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Anemones & Hydroids — 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: 13 October 2026

Anemones & Hydroids

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Pennaria disticha: tentacled animal reference. — Pennaria disticha: a branched colonial hydroid. This image does not show the separate digitate growth form.

Hydroids / digitate hydroids

Pest

Hydrozoa; multiple families

Treat irritating or spreading growth as a reef nuisance.

Quick ID: Tiny tentacled polyps on stalks or branching colonies. “Digitate” forms look more like a flexible finger or thread emerging from an attachment point.
Typical size: Individual polyps can be tiny; colonies or extended stalks can span centimetres. Growth form and size vary markedly.
Usually found: Live rock, frag plugs, algae, glass and sometimes shells; check shaded surfaces too.
Commonly confused with: Feather dusters, algae and spaghetti-worm tentacles. Hydroids have stinging polyps, rather than a feather crown retracting into a worm tube.
Worth knowing: Hydroids are predators using stinging cells. Risk depends on species, extent and nearby animals; isolated hydroids are not all equally destructive.

What should I do?: Isolate a newly affected frag. Remove a small accessible patch with its attachment material outside the display in tank water, avoiding injury to the coral. Monitor for regrowth; avoid blanket chemical treatment without a specific ID.

Research sources

Australian Museum / Bill Rudman: hydroids

A ball-tipped corallimorph identified at source as Corynactis globulifera (formerly Pseudocorynactis globulifera). The photograph is a reference example.: tentacled animal reference. — A ball-tipped corallimorph identified at source as Corynactis globulifera (formerly Pseudocorynactis globulifera). The photograph is a reference example.

Ball anemones

Caution

Ball-tipped corallimorphs; Corynactis / Pseudocorynactis-type

Small examples can be tolerated; larger predatory forms need care.

Quick ID: A small disc surrounded by translucent tentacles with conspicuous rounded tips, often pale or orange. The tentacle tips give the “ball” appearance.
Typical size: Many aquarium finds are centimetre scale. Related ball-tipped corallimorphs can grow much larger; do not assume all stay tiny.
Usually found: Shaded holes, rock undersides and areas where suspended food passes.
Commonly confused with: Aiptasia, small true anemones and mushroom corals. A rounded tentacle tip is useful, but not sufficient for an exact genus identification.
Worth knowing: These are corallimorphs, not true anemones. The hobby name covers more than one taxon; feeding risk cannot be inferred confidently from the name alone.

What should I do?: Observe growth and neighbouring livestock. Move sensitive frags away from contact. Isolate a large or actively troublesome specimen rather than assuming every small one needs eradication.

Research sources

Strawberry anemones

Caution

Corynactis spp.; common-name identification guide

Common names overlap; temperate Corynactis are not suitable additions to a warm reef.

Quick ID: Small, brightly coloured corallimorphs with conspicuous rounded tentacle tips; some form close groups of red, pink or orange individuals.
Typical size: Typically small: C. californica is approximately 2.5 cm wide; dimensions vary by species and whether the tentacles are extended.
Usually found: C. californica inhabits temperate Pacific rocky reefs; C. viridis is a temperate Atlantic jewel anemone. Tropical look-alikes also arrive on live rock.
Commonly confused with: Ball anemones, small mushrooms and true anemones. Strawberry is a common name, not a reliable species or temperature requirement.
Worth knowing: These are corallimorphs rather than true anemones. Do not buy or introduce a temperate strawberry/jewel anemone for a tropical reef based on this photograph.

What should I do?: Photograph and establish origin before treating or keeping it. Check the separate ball-anemone card for tropical finds. Protect neighbouring coral from contact and identify any growing colony.

Research sources