The spiky hermit crab, as its name suggests, is a type of hermit crab that lives in a shell resembling a spiky chestnut burr, and is on display in the "Deep Sea Creatures 1" tank.

Hermit crabs are known to use snail shells to protect themselves and move to larger shells as they grow, but this spiny hermit crab is a rare species that never moves shells throughout its life. The secret lies in the unique structure of its shell.

When very small, the spiny hermit crab carries a snail shell on its back, just like many other hermit crabs. However, after a while, a type of hydroid called *Hydrocotylum fuscipes*, which attaches to the surface of the snail shell, uses the snail as a foothold to create a spiny shell.

Specimen of the shell of the spiny hermit crab	The seashell I used when I was little is still attached to the tip.

Hydroids belong to the cnidarian group, which also includes jellyfish. Their appearance varies greatly depending on the species; some live individually, while others, like the spiny sea hydra, reproduce by dividing and multiplying, with many individual hydroids gathering together to form colonies.

The spiny hermit crab (Hydrocotylum fuscipes) grows and reproduces by feeding on plankton and other organisms drifting in the ocean, expanding its shell as the hermit crab grows. As long as the hermit crab is alive, the hermit crab is guaranteed a shell that is always the right size for it. The hermit crab can also move with the hermit crab, so the two are in a symbiotic relationship.

Now, let's get back to the topic of the spiny hermit crab.

I've seen these hermit crabs climbing on almost vertical walls and Goat (cnidarians, like corals) many times before, and I was always impressed by how agile they were. I thought there might be a secret in their shells, so I picked up a shell specimen, and although it was dry, I was surprised by how light it was. Hydroids make strong, thin shells out of chitin. This may have the advantage of being easier to move around in than using heavy snail shells as homes.

In an aquarium, pay attention not only to the fish's distinctive appearance but also to their behavior.


A spiky hermit crab climbing onto a slender Goat.
[Tokyo Sea Life Park]

(February 25, 2022)