Be careful with your crabs, they may be cryptic!


A longhorn decorator crab meditating

BIOl 326 at UBC doesn’t have an exam, and for that I’m grateful. What it has instead is a big, month–long independent research project. For the project, each student needs to choose a topic, come up with an experimental design and perform the experiments. After all of the data is collected, statistics must be run, and paper must be produced, a presentation must be given and a blog must be written.


This is my experience with my final research project.


A beautifully adorned decorator crab, covered in sponges. 


I chose crabs, but not just any crabs. I chose adorable, beautifully designed decorator crabs. I was interested in how aware the crabs were of their decorations and how well their decorations matched the surrounding environment.





Crab #14 was my favourite


Crab #1 pinched me every chance he got!

I went out with some friends to a wreck in Howe Sound, B.C. Canada. We went SCUBA diving to collect some adorable Longhorn decorator crabs and brought them back to the lab at UBC. I had also collected some seaweeds and sand, which I used as a substrate, or bottom, for the aquarium that I ran my experiments in. We I got back to the lab, I numbered and analysed each crab, recording sex, what decorations it had covering it, carapace (shell) size and the amount of stuff growing on it, in a percentage. I also noted if they were orange or brown in colour. Thankfully. Explanation to follow.




My experimental setup

I made half of the aquarium tank covered in sand and half of the tank covered in seaweeds. I then placed each crab in the tank, one at a time and recorded what each one did for 5 minutes. After all crabs went through this experiment, I added some water from a tank that held a big male Dungeness crab, Big Dan. Big Dan was my predator. I put each crab through the experiment again and recorded what each one did for 5 minutes. I changed the bottom of the tank with different substrates and ran the experiment twice again, once with normal seawater and once with Big Dan water.



Big Dan’s free-living cousin

I won’t bore you with the statistics that I did, but my results were a bit odd. When I was just about finished writing my paper, I came across a webpage that had some background info on the Longhorn decorator crab, including how to tell it apart from its cryptic cousin, the Graceful decorator crab.


Oh no. My brown crabs weren’t just brown Longhorn decorator crabs … they were Graceful decorator crabs. What I thought I was testing in my experiment wasn’t actually what I was testing. Luckily I recorded which crabs were which and I was able to distinguish the Longhorn decorators from the Graceful decorators. After 24 hours straight, I was able to change my stats to match the tests I actually did and finish my paper.



A Graceful decorator crab. Notice the spikes, one behind each eye. One of the distinguishing differences between the Graceful decorator crab and the Longhorn decorator crab

The results? Graceful decorators are more decorated, and surprisingly they run and hide faster than the less decorated Longhorn decorators. I’m sad to say that Big Dan just isn’t that scary. He had no effect on either species of my crabs, sorry Big Guy.


So what was the lesson I learned? Know your crabs! And, that there I so much more that has yet to be studied about decorator crab behaviour.

By Jake Dytnerski. All photos by Jake Dytnerski


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