And I've found several worms*, on the larger (about 1/4 inch) limpets. But they're not like the one I'd seen before.
|Limpet with passenger, just poking its head out from shelter.|
The worms* are shy; they stretch out just this little bit, wave around for a second, then retreat back behind the body of the limpet.
|Zooming in, showing the structure of the worm*. I darkened the background to make it more visible.|
As far as I can tell, this is a polychaete worm, aka bristle worm. There are many species of these, some free-living, many living on or in other organisms. Since my limpets are growing and appear to be healthy, their relationship with the worms is probably commensal; each one provides some benefit to the other.
The limpet in this case provides transportation, protection, and maybe even some food, whether crumbs from the limpet's diet of algae or excreted fecal matter (poop). It may be that the worm's contribution is basic housekeeping, keeping the place clean, taking the garbage out, etc.
But they're not the worm that started me looking in the first place. Here's the previous photo, for comparison:
|Long, thin worm. Not shy at all.|
I'll keep on looking.
*UPDATE: Not a worm. See next post.