Shell game, Part 2

(continuation of Shell game, Part 1)

The shell that the big female hermit had discarded (see Part 1) was too large. Too big for her, too big for her companion, who is slightly smaller than she is. I decided to remove it from the aquarium; it was in the way. I left it for the present in a bowl of water, to allow anything inspecting the inner reaches to leave.

A few hours later, I glanced at it, in passing, and saw the worm:

Red and white banded sea nymph, Cheilonereis cyclurus

I hadn't seen one like this before, but the pattern is quite distinctive; it took only a minute to find it in my Encyclopedia.

"This stunning red-and-white-banded worm has a special commensal relationship ... with large hermit crab species. Together they inhabit the shells of some large snail species, ... Seldom does this sea nymph emerge more than the length of its head, making detection of its presence difficult." 

Side view. The high "collar" around the head distinguishes it from other sea nymphs.

Googling for more information, I found only one good photo, a taxonomic record in the Encyclopedia of Life (with photo of long-dead worm) and WoRMS, and the name on various lists. One scientific article documents its relationship with hermit crabs in Alaska. It is found from Alaska to California, and on the far shore of the Pacific, in China and Japan.

But what does it eat? How does a big worm fit in a shell with a fat hermit? How does it get in there? How common is it? I found very few answers.

Tail end.

What do I know? It's a polychaete, a worm with a pair of paddle "feet" and several bristles on each body segment. It has four eyes (right-click the top photo). The "feet" on this worm are divided into two white paddles on either side.

Like other polychaetes, he sways back and forth along his length, keeping his space open, keeping the water moving.

Paddles (parapodia) and bristles (setae)

He belongs to the Nereid family. These have strong jaws and teeth. Some are carnivorous predators, while others eat algae or other vegetable matter. I tried to lure this one out of the shell with a piece of shrimp; he wasn't interested, even when the shrimp was in contact with his face. I gather that he's vegetarian.

About that relationship: it's called commensalism. The word comes from Latin, "cum mensa", meaning "with table", or sharing a table.

In ecology, commensalism is a class of relationship between two organisms where one organism benefits without affecting the other. (Wikipedia)

This brings up even more questions: Which one benefits, the hermit, or the worm? How do we know they don't both benefit? (This would be mutualism.)

Can the worm live in other places, like burrowing in the sand? Or must it live inside a shell? Does it do better if there's a hermit in the shell?

Does the shell provide shelter, or does the hermit provide food? Or, vice versa, does the worm help to remove the hermit's waste products? Or help with irrigation?

The hermit that moved out was one of the fattest I've ever seen. Was this because of some advantage the worm gave it?

Facebook has it right: it's complicated.

The worm has had the shell to himself now for two days. He is still there, still poking his head out at intervals. Other hermits have looked over the shell, and they keep rolling it about, but have not attempted to move in.