Hitchhiker IDs

Trapezia spp.

Confirmed Trapezia can provide protective services to their host coral.

Quick ID

A smooth, compact crab with conspicuous claws nestled among coral branches. Some have spots or bands; not all share one colour pattern.

Typical size

Usually centimetre scale. Species and age affect shell width, and larger individuals may have a much wider leg span.

Usually found

Most often Pocillopora and related branching corals, including Stylophora and Seriatopora.

Commonly confused with

Tetralia in Acropora and other coral-associated crabs. Host identity and markings support an ID but do not prove it alone.

Worth knowing

Different Trapezia species and sizes defend against different predators. Their documented benefits do not mean every coral crab is a guard crab.

What should I do?

Keep a positively identified resident with its healthy host. Avoid unnecessary handling; obtain close photographs if the identification is uncertain.

Cymo spp.

Coral associates with variable effects; identify and assess the host before deciding to remove.

Quick ID

A small crab nestled in coral branches, sometimes furry or granular, with striking blue eyes in some species. Eye colour alone does not confirm Cymo.

Typical size

Small, often around a centimetre across the shell; species and age vary.

Usually found

Within living branching corals, including Acropora and Pocillopora.

Commonly confused with

Gorilla-type crabs and beneficial Tetralia or Trapezia. Use body, claws, host and behaviour together.

Worth knowing

Cymo should not all be labelled pests. Research found C. melanodactylus slowed lesion progression in one study; that does not establish every species as beneficial.

What should I do?

Inspect the host for fresh tissue loss and photograph the crab. If repeated damage is associated with its activity, isolate it for identification.

Porcellanidae; Petrolisthes / Neopetrolisthes spp.

Generally useful particle feeders once correctly identified.

Quick ID

Broad, flattened claws and a flattened body. Watch for fine, fan-like feeding appendages sweeping particles from the water.

Typical size

Many hitchhikers are small, centimetre-scale crabs; apparent size increases substantially when the claws and legs spread.

Usually found

Live-rock crevices and undersides. Neopetrolisthes species are associated with suitable anemones; not all porcelains require an anemone.

Commonly confused with

Squat lobsters and broad-clawed true crabs. Feeding fans are a stronger clue than claw size or colour.

Worth knowing

Large claws do not automatically mean a predator. Porcelain crabs are anomurans, rather than true crabs, and can also accept other foods.

What should I do?

Usually keep in a peaceful reef. Provide shelter and suitable fine foods if normal feeding does not supply enough; protect from crab-eating fish.

Mithraculus sculptus

Often useful algae grazers, but reef compatibility is not guaranteed.

Quick ID

Typically green with a broad shell, sturdy claws and bristly legs. Inspect the whole crab rather than relying on its green colour.

Typical size

Several centimetres including the legs; reported maximum leg span is around 6.5 cm. Tiny arrivals may grow substantially.

Usually found

Caribbean live rock, rock crevices and algae-covered surfaces; commonly also introduced intentionally.

Commonly confused with

Green xanthids and other Mithraculus species. A green body is not proof of a reef-safe emerald crab.

Worth knowing

May graze bubble algae, but results vary. Hungry individuals can become opportunistic, so this is not an unconditional reef-safe label.

What should I do?

Monitor grazing and neighbouring livestock. Supplement with appropriate algae-based food when grazing is scarce. Remove if it repeatedly damages corals or attacks animals.

Majoidea; including Macrocoeloma spp.

Camouflage and feeding behaviour vary; some may damage valued sessile livestock.

Quick ID

Often a pointed or triangular shell with relatively long legs. Attached sponge, algae or other material can conceal its outline.

Typical size

Varies widely. Macrocoeloma trispinosum aquarium examples are reported around 2.5–7.5 cm overall; other decorator species differ.

Usually found

Live rock, rubble and sponge- or algae-rich surfaces; juveniles may arrive without much decoration.

Commonly confused with

Algae or sponge clumps, spider-like crabs and other majoids. Bare juveniles can look unlike decorated adults.

Worth knowing

“Decorator” describes a behaviour, not one species. Some use sponge or algae; others may take pieces of soft coral, so risk is species dependent.

What should I do?

Identify before deciding to keep. Offer suitable food and observe what it cuts or eats. Move to a suitable separate system if corals or small animals are damaged.

Lybia spp.

Usually small and peaceful, but their carried anemones can sting nearby livestock.

Quick ID

Tiny patterned crab often holding a small anemone in each claw, giving a pom-pom appearance. A crab can lose one or both anemones.

Typical size

Common aquarium examples remain small, roughly 1–2.5 cm overall. Species and measurement method vary.

Usually found

Sheltered live-rock crevices and rubble, often shy or active after dark.

Commonly confused with

Small patterned xanthids, especially when the anemones are absent. A missing pom-pom does not rule out Lybia.

Worth knowing

The anemones help collect food and may aid defence. This is a different association from the coral-protecting roles of Tetralia and Trapezia.

What should I do?

Keep with peaceful neighbours and feed small meaty foods. Observe contact with sensitive corals; rehome if repeated stinging causes damage.

Coralliocaris spp.

Coral-associated shrimp should be identified with their host; do not assume every small shrimp is protective.

Quick ID

Small shrimp living deep between coral branches, often with relatively substantial front claws. Colour patterns differ; host and close body photographs matter.

Typical size

Usually small, centimetre-scale shrimp; juveniles can be much smaller. Species-level ID may need a microscope.

Usually found

Especially living branching Acropora; may arrive concealed inside a colony.

Commonly confused with

Jocaste, Harpiliopsis and unrelated juvenile shrimp. Living in a coral is not sufficient to identify the genus.

Worth knowing

Examples to compare include C. graminea, C. superba and C. viridis. The photograph is explicitly labelled C. sandyi; it does not depict those three species.

What should I do?

Inspect the host and photograph the shrimp before removal. Keep a confidently identified healthy associate with its host; investigate repeated tissue loss or suspicious feeding.

Jocaste spp.

Usually close coral associates; appearance alone is not proof of a beneficial relationship.

Quick ID

A small coral-dwelling shrimp tucked between branches. Markings and claw details can help, but field photographs often cannot separate similar species.

Typical size

Small, commonly centimetre scale overall; species and age affect size.

Usually found

Branching Acropora, often hidden deep within a live colony.

Commonly confused with

Coralliocaris, Harpiliopsis and other palaemonid shrimp. The host is a supporting clue rather than a definitive ID.

Worth knowing

J. lucina and J. japonica are relevant examples, not the entire genus. The museum reference photograph shows J. japonica; preservation can change appearance.

What should I do?

Leave a confirmed healthy associate with its host where practical. Record the coral species and clear side/top photographs if an unfamiliar shrimp needs identification.

Harpiliopsis spp.

Coral association is well documented; protective benefits should not be assumed for every species.

Quick ID

A small shrimp with a compact, flattened profile that moves among tight coral branches. Patterns and structural details vary by species.

Typical size

Usually centimetre scale overall, with very small juveniles easily overlooked.

Usually found

Pocillopora and related branching corals; host associations vary with species and location.

Commonly confused with

Jocaste, Coralliocaris, other coral shrimp and juvenile pistol shrimp. A host photograph is useful alongside the animal.

Worth knowing

H. depressa and H. spinigera are two examples; the photograph shows H. beaupresii. Together with the other two cards, the seven named comparison species are examples, not a complete inventory.

What should I do?

Inspect new colonies carefully. Preserve a confidently identified healthy associate with its host, and seek an expert ID if there is unexplained damage.

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.

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.

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.