Found something in your reef? Compare the shape, movement and distinctive features below to identify the hitchhiker and decide what to do.
Friend: usually useful or harmless. Caution: identification or behaviour changes the risk. Pest: confirm the organism before managing it. Photographs show reference examples; similar-looking animals may be different species.
Written by Christian, Signature Frags · Last reviewed:
Search by common name, scientific name or abbreviation.
Stomatella impertusa: a live snail with its small shell and exposed soft body. Identification follows the source observation.
Stomatella snails
Friend
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.
Diodora aspera: a keyhole limpet example, not a claim that all limpet-shaped snails share its diet.
Limpets
Caution
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.
Chiton tuberculatus: a clear eight-plate body-shape reference; this species can exceed the size of a typical small hitchhiker.
Chitons
Friend
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.
Living vermetid colony in Réunion, showing fixed irregular tubular shells.
Vermetid snails
Caution
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.
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.
Turbonilla lactea shell, 5.3 mm long; a pyramidellid shape reference, not proof that this individual species attacks giant clams.
Pyramidellid snails
Pest
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.
Living Heliacus variegatus photographed in the Philippines.
Sundial snails
Pest
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.
Living Pollia undosa in Sulawesi; a whelk example, not a diagnosis for every pointed aquarium snail.
Predatory whelks
Caution
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.
Scutus sinensis, identified at source: a mottled Scutus example. Familiar aquarium specimens are often darker.
Elephant slugs
Friend
Scutus spp.; Fissurellidae
Confirmed Scutus are generally useful grazers.
Quick ID
A broad, often dark snail with a mantle covering most of a pale shield-like shell. A small patch of white shell may show through the back.
Typical size
Usually noticed at centimetre scale; some Scutus species can exceed 10 cm. A small hitchhiker need not stay small.
Usually found
Rock undersides and shaded crevices, coming out to graze in low light.
Commonly confused with
Nudibranchs, Stomatella and other shield limpets. Check for the partly hidden shell rather than relying on a black slug-like outline.
Worth knowing
Despite the nickname, Scutus is a shelled snail. It belongs to the keyhole-limpet family, whose other members do not necessarily share Scutus feeding habits.
What should I do?
Leave a confirmed, normally grazing Scutus and allow space as it grows. If tissue damage is observed, document the behaviour and recheck the ID before assuming every similar animal is safe.
Vayssierea felis photographed in Réunion; an example of the tiny orange-red spirorbid-eating nudibranchs.
Orange spirorbid-eating nudibranchs
Friend
Vayssierea spp.; including V. felis
Confirmed Vayssierea feed on small tube worms rather than corals.
Quick ID
A minute orange-red slug with a low smooth body and two small rhinophores. Look closely: orange colour alone does not identify Vayssierea.
Typical size
Only a few millimetres long; V. felis is reported up to approximately 5–6 mm.
Usually found
Glass, rock and other surfaces carrying tiny white coiled spirorbid worm tubes.
Commonly confused with
Other orange nudibranchs, flatworms and eggs. Host/prey association and a clear macro photograph are important.
Worth knowing
It eats the worms inside their calcareous tubes, not the hard tubes themselves. The genus contains similar forms, so a photograph or colour description may not establish species.
What should I do?
Usually leave a confirmed specimen in place if loss of spirorbids is acceptable. Do not purchase it as a general coral-pest cure; it depends on suitable small tube-worm prey.
Gammarus locusta: a marine amphipod body-shape example; not an identification of every reef “pod”.
Amphipods
Friend
Amphipoda
Common free-living aquarium forms recycle organic material.
Quick ID
A small, shrimp-like crustacean, usually curved and compressed from side to side. Many scuttle through algae or flex their body to move.
Typical size
Often a few millimetres to around a centimetre in aquaria; some species are larger.
Usually found
Rock pores, macroalgae, refugia, sand edges and underneath plugs, especially at night.
Commonly confused with
Copepods, isopods and tiny shrimp. Isopods tend to be flattened top to bottom; most copepods are much smaller.
Worth knowing
Amphipoda is a diverse order, not one guaranteed harmless species. An animal eating damaged tissue or trapped food does not prove it caused the original injury.
What should I do?
Leave common free-living forms unless you observe repeated damage to living tissue. Provide normal habitat and avoid overfeeding solely to increase numbers.
