Hermit Crabs and Shell Choice

I went to the second Lowell lecture at the NEAq last night which will apparently be posted on the PBS Forum Network sometime in the not too distant future. This technology is new to me, but apparently it’s where PBS hosts videoed lectures. Simple enough. The url is: http://forum-network.org/. I searched for the first lecture and couldn’t find it there and not everything is science related but maybe you’ll find something interesting.

In any event the lecture last night was on hermit crabs. I spend a lot of time with hermit crabs at my job right now so it was nice to learn a little bit more about them. The lecturer, Randi Rotjan, is a post-doc fellow at the NEAq and spends most of her time studying corals but during one trip to Belize she was struck with some bad weather that prevented her from diving. So she turned to studying the 1084 hermit crabs on the island of Carrie Bay Cay. Yes, she counted.

The basic focus of her research on these animals is on vacancy chains. This is what happens (not just with hermits but with any organism that uses discrete, limited and reusable resources) when one crab chooses a new shell and leaves his old shell for another crab to occupy who then leaves his old shell behind, etc. etc. Just one switch creates a chain of switches and she has found that around 10% of the population in the wild can end up in a better shell. This process has been compared to human behavior in real estate and also with job openings.

Before going off to Belize and introducing the new gastropod shells there she did a lot of work in the lab trying to figure out what factors hermits were using to decide on a new shell. Two major factors were looked at: crowding index (which is basically a discrete way to measure how well the hermit fits in the shell) and shell damage (how damaged the shell is, very technical). It turns out that damage has a lot more to do with choice than the CI (crowding index) and vacancy chains will generally end when one even moderately damaged shell is discarded. Also in ten out of thirteen trials a crab with a damaged shell won out in a dispute over a new shell against a crab with a slightly crowded shell.

Once studying these behaviors in the lab and in the field with terrestrial hermit crabs she (being a marine biologist) wanted to look at them in marine crabs. Luckily the Boston area has its very own native marine hermit crab, Pagurus longicarpus, the long-wristed hermit crab. Dr. Rotjan also wanted to learn more about the social nature of these vacancy chains. She had already discovered that there are two kinds of vacancy chains: synchronous (where crabs are all together and one switches and then the next then the next and so on in about five seconds) and asynchronous (where the crabs are further apart and shell switching happens with gaps of more than five seconds). Her data already showed that population density was a factor because the more crabs in total the more “waiters” were present which are little crabs that hang out by large shells waiting for a crab with the right sized shell to come along and drop it.

So gathering all that together she found two populations of pagurus longicarpus with different living conditions. One had somewhat poor conditions (more damaged shells) and the other had better conditions. It is still unclear why the different areas have these different conditions but it provided her with animals who were accustomed to different social pressures when choosing a shell. Interestingly she found no difference between the two groups in terms of how crowding and damage influencing their vacancy chains. She also found no difference in the average lengths of the vacancy chains. But what she did find were very different social behaviors in the poor condition group such as far more waiting, longer investigations of a single shell, more aggression and generally more interactions with their fellow hermits. More work is going on and I really can’t wait to see how it turns out. Seriously!

Just one more fun fact about hermit crabs: all of them, even terrestrial crabs, have a marine larval phase. I found it interesting to think that these larvae can be carried off to a new island to populate it or to introduce new competition for resources from a previously absent species. It’s a very different lifestyle from either fully marine or fully terrestrial arthropods.