The Cutest Critter You've Never Heard Of

Have some amazing photos to start your week

I accept the title might be overdoing it a bit. Maybe you have heard of this adorable little crustacean. Maybe it’s not actually the cutest of all the countless animals most folks have never encountered. Definitely not, in fact.

But, you know … I think the Granulate Shellback Crab is quite cute and I’m pretty sure not many people know it exists, so I’m going with it.

You find this, or something similar, on a beach (somewhere in the Americas):

Unremarkable. The discarded shell (half the shell - one valve) of a bivalve mollusc. Unremarkable until you turn it over:

What is that nestled safely and cosily within the shell? It’s a Shellback Crab. A Granulate Shellback Crab - Hypoconcha arcuata.

Shellback Crabs are examples of a whole pretty unusual family of crabs known as Sponge Crabs because they carry live sponges around on their backs.

Shellbacks, instead of sponges, carry around the valves of bivalve molluscs as portable shelters. And if you catch one, like the Granulate Shellback Crab, at the right moment they’re (I think) cute and photogenic:

Look - the adorable little thing is giving you a wave:

Previous two photos: © Austin Smith, CC BY 4.0

Crabs are decapod crustaceans, as in ‘ten feet’. So maybe you’d expect to see ten legs when you’re lucky enough to find a shellback crab in its shell. But if you count in the photos above, you don’t. You can see six legs, not ten.

That’s because the back two pairs of walking legs are specially adapted to hook onto and hold the shell on the crab’s back. I’ve helpfully and very beautifully arrowed them here (these are from a related species, not the granulate shellback crab, but the principle’s the same):

Photo adapted from: Noémy Mollaret - MNHN - Museum national d'Histoire naturelle (2020). CC BY 4.0

So you don’t see those four rearmost legs because they’re folded/twisted over the crab’s back to hold the bivalve shell in place. And they can do it so firmly - maybe aided by a bit of a suction effect - that it can be next to impossible to extract a shellback crab from its borrowed shell without damaging it.

So on the off chance you’re ever lucky enough to find such a thing, just bear in mind: that crab’s very attached to its mobile home.