One down, one to go. And a toe-biter.

I was wrong, I think. Yesterday's worm is not a worm at all, but a normal part of the limpet.

Not a polychaete. A gill.

Example # seventy-umpteen for my list, "Why I Love Blogging". There's always someone with better information, always something new to learn.

In the comments, yesterday, Neil Kelley suggested that the "worm" may actually be a gill. He supplied a link to a diagram I had not seen.

Ventral (underside) view of a true limpet, with part of the foot cut away to show the gill. (Ignore the two critters at the left.) (Marine Life Information Network, Fig. 18)

All true limpets have gills. (Wikipedia) They take different forms, sometimes forming a ring around the body, sometimes, as here, one pointed, feathery extension, which may at times be visible behind the foot.

Another anatomical diagram, this one from the top, so the gill extends to our right. Image from R. Fox, Lander University.

Other diagrams have not been as clear as these, usually showing the gill squished in alongside the other internal organs.

And I finally found a few photos of limpets with the gills showing. (It's amazing how knowing what to plug into Google turns up what you're looking for!) All of the photos are small, and mostly vague, but here's a clear one. The photo is for sale, so you'll have to click on the link.

Here's another:

Photo from Seashells of NSW.

So that's cleared up.

There's still that first worm to identify.

This one.

And a bonus: on Google+, Chris Mallory sent me a link to a page about another commensal scale worm in a limpet. The limpet is a keyhole limpet, quite distinct from the true limpets, but here's another instance of the benefit a limpet can receive from hosting a worm:

When Pisaster ochraceous* attacks Diodora aspera** in which this species is living, the worm moves around the pallial groove of the limpet to the side the seastar is attacking from, reaches out, and bites the tube feet or ambulacral area of the seastar.  This frequently results in the seastar withdrawing from its attack.

*A starfish
** The keyhole limpet

No end to mysteries

There's always something new . . .

I keep trying to get a decent photo of a limpet with its mouth open, scraping algae. This was better than most; the radula inside the mouth is visible, and even some of the internal organs show dimly through the meat of the foot. But what is that other tube around its "neck"?

1/4 inch long limpet, cleaning the aquarium wall.

I've spent several hours looking at limpet photos and reading anatomy pages. Nothing resembles that yellow tube that snakes out from the left side of the mantle and reaches out to the surrounding water. One source (beachwatchers.wsu.edu) mentions that keyhole limpets often carry a commensal scaleworm in the mantle groove. But this isn't a keyhole limpet, and that's no scaleworm.

Could it be a ribbon worm?

Any ideas?

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.