I think Flat-Clawed Hermit Crabs (Pagurus Pollicaris) are really cute. But the way they create a community to carry around is what makes them so endearing! They seem to like large, somewhat ugly shells. Each one, however, seems to support an entire mobile community. So these guys (and gals) would rather live in crowded squalor than pristine loneliness.
They always seem to choose the shells of large gastropods: knobbed whelks, murexes, etc. But often these shells have broken bits, and they are covered with colonies other marine organisms. These get some mobility from the crab, and the crab must get some benefit (camouflage or protection) from this little community he carries around on his back.
Tricolor Anemones often hitch along with hermit crabs, attaching themselves to the shell. Rumor has it that when a hermit outgrows his shell, he’ll take the anemone right along with him, moving the anemone to the bigger shell.
Worms in Little Tubes
Weirdly enough, there seem to be limited resources for identifying the polychaete worms I called “mason worms” in the above video. These worms are free floating when they are larva, and are attracted by the glue in the tubes of similar worms. They build their own tubes (hence the term mason) attached to the shell of the hermit crab. When the crab moves, they are able to feed on the stirred up detritus.
This video zooms in on the entrances to the worm tubes, and you can see them moving in there. At the edges of the tubes, the worms have long feathery feet that can stretch out into the water and grab particles to eat.
I grew up calling these cup shaped shells “slipper limpets” but the more common current terminology is slipper snails. Slipper snails often form stacks together. The Atlantic slipper shell can range in color from orange to pinkish to gray, while flat slipper snails are usually white. They’ll catch a ride on a wide variety of slow moving real estate, like horseshoe crabs and hermit crabs.
Other Encrusting Organisms
As you look at those photos, you’ll notice a few more organisms, like an encrusting bryozoan that leaves that scaly surface covering, and even some coral. And barnacles can be found on a lot of them as well.
We have long-wristed hermit crabs and thin-striped hermits here in our waters, but only the flat-clawed hermit can fold his front claws together like a puzzle and seal off the entrance in a way that keeps predators out. Something about these claws also makes them blend in amidst all the surrounding organisms so they’re not that noticeable.
Ability to Cling Inside the Shell
Like other hermit crabs, flat-clawed hermits use several rows of back legs to securely hold them inside their shells. We did a necropsy on one that washed up last year to see how it is anchored in there. As we removed the deceased crab from the shell, you can see how the abdomen has the smaller legs that can hold on.