Spirorbids are small polychaete worms characterized by coiled calcareous tubes. They are quite common along rocky shores and you may notice their tiny white spiral tubes on rocks, algae, or shells. The tubes measure only a few millimeters across, and the tube openings are often less than 1 mm in diameter.
Many people haven't had the opportunity to see spirorbids under water. These beautiful worms are filter feeders, using long tentacles on their heads to capture small particles from the water. Ciliary movement along the tentacles causes water to flow from underneath and up through the tentacles. Once captured, the particles are then transferred along grooves towards the mouth at the center of the tentacles.
Many marine invertebrates are broadcast spawners (releasing eggs into the water), but spirorbids are brooders. Some species brood embryos on the inside of their tubes, but others have special opercular brood chambers.
The operculum is a trumpet-shaped structure that seals the end of tube when the worm is withdrawn — see upper left corner in photo below: the operculum is disc-shaped with some whitish coloration and is being tilted to allow the tentacles to emerge.
Although I'm not positive, the operculum of the worm in the center of the photo may contain eggs or embryos! Look for the dark reddish spherical structures inside the orange chamber on the worm's lower right side.
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