Answer: When it’s a snail! Yes, there are snails that secrete and live in white calcareous tubes that look very similar to those of serpulid polychaete worms. Here, see for yourself:
9 May 2015
© Allison J. Gong
The worms secrete calcareous tubes that snake over whatever surface they’re attached to. When the worm is relaxed, it extends its delicate pinnate feeding tentacles and uses them to capture small particles to eat; they are what we call suspension feeders.
11 May 2015
© Allison J. Gong
But there are gastropods that secrete calcareous tubes, too. They are the vermetid snails, the local species of which is Thylacodes squamigerus. This is one of my favorite animals in the low intertidal, probably because it is so delightfully un-snail-like.
There are three individuals of T. squamigerus in this photo:
9 May 2015
© Allison J. Gong
9 May 2015
© Allison J. Gong
Thylacodes is also a suspension feeder, but it gathers food in a very different way. When submerged, it spins out some sticky mucus threads that catch suspended particles, then reels in the threads and eats them.
So how would you tell these animals apart if you see them? Here’s a hint: Look at the tubes themselves.
I invite you to use the comments section to tell me how you’d distinguish between Serpula and Thylacodes.
Okay, I’m going to give this a go. The shells of the Serpulorbis look to be tightly or loosely spiraled much like a regular snail’s shell. While the shell of the Serpula look like they are more randomly created. Am wondering if “orbis” means or is referring to an orbit or spiral?
Despite appearances to the contrary, Serpulorbis is still a snail, and its tube does retain some of the spiralling that characterizes gastropod shells. And “orbis” is indeed derived from “orb,” which of course is a sphere.
You’re right on, Pamela!
There’s one more feature that you can use to distinguish between these animals, even if you can’t see anything sticking out of the opening.
Yes, Serpulorbis is a great name, but no more for this species: Now Thylacodes squamigerus (Carpenter, 1857)
Huh. Damn taxonomists.