Leatherjackets: Australian Divers' ID Guide

Leatherjackets are the most underrated fish in Australian water. They appear at 21 of the dive sites we cover, they are endlessly approachable, and Australia has one of the richest leatherjacket faunas anywhere on earth. They are also the fish most often mistaken for something boring, which is a shame, because a leatherjacket does several things no other fish on the reef does.

## The name, and the spike

The name comes from the skin. Leatherjackets, family Monacanthidae, have no conventional scales. Instead the body is covered in tiny bristle-like denticles that give the skin a texture somewhere between suede and fine sandpaper. In older Australian usage they were called "leatherjackets" because that hide was tough enough to be genuinely difficult to skin.

The defining feature is on top of the head: a stout dorsal spine that locks upright. It is not decorative. A threatened leatherjacket wedges itself into a crevice, erects the spine, and locks it in place with a smaller second spine acting as a trigger. The fish becomes extremely difficult to extract, which is the same principle their close relatives the triggerfish use.

The body plan is the other giveaway. Leatherjackets are deep, laterally compressed and often almost diamond-shaped, with a small mouth carrying a set of prominent incisor-like teeth. They swim by rippling the soft dorsal and anal fins in opposing waves, which lets them hover, reverse and turn on the spot with a precision no tail-driven fish can match. Watching one manoeuvre around a sponge is watching a genuinely different solution to underwater movement.

## Species Australian divers meet

Australia has dozens of species and the southern temperate reefs are the diversity hotspot.

- **Six-spine leatherjacket** (*Meuschenia freycineti*), common through southern Australia, olive to brown with blue-green markings and dark blotching - **Horseshoe leatherjacket** (*Meuschenia hippocrepis*), with a bold curved band across the flank, a southern species - **Fan-bellied leatherjacket** (*Monacanthus chinensis*), which extends a large flap of skin below the body when displaying - **Mosaic leatherjacket** (*Eubalichthys mosaicus*), a spectacular large southern species with a strongly patterned body - **Yellow-finned and toothbrush leatherjackets**, both frequently seen around jetties and piers

Sexes often differ, and many species carry patches of bristles on the tail base in males, so as with wrasse, one photograph per species in a field guide will only get you so far.

## Where to find them

**NSW** is reliable across the board. [Little Bay](https://www.scubadownunder.com/dive-sites/little-bay), [North Bondi Rocks](https://www.scubadownunder.com/dive-sites/north-bondi-rocks) and [Terrigal Haven](https://www.scubadownunder.com/dive-sites/terrigal-haven) all hold several species, and the [Terrigal HMAS Adelaide](https://www.scubadownunder.com/dive-sites/terrigal-hmas-adelaide) wreck concentrates them around structure.

**Victoria** puts them on the piers. [Blairgowrie Pier](https://www.scubadownunder.com/dive-sites/blairgowrie-pier) and [Rye Pier](https://www.scubadownunder.com/dive-sites/rye-pier) are both excellent, and the submarine wrecks including [HMAS J4](https://www.scubadownunder.com/dive-sites/hmas-j4-submarine) hold resident populations.

**South Australia** is the diversity centre. The jetties are outstanding: [Port Lincoln Jetty](https://www.scubadownunder.com/dive-sites/port-lincoln-jetty), [Elliston Jetty](https://www.scubadownunder.com/dive-sites/elliston-jetty), [Marion Bay Jetty](https://www.scubadownunder.com/dive-sites/marion-bay-jetty) and [North Haven Wall](https://www.scubadownunder.com/dive-sites/north-haven-wall).

**Western Australia** adds its own southern species at sites like [Clarkes Reef](https://www.scubadownunder.com/dive-sites/clarkes-reef) near Albany.

The pattern is worth noticing: leatherjackets love structure. Jetty pylons, wrecks, kelp and sponge gardens. Open sand is where you will not find them.

## Diving with them

Leatherjackets are one of the easier subjects in temperate diving because they are curious and slow. They will often approach a stationary diver, hovering at a metre or so, working the fins to hold position while they decide what you are.

That hovering is the photograph. Let the fish come to you rather than chasing, because a leatherjacket that turns away will simply keep the sponge between you and it. Shoot slightly downward or level rather than up, since the body pattern is the point. And be patient around jetty pylons at dusk, when many species become noticeably bolder.

One caution: some leatherjacket species accumulate toxins, and there have been Australian poisoning cases from eating certain species. This is a fish to photograph rather than take.

## Why they matter to the reef

Leatherjackets are significant grazers of sponges, ascidians, bryozoans and algae, and in southern Australia several species eat the invertebrate growth that would otherwise dominate reef surfaces. Their small mouths and incisor teeth are specialised for exactly this: nipping pieces from attached, often chemically defended prey that few other fish will touch.

They are, in other words, one of the animals doing the maintenance work that keeps a temperate reef looking the way it does. Worth a second look next time one hovers in front of your mask.

## Sources

- [Australian Museum, Fishes](https://australian.museum/learn/animals/fishes/) - [Atlas of Living Australia](https://www.ala.org.au/)