Crustaceans — Hitchhiker IDs

Hairy gorilla-type crabs; Pilumnus spp. and look-alikes

Treat an unidentified hairy crab cautiously; the common name does not establish its diet.

Quick ID

Dense bristles over the body, legs and claws give a shaggy outline. Many are brown or mottled with strong claws; colour varies.

Typical size

Juveniles can be tiny. Adults may reach several centimetres across the shell; size depends on the species.

Usually found

Rock holes, beneath coral bases and shaded crevices. Often easier to see after lights out.

Commonly confused with

Blue-eyed Cymo, unknown xanthids and other hairy crabs. Hairiness or dark claw tips alone cannot confirm species or feeding risk.

Worth knowing

“Gorilla” is a hobby label covering several crabs. Pilumnus belongs to Pilumnidae, so gorilla and xanthid are not interchangeable scientific names.

What should I do?

Photograph the body and claws. Isolate an unknown specimen or use a monitored baited trap if it is damaging livestock; rehome to a suitable separate system.

Unidentified Xanthidae and similar true crabs

Unknown crab diets and adult size make isolation sensible.

Quick ID

A broad, often stout true crab with prominent claws. It may be smooth, granular or hairy; colour and claw-tip colour vary.

Typical size

May arrive only millimetres wide and grow to several centimetres. Measure carapace width rather than including the legs.

Usually found

Live-rock crevices, beneath coral bases and inside branching colonies, often active after dark.

Commonly confused with

Gorilla-type Pilumnus, emeralds, Cymo and beneficial guard crabs. Use the separate cards for comparison; appearance alone cannot establish diet.

Worth knowing

Xanthidae includes crabs with different diets and coral associations. An unknown family-level ID should not be treated as proof that the animal is either safe or predatory.

What should I do?

Photograph the upper body and claws. Isolate an unidentified crab in a secure container or separate suitable system while identifying it, especially if livestock is disappearing or fresh damage occurs.

Stomatopoda

A predatory hitchhiker unsuitable for many community reefs.

Quick ID

A shrimp-like predator with independently moving stalked eyes and folded striking appendages beneath the head. Species are broadly described as smashers or spearers.

Typical size

Hitchhikers may be small juveniles; many species reach several centimetres or more. Some wild species are much larger.

Usually found

Rock cavities and burrows, often watching from an opening before retreating.

Commonly confused with

Pistol shrimp and other crustaceans. Clicking alone does not distinguish them; look for the eyes and raptorial front limbs.

Worth knowing

Smashers commonly take hard-shelled prey; spearers catch softer prey including fish. Their striking appendages can injure fingers, so use a secure container and suitable tools.

What should I do?

Protect small fish, snails and crustaceans while confirming the ID. Trap or transfer the occupied rock without handling the animal directly. Rehome into an appropriate dedicated setup.

Alpheidae; Alpheus and related genera

Compatibility depends on species, size and tankmates.

Quick ID

A small shrimp with one conspicuously enlarged snapping claw. Some live with a goby, but not every pistol shrimp forms that partnership.

Typical size

Many familiar aquarium species are a few centimetres long; tiny and larger species also occur.

Usually found

Burrows beneath rock, sand edges and sometimes holes in live rock.

Commonly confused with

Mantis shrimp. A pistol shrimp snaps with a modified claw; a mantis shrimp uses folded raptorial appendages and has conspicuous mobile eyes.

Worth knowing

The snap results from a rapid water jet and cavitation, not claws simply clacking together. A clicking sound alone does not identify the animal or prove it killed missing livestock.

What should I do?

Identify before removing. A suitable goby-associated species can be welcome; isolate an unknown shrimp if small livestock is at risk. Keep rock foundations stable if burrowing undermines sand.

Cirripedia; ordinary acorn-type barnacles

Usually harmless attached suspension feeders.

Quick ID

A fixed, volcano-like shell made of several plates, with a central opening. When feeding, a fan of jointed limbs repeatedly sweeps out and retracts.

Typical size

Small attached hitchhikers are often millimetres to about a centimetre across; some species are larger.

Usually found

Rock, shells, coral bases, plugs and other hard surfaces.

Commonly confused with

Limpets and tube worms. Barnacles remain cemented in place; their opening has movable plates and jointed feeding limbs.

Worth knowing

Barnacles are crustaceans, not molluscs. Their feathery feeding limbs collect suspended food; a stationary shell with no visible feeding at one moment may simply be closed.

What should I do?

Leave ordinary small specimens unless they obstruct equipment or an animal’s normal movement. Avoid scraping a living coral merely to remove an adjacent barnacle.

Tetralia spp.

Confirmed Tetralia are coral associates with documented protective roles.

Quick ID

Small, relatively smooth crabs tucked tightly into Acropora branches. Colour and facial markings vary; the host and body shape help identification.

Typical size

Typically small, centimetre-scale crabs. Measure shell width rather than leg span; juvenile size is not a species diagnosis.

Usually found

Living branching Acropora, often in sheltered spaces deep within a colony.

Commonly confused with

Trapezia, hairy Cymo and juvenile predatory crabs. A crab living in Acropora is not automatically Tetralia.

Worth knowing

These crabs can defend their coral against some predators. Protection varies with crab, coral and predator; they are not a substitute for pest inspection.

What should I do?

Leave a confidently identified, healthy resident with its host. Inspect new colonies carefully and seek an ID before removing an unfamiliar crab.

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.