If You’re a Bird, I’m a Bird, but We Still Won’t Fool the Crabs

In grade 1, I had a teacher who was very into just letting us create and discover on our own. She would strum her ukulele and say things like “Okay children, I have some sticks and some yarn, make whatever you want with it for the next hour” or “Paint a picture of your house using only lunch leftovers, you magical beings of light”. That was essentially the vibe in BIOL 326 for our Shore Crab Lab, although more specifically it was “Okay stressed-out twenty-somethings, we have these buckets of Hemigrapsus oregonensis, do some science with them!”. And oh boy did we ever!

21Hemigrapsus oregonensis - yellow shore crab.jpg

The bucket of crabs in question contained Hemigrapsus oregonensis, a common shore crab here on the west coast of Canada. They come in a variety of colours, from green – purple, and are distinguished by the little hairs on their legs, called setae. Photo courtesy of micksmarinebiology.blogspot.ca

Hemigrapsus oregonensis is pretty much the poster organism for primary school field trips to the tide pools. You would flip over rocks, pick them up and they would skitter over the palm of your hand, distracting you from what ever the teacher was saying about tidal zones or not going to close to the water or whatever. These small shore crabs feed primarily diatoms and green algae, though as scavengers will eat pretty much anything. They are fed on by birds, fish, other crabs, and once by a boy named Cohen in my elementary school (who was dared to do so).

So my lab partner and I were handed this bucket of shore crabs, and told to do science. What we came up with was this: Would a crab’s reaction in the face of death-by-bird change if they were stranded out in the open, vs if there was a shelter just out of reach? We felt that left totally exposed, they would probably stay still and hope to go unnoticed, but with a shelter present, they would possibly make a dash for safety. However, all of these questions are predictions hinged on one simple assumption: that we could trick the crabs into thinking there was a bird overhead. We decided to do this by waving a notebook between them and a light, creating a sudden swooping shadow. It looked pretty good to us, but when you assume, you make an ass out of u and me.


Image credited to The Little Mermaid, gif courtesy of ohmydisney.com

80 trials later, 4 treatment groups, shelter vs no shelter, “bird” vs no “bird, my arm nearly falling off from all the notebook swooping and our data was as insignificant as it could get. Sometimes when you have collected data, and you are running your statistics you think for sure what you are seeing has to be significant, only to be crushed when you realize that it may have only been due to chance. This was not one of those times. In every one of our treatments, exactly half of the crabs stayed still, and half ran around, shelter or no shelter, “bird” or no “bird”. So what did we discover in our five hours of scientific crab discovery? That these intertidal dwellers were SO not fooled into panicking by me waving a notebook around, pretending to be a bird. And that my friends, is science.




Image and cheesy quote courtesy of The Notebook, gif credited to Sharegif.com


Image credited to The Little Mermaid, gif courtesy of Your Tang0




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