Illustrative orange fireworm with white bristles on purple live rock

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Sea slugs — Hitchhiker IDs

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 FragsLast reviewed:

Sea slugs

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Scutus sinensis, identified at source: snail or mollusc identification reference.

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.
Research sources
Vayssierea felis photographed in Réunion; an example of the tiny orange-red spirorbid-eating nudibranchs.

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.
Research sources
Close-up of coral tentacles in purple, pink, blue and green

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