Please see photo below.
You are looking at two animals, not one.
This is an Alaskan Hermit Crab (who resides in front of Port McNeill, not Alaska).
Living on his/her back is a “Hermit Crab Sponge”.
This sponge species (Suberites latus) settles on the shell home of some hermit crab species and can completely dissolve the shell away.
Having a sponge home has its advantages. It is light. Also, the sponge will grow whereby the hermit crab need not find a new home as would be the case if it outgrew a shell home.
But, it can be awkward to tote around when it gets really big. See an example below.
Yes, the hermit crab could leave the sponge and get another home if one were available. But, there is risk when outside your home, be it ever so brief.
Another disadvantage is when you have unwelcome house guests.
See below to get a sense of the inconvenience when a sponge predator crawls on your back.
From top to bottom: the big yellow animal is a Monterey Dorid (nudibranch species – gills are on left). This nudibranch is feeding on the Hermit Crab Sponge (tan colour) and then, see the tiny face? That’s a Mud Hermit Crab who isn’t going anywhere for a little while (Pagurus capillatus to 4 cm).!
Here’s another Mud Hermit Crab. See the bite out of the sponge? I initially found this individual upside down. The resulting photo of the underside of the sponge gives you a sense of how the sponge is shaped to the hermit crab’s body.
Note too how all the hermit crabs included in this blog have one claw bigger than the other? This is the case for many hermit crab species and it allows them, when they retreat into their home, to seal off the opening to the shell or sponge with the bigger claw. They close the door to their home.
In the photo below, see how the larger claw seals off the hole for the hermit crab on the right? I suspect this interaction captured in this photo more about mate selection that it is about home envy.
I hope this “who is sponging off who” interaction provides some wonder for you at a time when safety in homes is such a reality for our species too (re COVID-19).
Be safe whatever, and wherever, your chosen home. 💙
The Hermit Crab Sponge is Suberites latus to 20 cm long, 6 cm wide and 4 cm high.
Source for these dimensions is “Beneath Pacific Tides” by Greg Jensen.