OK, so WHAT is going on here?
Last month I was walking down the beach with a group of students and this is what we saw – Strawberry Hermit Crabs up in the branches of the Beach Heliotrope (Heliotropium foertherianium).
I have seen this before, but this was a particularly large group scattered about the branches and the students of course were fascinated. The questions came fast a furious. What are they doing? Why are the up there? Why are they lined up on the branch? What is that one doing way at the top?
Other than this communal tree climbing thing, Strawberry Hermit Crabs are similar in their biological and behavioral characteristics to other hermit crabs and their large cousin, the Coconut Crab. They are all in the same taxonomic family and all (with the exception is the Coconut Crab) use gastropod shells most of their life cycle. Hermit crabs are scavengers and eat a variety of plant and animal material. They seem remarkably cognizant of the world around them, and in fact use a keen sense of smell to locate food. They also watch each other and follow the crowd to potential food sources. It is common to see a huge pile of hermit crabs on a broken coconut, or a dead bird.
But this brings us back to the trees. So why are they up there? There are a reasonable number of hypotheses for this behavior, but no definitive answers. They could be up there to eat fresh leaves. They could also be feeding on bark or something growing on the bark. It could be a social behavior that gets them all up there. Maybe they are gathering to mate? Hermit crabs spend a lot of their time looking and competing for new, larger shells (see this great video), so maybe it is about that. Or it could be a reaction to the weather, or a big swell on the way.
The fact is we do not know what is going on, and this is a great example of a question that a field scientist might ask, and then go on to investigate. Maybe one of the students that were there that day will want to figure this out. Questions like this generally lead to more questions, and field investigtions often lead researchers along branching paths that require observations on multiple organisms at the same time. This is how we come to understand ecosystems and figure out ways to protect them. For the moment though only the Strawberry Hermit Crabs know what gets them up in the trees.
