Stars — Hitchhiker IDs

Aquilonastra spp.; Asterinidae

Often harmless grazers; reported coral interactions need observation.

Quick ID

A tiny, chunky star with short arms. Uneven arm lengths and odd arm counts are common after splitting and regeneration.

Typical size

Usually millimetres to around a centimetre across in aquaria; the group contains several species.

Usually found

Glass, live rock, coralline-covered surfaces and sometimes around coral colonies.

Commonly confused with

Mini brittle stars, which have a separate small disc and long, thin arms. The aquarium nickname “Asterina” does not reliably identify a genus or species.

Worth knowing

Aquilonastra corallicola has been studied feeding on algae in aquaria. Coral or zoanthid feeding is also reported by hobbyists, but cannot be predicted reliably from colour alone.

What should I do?

Observe what they actually eat. Isolate individuals repeatedly associated with fresh coral damage; manually reduce a troublesome population. Do not buy a specialist star-eating shrimp without a long-term feeding plan.

Research sources

• Original aquarium study: Aquilonastra fission
• Firsthand observations of possible zoanthid feeding

Small Ophiuroidea; Ophiactis and related genera

Small hitchhiking forms are generally useful particle feeders.

Quick ID

A tiny central disc clearly separate from long, thin, flexible arms. Often only striped or bristly arms project from a rock hole.

Typical size

Discs are commonly millimetres across; the total arm span may reach several centimetres. Larger traded brittle stars require a separate assessment.

Usually found

Rock pores, sponges, sand and shaded crevices, with arms reaching into the water or across surfaces.

Commonly confused with

Aquilonastra stars and worm tentacles. Brittle-star arms are jointed and attach to a distinct central disc.

Worth knowing

Some Ophiactis reproduce by splitting, leaving unequal arms while they regenerate. An unusual arm count does not automatically mean disease or a different type of animal.

What should I do?

Leave small established hitchhikers. Avoid grabbing arms or crushing occupied rock; do not assume a much larger green brittle star has the same compatibility.

Research sources

• University of Michigan: Ophiactis savignyi
• Original research: brittle-star diversity and fission