I first posted about Sandcastle Worms (Phragmatopoma californica) on 8 January 2012. They're amazing worms, and it's worth sharing a few more things about them.
After 8 years, we've noticed that the abundance of Sandcastle Worms on Bodega Head varies quite a bit among years. Many were visible when we first moved here in 2005, but they've been relatively sparse during the years since then. This, however, is a banner year for them. In some places in the middle intertidal zone (near mussel beds), it's difficult to walk without stepping on a cluster of them. Here are a few recent photos showing dense aggregations of their tubes. (Most of the tube openings are ~5 mm across.)
Sandcastle Worms feed by extending a crown of purple tentacles into the water. Outside of tidepools, the tentacles won't be visible unless it's high tide. But in pools where the worms are submerged, you can see their beautiful tentacles.
I think the electric blue highlights are rows of cilia that are reflecting light at certain angles (see below). The worms use the cilia to move captured particles (plankton, organic detritus, sand grains) towards the mouth. Food is eaten, while sand grains are transferred to a special organ that adds liquid cement to bind the sand to the tube.
The operculum that seals the opening of the tube when the worm is withdrawn is made of fused setae (bristles).
Below is a close-up of a tube as seen under the microscope. Note that along with sand grains and shell fragments, there are also quite a few forams, or foraminifera. They're the very round, white, spherical shells with a nice chambered pattern (the forams are scattered across the tube, but see lower left corner for the clearest individual).
Forams are amoeba-like protozoans with hard shells or tests. There are over 200 species present in shallow coastal waters of California. I haven't tried to identify this species, but if someone out there knows what it is (Tessa?), please let me know and I'll post its name.
Interestingly, I don't know if the worm attached the forams to its tube, or if the forams nestled into the wall of the tube on their own.