H. Zell (CC BY-SA 3.0)
Sundial snails
Heliacus spp. (family Architectonicidae)
Specialist zoanthid predators that hollow out polyps from inside the colony and can strip a colony over days to weeks.
Identification
A small snail, roughly 5-15 mm across, with a low, disc- to dome-shaped shell boldly patterned in alternating tan, brown and cream bands or checkerboard markings. Mostly nocturnal: by day it wedges itself down between zoanthid polyps or under the colony edge, and at night it sits on the polyps feeding. A flashlight check a few hours after lights-out is the standard way to find them.
Often confused with
Collonista snails (mini turbos), which are harmless algae grazers with a similar size and mottled pattern. Check the operculum, the trapdoor closing the shell: Heliacus carries a raised, conical, pagoda-shaped operculum, while Collonista has a flat operculum with a small central pit. Pointed means sundial.
What it targets
Zoanthids: Zoanthus, Palythoa and Protopalythoa colonies. The snail punctures the base of a polyp and feeds on its tissue and fluids, leaving an empty polyp husk that detaches and decays.
How it spreads
Arrives hidden inside or under imported zoanthid colonies. Populations grow slowly compared with many pests, which is why persistent hand removal can clear them.
Control (safest first)
Manual removal with tweezers during night inspections: lift each snail out of the colony and check the colony base and underside for more. Reported as sufficient by most sources because reproduction is slow; repeat weekly until night checks stay clean. Inspect and dip new zoanthid colonies before placement and quarantine them where possible: dips may dislodge adults, but eggs can survive dips, so a follow-up inspection about two weeks later matters more than any single dip. Remove and inspect a declining colony out of the tank: sundials tuck deep between polyps and are easy to miss in place; working over a container of tank water reveals them as polyps close.
Natural predators
No natural predators are listed.
Field notes
In one forum case a keeper worked through a long list of suspects for vanishing zoanthids before another member named sundial snails, and the keeper then found them hidden in the colony and confirmed the match. A WetWebMedia correspondent only spotted a tiny suspect snail by chance during morning coffee, and by the follow-up message a day later reported the zoanthid colony already reduced to a rock with short red stubs, showing how much damage a small snail can do before it is seen. Keepers are often warned the suspect may actually be a harmless Collonista, and several threads turn on the operculum check: relief when the trapdoor is flat, removal duty when it comes to a point.