Snails — Hitchhiker IDs

Stomatella spp.; Trochidae

Useful small grazers when the identification is correct.

Quick ID

A slug-like snail with a small, flattened ear-shaped shell on its back. Much of the soft body and elongated foot remain outside the shell.

Typical size

Usually small; roughly centimetre scale, with many aquarium examples around 1–2 cm long. Juveniles are much smaller.

Usually found

Live rock and glass, often grazing in dim light or after lights out.

Commonly confused with

Nudibranchs, flatworms and Scutus. Look for the small external shell; nudibranchs and flatworms lack one, while Scutus has a different shield-like shell.

Worth knowing

Colour and pattern vary substantially. They can establish reproducing aquarium populations; a slug-like appearance alone is not a reason to remove them.

What should I do?

Leave confirmed Stomatella to graze. Avoid pulling them off rock and protect them when cleaning glass or transferring frags.

Limpet-shaped Gastropoda; several unrelated lineages

Many graze algae; diet varies, especially among keyhole forms.

Quick ID

A low cone or cap-shaped single shell with a broad gripping foot beneath it. Some keyhole limpets have a hole or slit near the shell’s apex.

Typical size

Common hitchhikers are millimetres to a few centimetres across. Larger species exist; shell size does not reveal diet.

Usually found

Rock, glass, shaded surfaces and sometimes around sponge growth.

Commonly confused with

Chitons, barnacles and small bivalves. Chitons have eight dorsal plates; bivalves have two hinged valves; barnacles stay fixed.

Worth knowing

“Limpet” describes a shell shape shared by multiple groups. Treating every cap-shell snail as the same harmless algae eater overlooks species with different diets.

What should I do?

Leave a small limpet observed grazing film algae. Identify or isolate one repeatedly feeding on valued sponge or living coral; avoid forcing the strongly attached foot from a surface.

Polyplacophora

Most small reef hitchhikers are useful surface grazers.

Quick ID

A flattened oval mollusc with eight overlapping shell plates down its back, surrounded by a flexible girdle. It clings tightly to rock.

Typical size

Small hitchhikers are commonly millimetres to a few centimetres long; some chiton species grow much larger.

Usually found

Rock undersides, crevices and hard surfaces, often active in low light.

Commonly confused with

Limpets and isopods. The row of eight plates is the most useful visible distinction; the surrounding girdle may partly conceal them.

Worth knowing

Many chitons scrape algae with a specialised radula. Polyplacophora contains diverse feeding strategies, so the Friend badge describes typical small aquarium grazers rather than every species.

What should I do?

Leave identified small grazers. Avoid prying them off rock; if relocation is necessary, transfer the occupied surface where possible.

Vermetidae; sessile worm snails

Mucus nets can irritate nearby corals; assess density and contact with living tissue.

Quick ID

A fixed, irregular hard tube attached to rock or a frag. A snail lives inside and releases a sticky feeding web, especially when food is suspended.

Typical size

Tube openings may be only a few millimetres across; shells extend farther along the surface. Species and colonies vary considerably.

Usually found

Live rock, frag plugs, coral skeleton and shells, often close to places where food passes in the current.

Commonly confused with

Spirorbid or other tube worms. A mucus net rather than a feathery crown or paired palps is an important clue.

Worth knowing

Effects vary between species and conditions. Avoid damaging live coral during removal; predatory snails and dips are not a guaranteed cure.

What should I do?

Remove troublesome specimens on accessible frags in a separate container. Remove the occupied base, not only the tube tip, then rinse debris in tank water and recheck.

Coralliophila spp.; coral-associated muricid snails

Confirmed coral-feeding species should be removed from valued host corals.

Quick ID

Small, often rounded or sculptured snails remaining closely attached to coral. Some species have a purple aperture; the shell alone may be misleading.

Typical size

Often centimetre-scale adults. The pictured C. violacea shell is 3.2 cm long; other species and juveniles differ.

Usually found

Around coral bases, underneath branches and at living tissue margins, sometimes concealed by growth on the shell.

Commonly confused with

Drupella and non-corallivorous marine snails. A purple aperture or nearby coral does not independently prove Coralliophila or predation.

Worth knowing

Host preferences differ by species. Some cause subtle tissue loss while staying in place, so look for feeding activity as well as obvious bite scars.

What should I do?

Manually remove confirmed coral-feeding snails with fine tools in a separate container. Inspect underneath and around the host, then repeat checks for overlooked individuals.

Collonista spp.; tiny turban-like grazers

Correctly identified Collonista are generally welcome algae and film grazers.

Quick ID

A very small, rounded coiled shell with a low spire. Individuals may be pale, mottled or patterned and often appear on glass after dark.

Typical size

Usually only a few millimetres across; the reference shell is 4.1 mm in diameter.

Usually found

Live rock, algae-covered surfaces and glass, particularly at night.

Commonly confused with

Juvenile turbo snails, sundial snails and other small gastropods. Photograph the shell opening and underside as well as the top.

Worth knowing

A large night-time population can look alarming while providing useful grazing. Shape and observed feeding are more informative than size alone.

What should I do?

Usually keep a confirmed grazer. Inspect tiny snails found persistently on irritated zoanthids or clams before assuming they are Collonista.

Pyramidellidae; including Turbonilla-like forms

Some parasitise clams and other molluscs; confirm the identification and host association.

Quick ID

Often minute pale snails with a tall, pointed, many-whorled shell. Look for repeated attachment around a clam’s mantle, shell margins or byssal area.

Typical size

Many aquarium pests are only a few millimetres long. Other harmless tiny snails overlap in colour and size.

Usually found

On affected clams or snails, underneath shells and around the byssal opening; inspect after lights out.

Commonly confused with

Small ceriths and other white snails. Shell-tip structure under magnification and the animal’s host association help identification.

Worth knowing

Pyramidellids use a feeding proboscis, and hosts differ by species. A white pointed shell alone does not establish that a snail attacks Tridacna.

What should I do?

Quarantine affected molluscs and obtain a confirmed ID. Carefully remove confirmed parasites and inspect repeatedly for more animals and egg masses; protect the clam’s mantle and byssal tissues.

Heliacus spp.; zoanthid-associated snails

Confirmed zoanthid-feeding Heliacus can damage colonies.

Quick ID

A flattened, rounded spiral shell, often with alternating pale and dark markings. Check the underside and distinctive operculum rather than relying only on stripes.

Typical size

Often small, millimetre to centimetre scale; juveniles can be easily missed.

Usually found

Between or beneath zoanthid polyps, on colony bases and nearby rock.

Commonly confused with

Collonista and other small patterned snails. A low spiral shell by itself is not a confirmed sundial ID.

Worth knowing

These are snails, not flatworms. Closed polyps can have many causes, so find and identify the animal rather than diagnosing from coral behaviour alone.

What should I do?

Isolate suspect zoanthid colonies and manually remove confirmed pests. Reinspect sheltered areas and egg deposits; use gloves when handling zoanthids and avoid damaging the polyps.

Whelk-like predatory gastropods; several families

Some prey on valued molluscs; similar-looking scavengers require careful identification.

Quick ID

A sturdy, often sculptured spiral shell and an extended siphon. Watch for repeated attacks on a living snail or clam rather than only scavenging a dead animal.

Typical size

From millimetre-scale juveniles to several centimetres. A small arrival can grow considerably.

Usually found

Live-rock crevices, sand and around molluscs or food, often active at night.

Commonly confused with

Nassarius, ceriths and other snails with siphons. A long siphon or pointed shell alone cannot distinguish a predator.

Worth knowing

“Whelk” is a broad common name. Scavenging a carcass does not prove the animal killed it, and no single shell photograph represents every hitchhiker species.

What should I do?

Isolate an unidentified whelk-like snail if valuable molluscs are at risk. Photograph its shell opening and living body, and rehome confirmed predators to a suitable separate system.