Today’s post is a little unusual; as you might have noticed, the title mentions two animals. The reason for this is simple: Leach’s hermit crabs and cloak anemones almost never appear without the other. Since they live together and gain equally from their association, it didn’t seem fitting to pick one over the other as the star of this blog post. I don’t want all the world’s anemones or all the world’s crabs hating me, so this way everyone is happy (this is of course, assuming that crabs and anemones read my blog. Which I’m sure they do).

Anyway, these two animals have quite the interesting symbiotic relationship. It starts with the anemones, and their birth. Well, that’s kinda of like saying which came first, the anemone or the crab. But I’m starting my story with the little baby anemones. Each anemone larva somehow finds its way to an empty gastropod shell, where it settles and starts to grow. The anemone eventually envelops the entire shell, which is almost always by this point inhabited by a hermit crab.

So why do these two species do this? Hermit crabs, of course, usually live in shells, but these are usually empty, because living with roommates can be unbelievably annoying. So what makes this odd couple pair up and live so happily together? Well, there a number of good reasons, and because of these leach’s hermit crabs and cloak anemones are rarely found alone.

Image By © Hans Hillewaert, CC BY-SA 4.0

The crab benefits from the protection it gets from the shell, but the addition of the anemone to the shell offers extra safety. As is the case with most anemones, the cloak anemone has stinging nematocysts. The anemone is capable of firing these at an approaching enemy, protecting both itself and the crab. It’s kind of like living someone trained in the use of firearms. I’d feel pretty safe. In addition to this defence the crab gets an added benefit: it never has to change shells.

For most hermit crabs, the most dangerous part of their lives is when they outgrow their shells and have to wander unprotected until they find a new one. But the leach’s hermit crab doesn’t have to do this; as the crab grows, so does the anemone. And once the anemone outgrows the shell, it deposits a chitinous layer at the bottom of the shell, which serves to increase the volume of the shell. And because the anemone and the crab grow at the same rate (hooray evolution!), the crab never needs to find a new shell.

In terms of benefits to the anemone, these are fairly straightforward. The anemone gains a mobile lifestyle, which most anemones could only dream of. As well, any food scraps that the crab doesn’t eat are scarfed down by the anemone. So basically all the anemone has to do is guard itself and the crab and it gets free rides and free meals. Not a bad deal, if you ask me.

The fact that such different species can live and work together so well is a marvellous thing. If we could all be more like the leach’s hermit crab and the cloak anemone, maybe the world would be a better place. Then again, maybe not. Anemones and crabs are pretty weird.