose to burrow into the sand and do their best impression of a statue. But it gets more interesting!
The underwater crabs weren’t very good at telling apart the regular slow threat from the swooping bird, so they weren’t more scared of the swinging pendulum. But the dry sand crabs burrowed even deeper and stayed even more statuesque when under the pendulum. Why is this?
Well, I think that the underwater crabs are using both their eyes and their keen sense of underwater smell to sense their surroundings, so they aren’t relying on vision alone. This might make it harder to tell if you should be scared of something above the surface of the water, because you can’t smell it! They might also have a harder time seeing the shadows from below the surface, which would make them less scared of the swooping bird.
I had a pretty awesome time working on this project, and I got to learn that not only does an animal’s decisions when it’s scared impact the environment, but an animal’s environment impacts its decisions when itis scared! Science rules!
If you’re interested in being scared yourself, here are some crabby links:
Run, crabs, run!
Birds have some things to be scared of too!