Benjamin Smith (CC BY 2.0)
Chitons
Class Polyplacophora
Slow, herbivorous molluscs that graze filamentous and film algae from rock and glass and ignore corals and livestock.
Identification
Oval, flattened molluscs, commonly 1–3 cm long as hitchhikers, in mottled grey, brown, or green. Eight overlapping hard plates run down the centre of the back, ringed by a fleshy, sometimes scaly or fuzzy girdle. They clamp onto rock and glass with a strong muscular foot, stay put or hide by day, and creep out at night to graze; a dislodged chiton can flex and curl thanks to those separate plates.
Often confused with
Pest snails and flatworms. Count the plates to separate them: a chiton carries exactly eight hard, overlapping shell valves in a row, which no snail or flatworm has. Snails carry a single spiral or cap shell with a visible head and antennae; flatworms are completely soft, thin, and smooth, ripple as they glide, and show no armour at all. A slow oval with eight plates is a grazer, not a parasite.
Natural predators
No natural predators are listed.
Field notes
Tireless film-algae grazers that work rock and glass without the bulldozing habits of urchins and big snails. One caution: their scraping teeth can leave fine scratches on acrylic, so glass-tank keepers get the better deal.