They shore like rocks!

If we give Mr. Krabs a choice between a refuge made with rocks and no refuge, what would he choose? What if we shock Mr. Krabs with super salty water and freshwater, will his decision be the same? Let’s find out!

crab

Hemigrapsus oregonensis in its natural habitat (PC: http://bugguide.net/node/view/521931/bgpage)

When you walk along the beach during low tide, have you ever lift up a rock and look what is hiding underneath? The green shore crabs (Hemigrapsus oregonensis) in the intertidal zone does not live inside an anchor like Mr. Krabs, instead, they tend to live under rocks. Underneath the rocks, the crabs have protection from predators like birds, fish, and larger crabs. Since they heavily rely on rocks for shelter, we decided to test if there are any circumstances when Mr. Krabs and his friends forget how to get home.

In Vancouver, the organisms that live in the intertidal zone experience different salinities depending on the different seasons. Unlike Mr. Krabs’ home at Bikini Bottom, the water near the water surface goes through salinity fluctuations due to freshwater that rushes into the ocean. The melted snow from the top of the mountains drains into Fraser River in Vancouver, then the freshwater is dumped into the Pacific Ocean. Therefore, in the summer, intertidal water is not as salty as in the winter. With that in mind, we were curious about how well our crab friends can find their shelters at different salinities.

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In a small aquarium, we built a nice and cozy rocky home for the crabs. One crab at a time (so they are not distracted by their friends), we give each of them two minutes to choose between the empty side and the home side. We did the same thing for three different salinities: 15 ppt, 31 ppt (normal), and 45 ppt.

At the end of the experiment, we found that saltier water did not confuse the crabs’ instinct to go home because we got the same result for saltier water (45 ppt) as for normal water (31 ppt). However, low salinity (15 ppt) seemed to influence the crabs. We found that some of the crabs did not frantically run for their lives and hide under the rocks right away in the low salinity treatment. Instead, they seemed to have forgotten which way was home. There are a few different possible explanation that can help us make sense of such behaviour. Maybe Mr. Krabs’ friends’ activity rate is lower in low salinity because they are using their energy trying to adjust to the new salinity. Follow-up experiments should be done to investigate the causation of such confusion.

The green shore crab is an interesting organism to study! If you like to learn more about their natural habitat and life history, check out this link:

http://people.oregonstate.edu/~yamadas/crab/ch5.htm

Next time you are at a beach during low tide, make sure you gently flip over some rocks and say hi to colonies of cute shore crabs!

 

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