Crabby Crabs on Sandy Shores

Poke, spin, shake, and feed, were some of the many possible things we could do to shore crabs collected from Tower Beach, near the University of British Columbia in Vancouver, Canada. We had 3 hours to come up with and carry out an experiment.

We knew that some crabs would burrow to avoid predators as crabs found above sand make for an easy meal for birds, fish, and larger crabs. The question we wanted to answer was whether crab size and wet or dry sand had an effect on shore crab burrowing times.

Green shore crab using claws to pinch a piece of paper

Green shore crab using claws to pinch a piece of paper

We worked with Hemigrapsus oregonensis, the green shore crab, and Hemigrapsus nudus, the purple shore crab. The purple shore crab was about 0.5 to 1cm larger with much larger claws. When the crabs pinched me while I was handling them, it was a shock! The pinch was strong enough to make me uncomfortable enough and let them go, though not strong enough to hurt me.

We had two tanks containing either wet or dry sand, which we let the crabs roam around in. After they burrowed, we let them rest and switched them to the other tank.

Crabs in wet sand

Crabs in wet sand

Out of 12 crabs, we had 10 crabs burrow in wet sand and only 2 in the dry sand. We found that the burrow times were much higher in wet sand than dry sand and that their size had no effect.

In the dry sand, they looked crabby as some had been roaming in it for 2 hours. We made sure to place them back in seawater every 30-45 minutes so that they could rehydrate.

At one point, we buried a crab in dry sand, to see how it would react. It didn’t have any response after being buried. After 5 minutes, we were worried it was feeling claws-trophobic and possibly drying out. I would be if I was buried and kept in a pitch black place! We removed it from the sand and placed it back in seawater. To our relief, it started moving around and looked to be back to normal.

Crabs in dry sand

Crabs in dry sand

The two crabs that did not burrow in the wet sand were our two largest crabs. One of them looked to be angry and aggressive as it would constantly stand on its back legs. Maybe it wasn’t afraid like the smaller crabs and that’s why it didn’t burrow. Or maybe the larger crabs were too large to burrow.

Why did most of the crabs’ dislike burrowing in the dry sand? Maybe because there is less/no moisture in the dry sand, and burrowing would cause them to quickly dry out. Maybe it’s because it’s easier for the crabs to move in the wet sand as the sand clumps together and forms spaces, while dry sand is loose and will immediately fill in any gaps.

In the future, it would be interesting to try this experiment with varying levels of water saturated sand.

Here is an interesting paper on oxygen distribution (in pore waters) in complex invertebrate burrow walls: http://onlinelibrary.wiley.com/doi/10.1111/j.1472-4669.2006.00074.x/abstract

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