FWC Fish and Wildlife Research Institute (CC BY 2.0)
Limpets
Patellogastropoda; keyhole limpets family Fissurellidae
Common limpets are harmless algae grazers, but keyhole limpets can occasionally graze soft or large-polyp stony coral tissue when other food runs short, so they earn watching rather than blind trust.
Identification
Snails wearing a single low cone- or volcano-shaped shell with no spiral, typically 5–25 mm across, with radiating ribs and concentric growth rings in grey, tan, brown, or algae-tinted colours. Keyhole limpets show a small hole at the very top of the cone. They clamp hard onto rock and glass with a broad foot, move slowly, and graze mainly at night; prising one off without damage is genuinely difficult.
Often confused with
Pest snails, because an unfamiliar cap-shaped shell reads as suspicious. The tell is the single unbroken cone: no spiral whorls, no operculum, no separate head poking out front — just a ribbed cap clamped flat to the surface, with the keyhole at the apex on Fissurellidae species. Whelks and other predatory snails show a coiled shell and an obvious roaming head and siphon by contrast.
Natural predators
No natural predators are listed.
Field notes
Strong grazers of film algae and coralline algae that keep rock surfaces cropped. Keep standard limpets without a second thought; with keyhole limpets, enjoy the grazing and move an individual to the sump only if you actually catch it rasping on a coral.