A few weeks ago I found the broken shell of a tun snail, a relative of triton and helmet snails that grows to about 7 inches long. And on that fragment clung a live marine animal so unfamiliar to me that I didn’t know where to start in looking it up.

I often find tun shell pieces, exquisite with their gold-and-rust-colored patterns, but I’ve never seen a live tun. In daytime, these snails bury themselves in sand 2- to 30-feet deep. When a tun rises from its sand bed at night, it walks around on an enormous foot that looks like a circle skirt around the shell.

Tun snails hunt for warty sea cucumbers. The species name, horrens, means dreadful in Latin, probably due to its rows of wartlike bumps.

A full-grown tun can swallow an entire 10-inch-long sea cucumber in about 15 seconds. But the warty sea cucumber has a trick up its lumpy sleeve. It detaches the flap of skin the tun has seized. In this way, the sea cucumber often escapes, crawling away on its tube feet.

But back to my mystery creature.

When I picked up the tun shell, the round creature that was stuck to it began spewing pink threads from its center and from white spots that lined its edge. Thinking the threads likely contained stinging cells (they do), I kept my hands clear, took photos and replaced the piece in a pile of coral debris.

After paging through my books and getting nowhere, I sent my pictures to John Hoover, author of several excellent local marine guides. As always, John came through, emailing: “It’s a hermit crab anemone.” Aha.

A few types of hermit crabs, called anemone crabs, like to move into empty tun shells because they’re lightweight, and therefore, easy to tote around.

When an anemone crab moves into a new shell, tun or other, it searches for the little anemone called Calliactis polypus (no common name). Growing to about 3 inches in diameter, the anemone also lives on coral rocks and pumice.

When the crab finds an anemone, it doesn’t just hop aboard. The crab must tap and massage the anemone’s adhesive foot, called a pedal disc. When the anemone relaxes its grip, the crab picks it up and holds it onto its shell until it sticks.

The anemone stays there because it gets free rides and crab leftovers. When covered with stinging anemones, as crabs sometimes are, the crab gets protection. But it’s limited. John guessed that a predator, such as an octopus, had eaten the crab living in the tun shell, leaving its anemone homeless.

My little anemone might live happily among the rocks where I placed it, or another crab may find and adopt it. In either case, if I see it again, I’ll know what it is.

Reach Susan Scott at susanscott.net and click on “Contact” at the top of her home page.