Bony fishes

FilefishesMonacanthidae

Family10 species

One filefish here hides by smell. The longnose filefish eats nothing but the polyps of Acropora coral, and it takes on the chemical signature of what it eats closely enough that in trials a reef predator hunting by odor responded to the fish no more than it responded to the coral. It was the first vertebrate shown to disguise its smell rather than its outline, and the disguise is made entirely of dinner.

The bill for that specialization comes due when the coral does. A fish that eats one genus of coral has nowhere to go when a bleaching event takes it, and populations of these filefish drop out of reefs where the Acropora has died even though the reef structure is still standing. This sea has its own version of the animal, found here and nowhere else, alongside the wider Indo-Pacific one.

The family as a whole is triggerfishes with the armor taken off: same order, same body plan, thinner and more flexible. The skin has no ordinary scales, only minute bristles set into it, and the result is rough enough that dried filefish hides were used to smooth wood, which is where the English name comes from. The first dorsal spine locks upright, and the catch that holds it is described on one of the species pages. Below, the pelvis swings down a flap of skin that makes the fish abruptly twice as deep, so one that has reversed into a crevice is wedged there deliberately and cannot be pulled out by the tail.

The rest of the family eats what nothing else will: sponges, hydroids, seagrass, algae, gorgonian polyps, tunicates. They travel by rippling the dorsal and anal fins, tail folded away, which gives them a slow hover, the ability to reverse, and a habit of hanging head-down beside a coral head while working over it.

Ten species are listed here, from fish the length of a finger to one over a meter, and several are small, plain, brown and effectively unrecorded in this sea. Their eggs are laid on the bottom rather than released into the current, and in some species the male stays with them, which is unusual company for the group they belong to.

How to recognise filefishes

OvalEgg-shaped body, deeper than it is thick - damselfishes, surgeonfishes.

rhomboid body, laterally compressed
single prominent dorsal spine
small terminal mouth

10 species in Filefishes

Aluterus

The big filefishes, drawn out long and flat: up to a meter, with a broom-like tail and a habit of drifting head-down in open water beside the reef, moving with the surge rather than against it. They read as debris until they turn.

Amanses

A genus of one species, and it is built around a difference between the sexes that nothing else in the family shows: males grow five or six long stiff quills on the side of the tail base, females a dense pad of bristles in the same place. Both are made of the same skin spinules that give the family its name; only here are they drawn out far enough to sex the fish at a glance.

Broom Filefish

Amanses scopas

Cantherhines

Broad, deep-bodied filefishes that sleep wedged into the reef. The first dorsal spine hinges up and a second, smaller one slots in behind to jam it there - reversible for the fish and not for anything pulling on it - so an animal backed into a hole with the spine raised against the roof does not come out. Triggerfishes are named for the same mechanism; the filefishes inherited it.

Honeycomb Filefish

Cantherhines pardalis

Oxymonacanthus

The coral pickers. A long tube of a snout with a minute mouth at the end, used to take single Acropora polyps one at a time, and rows of yellow spots along the body that read as the coral itself. They go in pairs and they eat nothing else, which is why the genus disappears from a reef before the coral skeletons do.

Orange Spotted Filefish

Oxymonacanthus longirostris

VUEndemic

Red Sea Longnose Filefish

Oxymonacanthus halli

Paramonacanthus

Small, deep, leaf-shaped filefishes of weedy rubble and soft bottom rather than clean coral, and the genus where the sexes diverge: males run to longer snouts, drawn-out fin lobes and patches of enlarged spinules on the tail base while females stay plain, so two fish of one species can look like two species.

Faintstripe Filefish

Paramonacanthus pusillus

Fuzzy Filefish

Paramonacanthus nematophorus

Pervagor

Small filefishes, most under ten centimeters, where the tail carries the color the body does not - orange or red, often crossed by darker stripes, on an otherwise plain brown fish. The genus name means to wander, which fits the family better than it fits these: they keep to rubble and the undersides of coral heads.

DDEndemic

Randall’s Filefish

Pervagor randalli

Stephanolepis

A genus from cooler water than this, and the one that left. These are filefishes of subtropical and temperate seas, and the species here went north through the Suez Canal into the Mediterranean, where it is now established - one of several hundred species that have made that crossing in one direction only.

Lozenge Filefish

Stephanolepis diaspros

Other oval-bodied groups

Similar silhouette underwater - worth a look if this was not your animal.

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Last Update: September 11, 2026