H. Zell (CC BY-SA 3.0)
Collonista Snails
Collonista spp. (family Colloniidae)
Tiny herbivorous grazers, often called mini turbos, that feed on algae film and never eat zoanthids.
Identification
Very small round turban-shelled snails, usually around 3 mm and rarely over 4–5 mm, in white with mottled tan, brown, pink, or red checker-like markings (some are nearly all white). Nocturnal: they pour out over rock and glass at night and hide away by day, and they reproduce readily so numbers can look dramatic.
Often confused with
Sundial snails (Heliacus spp.) — the dangerous mix-up, because sundials share the checkerboard pattern and genuinely eat zoanthids. Check the operculum (the trapdoor closing the shell opening) under a magnifier: a Collonista has a flat operculum with a tiny pit in the centre, while a sundial has a raised, cone- or pagoda-shaped operculum. Sundials are also flatter and heavier-looking with beaded shell texture, grow larger, and stay buried at the base of zoanthid colonies rather than wandering rock and glass. Collonista also get mistaken for baby trochus snails, which grow much larger and develop a taller conical shell.
Natural predators
No natural predators are listed.
Field notes
Useful micro-grazers that reach tight spots bigger snails miss and help keep film algae down. Confirm the flat, pitted operculum once, then leave the whole population alone.