A few days ago, I was trying to photograph this beautiful tubeworm under a microscope. This is Chone sp. (probably Chone minuta), commonly found among surfgrass in the rocky intertidal zone.
When you first encounter them in the field, you might mistake these worms for mats of accumulated shells and sand. But when you inspect this material closely, you can see that it's made up of many densely packed individual worm tubes that are covered with small shell fragments. I need a better picture, but this is what they look like at the base of the surfgrass when the tide is out:
The worms use their feather-like tentacles for feeding. While I was looking at them under the microscope, something on the left side of the tentacles caught my eye. Can you see the horseshoe-shaped piece at about 9 o'clock in the image below?
I wasn't sure what it was, so I started searching to see if it showed up in other places (next image).
Sure enough, those horseshoe-shape pieces extended between each tentacle. I didn't know it before that moment, but Chone has webbing between its tentacles!
Below is a view from the side. You can see how the transparent webbing forms a membrane below the tips of the tentacles to the opening of the tube.
I'm not sure what function this webbing serves, but it has been suggested for another species (Myxicola) with a similar structure that it prevents fine sediment from being drawn up from below and fouling the feeding tentacles.
One more fun and somewhat puzzling observation about Chone. Sometimes they would pull in their tentacles, leaving only the tips projecting outward at the edge of the tube (see below).
I started calling this the "sun position" because they looked like little suns when they did this. While in this position, the tentacle tips flicked regularly in random sequence. I don't know what's going on with this behavior, but it was interesting to watch and to wonder about!