East Coast - Hermit Crabs And The Shell Game
Hermit crabs are among the most successful shoreline critters, adaptable to changing conditions and world-class scavengers. In fact they even scavenge for their homes. They’re also a surprising mix of combative and sociable, especially where those homes are concerned. So today we’ll see just how important a shell is to a hermit crab and how sometimes these aggressive creatures can all cooperate so everyone wins.
EPISODE NOTES
Sad thing I learned researching this episode: hermit crabs will not breed in captivity. So every hermit crab that is sold in a pet store has been captured in the wild. I know the hermit crab craze is in the rearview mirror (that was like a mid-90s thing, right? Or did it have a social media resurgence?) but in any case: don’t buy hermit crabs as pets! And… if it’s too late for that, for the love of god don’t bedazzle them. It’s not just tacky, it can cause real health issues.
TRANSCRIPT
The intertidal is not for the faint of heart. With your environment constantly ping-ponging between heat, cold, sun, rain, dry, wet, it takes a particularly hardy sort of life to thrive here.
And if you’re poking around at low tide, it usually doesn’t take long for one of them to announce themselves: a snail shell suddenly growing legs and taking off. You have found a hermit crab.
And when you find one there’s usually a hundred more. Hermit crabs are a weird combo of intensely communal and just as intensely competitive. They group together and then fight over everything. When you get enough of them in one place it is a madhouse.
But they’re also among the most successful shoreline creatures, with more than 1200 species worldwide. Able to weather changes in temperature and salinity, and they’re super adaptable scavengers. A large part of their nearly microscopic brains are dedicated to processing smells to better locate food. They scoop up sand in their claws and pick through it for any edible tidbits, or descend on - and naturally fight over - carrion.
But they don’t just scavenge for food. That’s how they find their homes. Because hermit crabs belong to the family paguridae, animals that live in the empty shells of snails.
To say shells are important to hermit crabs is obvious and still an understatement. It’s their big evolutionary advantage, getting the benefit and protection of the shell without having to spend the energy growing one.
The problem is: hermit crabs themselves want to keep growing. And their shells… don’t. So they need to trade up around twice a year for a bigger model.
When they go out looking for a new shell, they’re looking for something really specific.
If the shell is too big, they’ll be hauling around extra weight everywhere, expending pointless energy in a hyper-competitive environment.
But if it’s too small, they won’t be able to fully retreat into it, leaving them vulnerable to predators and drying out.
On top of that, hermit crabs will flat-out not eat unless they’re in a properly sized shell, so it really is a life-or-death issue.
And to make it even trickier, the ideal shell does need to be too big - but just a bit too big. That’s what allows them to keep growing inside it, and reproductive success in hermit crabs is correlated with shell size. So a sequence of shells, each with a little more room to grow, is ideal.
To big - no good. Too small - no good. Just right - the best chance to survive and reproduce. So you can see how important this is and how narrow the band of ‘acceptable next shell’ really is.
And so no surprise that competition is fierce. Hermit crabs fight hard over shells. If they see one they want occupied by another hermit crab, they might actually start hammering on it with their claws, trying to get the occupant to come out far enough that they can grab them, yank them out and move in.
But in the endless competitive / sociable flip-flop of hermit crabs, sometimes the stars - or, shells - align. A shell swap. Or to use its official name, a ‘vacancy chain’.
When a hermit crab moves to a new shell it leaves its old one behind. Which might be perfect for another crab looking for a trade-up - which leaves their shell behind. And so on.
The coolest of these are synchronous vacancy chains. Say a predatory snail is devouring another snail, about to leave a nice big empty shell - perfect for a nice big hermit crab. When that crab shows up to check out the new real estate, an actual queue can form behind them in descending size. And if the original crab decides to move in, everyone follows suit in a matter of seconds, each trading up in turn.
It’s a reminder that even these little guys with their nearly microscopic brains dedicated largely to olfactory processing can still have these beautifully complex behaviours and social rituals where everyone wins. A rare moment of mass cooperation in the cutthroat world of the intertidal.