Sandcastles? Worms? These go together? Well, sandcastles are not just for kids. The sandcastle worm, Phragmatopoma californica lives in massive sand formations on rocks in the mid intertidal area. You rarely see one tube; more often you will find thick masses up to 2 meters across, arranged in a honeycomb pattern (Fig 1).
The worm itself is small, 30 to 50 mm, segmented like an earthworm, and ends in an anterior tuft of lavender tentacles and a dark operculum which serves to block the tube opening like a trap door (Fig 2). The tentacles,
extended during high tide, trap plankton, organic matter and sand. The plankton and organic matter are taken into the mouth and the sand is transported to a small gland behind the mouth where glue is applied and the particles attached to each other to form the tube. As more tubes are formed, the mass increases to form the “castle” (Fig.3). These masses are firm, yet very delicate, and to step on a mass crushes it easily.
These worms are not fastened in their tubes but they never leave them. Because of this, you might ask, how do they keep their home clean? How do they poop and not foul the tube? As with most organisms, the anus is at the posterior end of the body, but in the case of Phragmatopoma, the anus is at the end of a long, thin rectum which curves back toward the front end of the body. Fecal pellets are released about halfway up and water currents produced by the tentacles wash the pellets out of the tube. Amazing, and quite cleansing.
And, in case you didn’t know, these worms have separate sexes, male and female, which release eggs and sperm into the water at the same time to insure fertilization. The planktonic larvae swim until ready to settle on rock, 18-35 days, depending on the water temperature. Once settled on rock, the larvae undergo a continuous metamorphosis to the adult form and begin to form tubes.
The glue produced by the worm is quite interesting. Researchers have found it contains 3 proteins, magnesium and calcium; is produced and sets up underwater; but does not dissolve in water. The glue is being used to mend and align broken bones, and it is biodegradeable by the human body when not needed any more (http://www.authorstream.com/Presentation/orangelaces-1016707-sandcastle-worm/).
A really good article on the sandcastle worm can be found in Bay Nature (June 6, 2019), by Allison J. Gong at this site: https://baynature.org/2019/06/06/meet-a-beach-worm-that-builds-
sand-castles/. Many sandcastles are found at North Point in Morro Bay, which is where the photo in Fig 1 is taken. No moats, no dragons, but LOTS of worms!