Most millenials in Vancouver can relate: the low availability of decent, affordable housing in the city can sometimes make house-hunting feel like a brutal competition. I’ve had personal experience going to view an apartment and waiting in line amongst upwards of twenty other students to interview with a landlord, with the hope of “outcompeting” the others by proving that I am the best prospective tenant.
For invertebrates living in the rocky intertidal zone, competition for a nice place to live can be just as intense. Mobile organisms like shore crabs compete for shelter under cool, shady rocks, where they can avoid predators and direct sunlight (to avoid overheating)¹. Stationary animals like mussels and barnacles compete for space on rock surfaces to attach themselves; like those wealthy UBC students who are able to afford a one-bedroom apartment for over $3000 a month, some of these animals are able to live in more exclusive spots than others like gooseneck barnacles
However, no intertidal critter knows the struggle of finding a place to live like hermit crabs do. Not only do they still need to find spots under rocks, tidepools, and seaweed² like their cousins the shore crabs- they also need to find a gastropod (snail) shell to house their soft, de-calcified abdomen. This diagram shows what I am talking about- hermit crabs have a soft, asymmetrical abdomen (it looks like a tail) so that it can fit inside a snail shell. Which means that without a shell, they are a lot more vulnerable than their fully calcified (hard-shelled) cousins.
Seems pretty inconvenient, right? Not necessarily- many hermit crabs, such as Pagurus hirsutiusculus, can leave their shells behind as a defense mechanism when they feel threatened³. This is a great defense tactic if, for instance a fish had the snail shell in its jaws and the crab needs to get away.
In lab last week, we performed experiments on the hairy hermit crab Pagurus hirsutiusculus to determine the presence of interactive effects of temperature and salinity stress on behaviour (read more here!). One behaviour we measured was length of time it takes for a crab to retrieve its shell- a very important survival behaviour.
However when we tried to shake one little crab out of its shell, we noticed something hilarious- another larger, naked crab was trying to forcibly remove the littler one with its claws! I was astonished at how persistent the larger one was, even when we lifted it out of the water. I suppose it just goes to show competitive these animals are willing to be to get a suitable home.
However, house-hunting for these rocky intertidal crustaceans is not all ruthlessness… This hilarious BBC snippet shows tropical hermit crabs lined up in order of size to switch shells that they all have outgrown- this level of cooperation is remarkable for backbone-less critters!
Unfortunately, at the end of the switcheroo, one individual is without a shell and has to settle for one that’s a bit too big and has a hole… as a broke university student who’s had to settle for suboptimal basement suites in this real estate market, I can relate.
1. Steinberg, M. K. & Epifanio, C. E. Three’s a crowd: Space competition among three species of intertidal shore crabs in the genus Hemigrapsus. J. Exp. Mar. Bio. Ecol. 404, 57–62 (2011).
2. Meschkat, C., Fretwell, K. & Starzomski, B. Hairy hemit crab Pagurus hirsutiusculus. Biodiversity of the Central Coast (2014). Available at: http://www.centralcoastbiodiversity.org/hairy-hermit-crab-bull-pagurus-hirsutiusculus.html. (Accessed: 12th March 2017)
3. Story, R. M. & Steitz, T. A. © 19 9 2 Nature Publishing Group. Nature 355, 242–244 (1992).