Stephen James McWilliam (CC BY 4.0)
Spirorbid Worms
Family Spirorbidae
Sessile filter feeders in tiny calcareous coils that strain particles from the water and harm nothing in the tank.
Identification
Tiny white hard tubes, 1–2 mm across, cemented flat to glass, rock, snail shells, plumbing, and sump walls in a neat, tight spiral like a miniature coiled shell or ammonite lying flush against the surface. A close look may show a tiny crown of fine filter-feeding hairs at the centre of the coil. They never move and often appear in numbers in high-flow spots as a tank matures.
Often confused with
Vermetid snails, the mucus-netting pests. The coiled-tube tell separates them: a spirorbid stays a small, flat, perfect spiral pressed to the surface, sends out no mucus, and stays at 1–2 mm. A vermetid builds an irregular, uncoiled tube that rises off the rock, grows larger and messier, and casts sticky mucus strands that drape over nearby corals. Flat white pinwheel on the glass: spirorbid, and welcome.
Natural predators
No natural predators are listed.
Field notes
Free water polishers and a classic sign of a healthy, maturing tank that can support filter feeders. Leave them in sumps and on rock; scrape them off viewing glass only if the look bothers you.