Hitchhiker IDs

Isopoda; including munnids, sphaeromatids and cirolanids

Many are harmless; some isopods parasitise fish.

Quick ID

An often oval body flattened top to bottom, with seven pairs of walking legs as an adult. Free-living munnids can be tiny; fish-attacking forms may swim rapidly or attach to a fish.

Typical size

Many aquarium finds are only millimetres long. Fish-associated parasitic isopods can be much larger; there is no universal size cutoff.

Usually found

Algae, rock and sediment for many free-living forms. Inspect fish skin, fins, mouth and gill areas if attachment or injury is suspected.

Commonly confused with

Amphipods, tanaids and juvenile shrimp. Tanaids have a separate card; eyes and swimming speed alone do not prove that an isopod attacks fish.

Worth knowing

Munnid and some sphaeromatid isopods are familiar free-living forms. Cirolanids include scavengers and fish-attacking forms; neither “all pods are safe” nor “all cirolanids are parasites” is reliable.

What should I do?

Leave confidently identified free-living grazers. If an animal attaches to a fish or repeatedly attacks it, isolate the affected livestock and seek expert identification and safe removal advice.

Copepoda; harpacticoids, cyclopoids and calanoids

Confirmed free-living copepods are useful food-web animals; parasitic forms require a separate assessment.

Quick ID

Tiny moving specks with a segmented body and antennae under magnification. Adults may carry egg sacs; young nauplii look different from adults.

Typical size

Many aquarium forms are below 1 mm to a few millimetres. A phone image often cannot resolve species.

Usually found

Glass, biofilm, rock, sand and refugia, or swimming in the water column. Harpacticoids often use surfaces; calanoids are mainly planktonic, while cyclopoid habits vary.

Commonly confused with

Ostracods, juvenile amphipods, mites and coral-associated parasitic copepods. Movement alone does not identify a species.

Worth knowing

Tisbe and Tigriopus are familiar harpacticoid examples; Apocyclops is a cyclopoid. “Crawler” versus “swimmer” is a helpful observation, not a strict identification test.

What should I do?

Usually leave free-living populations in place and provide varied habitat. If tiny animals are consistently attached to damaged coral or fish, obtain an ID before treating.

Ostracoda

Most familiar free-living aquarium seed shrimp are inconspicuous scavengers or grazers; diets vary between species.

Quick ID

A tiny oval or bean-shaped shell enclosing the body, with legs or antennae projecting while it moves. It can look like a seed that suddenly walks.

Typical size

Many hitchhikers are roughly submillimetre to a few millimetres long; the group includes larger species.

Usually found

Rock pores, sediment, algae, glass and refugia. Some crawl, while others swim.

Commonly confused with

Copepods, snail eggs and tiny bivalves. The enclosing two-valved shell and moving appendages are useful clues.

Worth knowing

Despite the shell, these are crustaceans. The marine Vargula photograph illustrates the body plan; it does not imply that all aquarium seed shrimp are Vargula or glow.

What should I do?

Usually observe and leave a normal population in place. Identify unusual large animals or any suspected livestock damage before making a management decision.

Mysida; not the same order as true shrimp

Small free-living mysids are usually useful members of the aquarium food web.

Quick ID

A tiny transparent shrimp-like animal with prominent stalked eyes and a clear tail fan. Often darts or hovers in small groups near shelter.

Typical size

Common small aquarium forms are a few millimetres to around a centimetre; species vary.

Usually found

Rock crevices, shaded spaces, refugia and the water immediately around them, often conspicuous after dark.

Commonly confused with

Juvenile true shrimp, amphipods and fast-swimming isopods. A clear shrimp shape with stalked eyes helps distinguish them.

Worth knowing

Females brood young in a pouch. The frozen food sold as “mysis” is not evidence that a living hitchhiker is the same species.

What should I do?

Usually leave in place. Provide refuges and monitor compatibility with very small livestock; mysids are omnivores, not exclusively algae eaters.

Tanaidacea; tiny pincer-bearing crustaceans

Most small free-living aquarium tanaids are unobtrusive; feeding habits vary.

Quick ID

A slender, segmented body with conspicuous small front pincers. Many remain close to the surface or retreat into a tiny sediment or mucus tube.

Typical size

Common finds are only a few millimetres long. Magnification is often needed to see the pincers clearly.

Usually found

Sediment, rock, macroalgae and small tubes in refugia or quiet crevices.

Commonly confused with

Isopods, amphipods and juvenile shrimp. Tanaids are a separate crustacean order, not a type of isopod.

Worth knowing

Some graze or collect detritus, others filter-feed or take small prey. Tube building and species-dependent feeding make a single universal diet misleading.

What should I do?

Usually leave an identified population alone. Photograph with magnification if uncertain, and assess behaviour before assigning a safe or pest label.

Caprellid amphipods; Caprellidae

Usually unobtrusive food-web animals; some also take small prey.

Quick ID

A very slender stick-like segmented body with grasping limbs. Often clings by the rear legs and reaches or sways like a tiny praying mantis.

Typical size

From a few millimetres to a few centimetres, depending on species; many hitchhikers are small.

Usually found

Macroalgae, hydroids, bryozoans and other surfaces to which they can cling.

Commonly confused with

Tanaids, hydroids and tiny shrimp. Watch for a jointed moving body and gripping legs rather than a fixed polyp.

Worth knowing

Despite the name, these are amphipods, not true shrimp. They can graze, collect detritus or capture small organisms; diet is species dependent.

What should I do?

Usually observe and leave in place. Identify unusually large individuals or any suspected predation before deciding on removal.

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.