Common Hitchhiker IDs

Polychaeta; common scavenging amphinomids

For common scavengers; identify unusual or damaging worms separately.

Quick ID

A flexible, segmented worm with bristles along both sides. Common scavenging forms are pink, orange or grey and retreat into rock when disturbed.

Typical size

From a few millimetres as juveniles to many centimetres. Size alone cannot separate a helpful scavenger from a predator.

Usually found

Rock crevices, sand, beneath frag plugs and around leftover food, especially after lights out.

Commonly confused with

Coral-eating fireworms and eunicids. Eunicids have a conspicuous head with antennae and jaws; colour and white bristles alone are unreliable.

Worth knowing

“Bristleworm” is a broad description. Eurythoe is also called a fireworm, so the hobby’s good-worm/bad-worm naming split is imperfect. Feeding behaviour matters more than the nickname.

What should I do?

Leave ordinary scavengers that show no damaging behaviour. Remove uneaten food if numbers surge. Use tools and gloves when moving occupied rock; avoid touching bristles.

Amphinomidae; Hermodice carunculata example

For identified coral-eating fireworms in a community reef.

Quick ID

A robust segmented worm with prominent white bristle tufts. Hermodice has a branched fleshy structure, the caruncle, behind the head; a close view is needed.

Typical size

Can reach tens of centimetres. Small individuals can already be difficult to distinguish from other amphinomids.

Usually found

On and beneath live rock, including around corals and other animal food sources.

Commonly confused with

Common scavenging bristleworms, many of which are themselves called fireworms. Red colour, large size or fluffy bristles do not establish Hermodice.

Worth knowing

Hermodice carunculata is an opportunistic feeder that can consume living coral and other invertebrates. Seeing any worm on already dead tissue does not prove it killed the animal.

What should I do?

Photograph the head and body, confirm the ID, then isolate a coral-eating animal with a baited trap or long tools. Do not handle it bare-handed.

Terebellidae and related terebelliform worms

Surface deposit feeders that collect small organic particles.

Quick ID

Many long, thin, flexible feeding tentacles radiate from one hidden point. The thicker segmented body usually stays inside a tube or crevice.

Typical size

Tentacles may stretch several centimetres or more. Visible threads do not reveal the full body length, which varies widely by species.

Usually found

Sand, rubble and rock crevices, with tentacles sweeping adjacent surfaces.

Commonly confused with

Digitate hydroids and vermetid mucus nets. Terebellid tentacles are numerous, mobile threads emerging from a worm’s hiding place.

Worth knowing

Feeding tentacles transport gathered particles back to the mouth. A thread crossing coral is not, by itself, evidence that the worm is eating living coral.

What should I do?

Leave them to collect particles. Avoid tearing tentacles or repeatedly exposing the tube; move a frag away if contact seems to irritate it.

Sabellidae and Serpulidae

Usually harmless suspension feeders.

Quick ID

A symmetrical fan or feather-like crown protrudes from a fixed tube and retracts quickly when disturbed. Tubes may be soft or calcareous.

Typical size

Small hitchhiker crowns are often only millimetres to a few centimetres across. Larger purchased feather dusters are different care propositions.

Usually found

Rock, shaded crevices, undersides of frags and sometimes sump surfaces.

Commonly confused with

Hydroids and Aiptasia. A tube worm withdraws its organised crown into a tube; hydroids have individual stinging polyps or branches.

Worth knowing

They catch fine suspended food, not dissolved nutrients directly. A lost crown can regrow, but repeated crown loss or abandonment can indicate stress rather than normal thriving.

What should I do?

Leave established small worms. Avoid damaging the tube or adding food solely because they are present; observe growth and condition first.

Spirorbinae; Serpulidae

Tiny harmless tube-dwelling suspension feeders.

Quick ID

An attached white, tightly coiled calcareous tube, sometimes with a minute feeding crown visible at the opening.

Typical size

Tubes are typically a few millimetres across; Spirorbis spirorbis reaches roughly 3–4 mm in diameter.

Usually found

Glass, pumps, shells, rock, frag plugs and shaded hard surfaces.

Commonly confused with

Snail eggs and vermetid tubes. Spirorbid tubes are hard, fixed coils with a worm opening; eggs do not deploy a feeding crown.

Worth knowing

These are worms, not snails. Aquarium spirorbids are not necessarily Spirorbis spirorbis; a white coil alone is insufficient for species identification.

What should I do?

Leave on rock and plugs. Scrape viewing panels or equipment during routine cleaning if needed, keeping debris out of moving parts.

Sipuncula; sipunculans within Annelida

Usually harmless deposit feeders hidden in rock or sediment.

Quick ID

An unsegmented, bristle-free worm that extends a thin front section, the introvert, then pulls it back inside a thicker body like a glove finger.

Typical size

Aquarium-visible extensions range from millimetres to several centimetres. The concealed body and larger wild species may be substantially longer.

Usually found

Rock holes, rubble and sediment, often extending to feed after dark.

Commonly confused with

Eunicids, bristleworms and a single spaghetti-worm tentacle. Peanut worms lack obvious external segments and lateral bristle tufts.

Worth knowing

The feeding end may show a ring of small tentacles when fully extended. Retraction can make the same animal look very different from one moment to the next.

What should I do?

Leave a confirmed sipunculan. Avoid pulling the protruding section out of its hole or breaking rock merely to expose the body.

Eunicidae; large predatory Eunice-type forms

Eunicids vary; confirmed large ambush predators are pests in a community reef.

Quick ID

A segmented worm with a distinct head, antennae and strong jaws. Many familiar Eunice-type forms have five head appendages; a clear head view is important.

Typical size

From small rock-dwelling worms to very large species. The notorious ambush predators can reach metre scale; that is not typical of every eunicid.

Usually found

Rock burrows and sediment, often with only the head exposed at night.

Commonly confused with

Ordinary bristleworms and peanut worms. Jaws and head antennae are more informative than body colour, iridescence or length.

Worth knowing

“Bobbit worm” is used loosely. Not all Eunicidae are the same large fish-catching predator; remove a confirmed dangerous individual without labelling every small eunicid a fish killer.

What should I do?

Confirm the ID and behaviour. Isolate an occupied rock or use a monitored trap for a troublesome animal. Avoid grabbing the head or tearing a worm out of its burrow.

Spionidae; including Polydora-type worms

Often particle feeders; some species bore into shells or calcareous substrate.

Quick ID

Usually two slender feeding palps extend from one tube or burrow. They sweep particles and may curl or retract when disturbed.

Typical size

Visible palps may extend millimetres to centimetres; the hidden body and adult size vary between species.

Usually found

Rock, sand tubes, shells, frag bases or calcareous coral skeleton. Location alone does not establish harm.

Commonly confused with

Spaghetti worms with many tentacles, fan worms with a crown, and vermetid snails that release mucus webs.

Worth knowing

Not every spionid is a coral pest. Palps alone cannot identify a boring species; treatment choice needs evidence of damage and a more reliable ID.

What should I do?

Leave harmless rock dwellers alone. If a coral is affected, isolate and inspect it; remove accessible infested plug or dead substrate without cutting living tissue. Recheck for activity.

Small transparent acoels; including Amphiscolops-like forms

Many free-living forms cause little trouble; clear colour does not establish a safe identification.

Quick ID

A very thin, transparent or whitish animal gliding smoothly over glass or rock. Internal contents may be visible; use a macro photograph to inspect its outline.

Typical size

Often only a few millimetres long. Size and colour overlap with other flatworms.

Usually found

Aquarium glass, rock, algae and sheltered surfaces, sometimes near concentrations of small pods.

Commonly confused with

Pale coral-eating flatworms, juvenile polyclads and tiny slugs. The absence of brown colour does not rule out a pest.

Worth knowing

Many feed on small organisms rather than coral tissue. Acoels are commonly called flatworms in reefkeeping but belong to a different lineage from true polyclad flatworms.

What should I do?

Observe a small stable population and obtain an ID. If animals sit on damaged coral, isolate the coral for closer inspection rather than assuming they are harmless.

Convolutriloba spp.; hobby name “red planaria”

Large populations can cover surfaces, and mass die-offs can harm livestock.

Quick ID

Small reddish or rust-brown flattened bodies gliding on surfaces. Convolutriloba typically has three lobes at the rear; magnification helps.

Typical size

Usually a few millimetres; the photographed C. retrogemma is approximately 3 mm long.

Usually found

Light-exposed rock, substrate and glass; dense populations may also spread over corals.

Commonly confused with

Waminoa, other brown acoels and detritus. Red, rust and brown planaria are overlapping hobby names, not three separate diagnoses.

Worth knowing

These are not the same as Acropora-eating flatworms. Asexual reproduction can build numbers quickly; released toxins are a particular concern when many die together.

What should I do?

Siphon accessible animals and inspect new arrivals in quarantine. For a heavy infestation, plan management carefully: avoid an uncontrolled mass die-off or unverified chemical dosing.

Waminoa spp.; coral-associated acoels

Heavy coverage can interfere with coral feeding and light; they are distinct from tissue-eating coral flatworms.

Quick ID

Small tan or brown flattened discs resting on living coral. The outline is often rounded or notched, rather than the three-tailed Convolutriloba shape.

Typical size

Typically millimetre scale; a close photograph is more useful than colour alone.

Usually found

Living coral surfaces, including bubble and other fleshy corals. Look for individual discs moving across the tissue.

Commonly confused with

Red planaria, coral pigmentation and tissue-eating flatworms. Host location alone cannot confirm the genus.

Worth knowing

Waminoa can consume coral mucus and intercept food. Research documents reduced host feeding efficiency; this is different from directly consuming coral tissue.

What should I do?

Monitor coverage and coral response. Isolate affected new coral and seek a coral-appropriate removal plan; avoid scraping living tissue or treating the entire tank casually.

Polycladida; leopard-type and other larger flatworms

Some species prey on snails or bivalves; a spotted pattern does not establish diet.

Quick ID

A broad, very thin leaf-like animal with a rippling edge, gliding beneath rock. Some have leopard-like spots; others are plain or differently patterned.

Typical size

From small juveniles to several centimetres long, depending on the species. They can stretch and fold dramatically.

Usually found

Live-rock undersides, crevices, empty shells and areas near molluscs, especially after dark.

Commonly confused with

Nudibranchs, acoels and harmless-looking mottled film. A flatworm lacks a slug’s typical raised body and obvious external appendages.

Worth knowing

Polyclads have varied diets. Some consume bivalves, while others specialise on different prey; do not label every colourful marine flatworm a clam eater.

What should I do?

Isolate an unidentified larger specimen for photographs. If molluscs are disappearing, inspect rock and clam surroundings at night and remove confirmed predators to a separate system.