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Southern Pygmy Leatherjacket
Brachaluteres jacksonianus (Quoy & Gaimard, 1824)

A Southern Pygmy Leatherjacket at a depth of 5 m, Fly Point, Nelson Bay, New South Wales. Photo © G. McNeil.

Common name

Southern Pygmy Leatherjacket

Scientific name

Brachaluteres jacksonianus
brachys - short (Greek)
luteus - yellow (Latin)
jackson - named after Port Jackson (Sydney Harbour)
ianus - pertaining to, belonging to (Latin)

Presumably the genus name was chosen to describe the shape and common colouration of this species. The species name refers to Port Jackson (Sydney Harbour), the place where the fish used in the description of the species was caught.

Family name

Monacanthidae
mon as - single (Latin)
canthus - spine (Latin)
idae - suffix meaning that this a family name. All animal family names end in -idae.

The family name refers to the large first dorsal fin spine.

Size

The Southern Pygmy Leatherjacket grows to 9 cm in length.

The smallest leatherjackets are in the genus Rudarius. The Diamond Leatherjacket, Rudarius excelsus only grows to 2.5 cm. The largest leatherjacket is the Scribbled Leatherjacket. This species reaches a length of about 1 m.

Number of species

The Southern Pygmy Leatherjacket is a member of the family Monacanthidae. This family includes about 100 species. They range in shapes and sizes, but are all compressed fishes without pelvic fins, and most have a prominent first dorsal spine.

Other Australian leatherjackets include the Six-spine Leatherjacket and the Black Reef Leatherjacket.

The monacanthids bear a strong resemblance to the Triggerfishes, family Balistidae. Both families have a large locking first dorsal spine, however the balistids generally have three visible dorsal fin spines, whereas the monacanthids only have two, one of which is visible.

Distribution

The Southern Pygmy Leatherjacket is endemic to Australia occurring in temperate coastal waters from southern Queensland to south-western Western Australia and south around Tasmania.

Fishes in the genus Brachaluteres occur in tropical marine waters of the Western Pacific. The genus contains four species, two of which are known from Australia, the Southern Pygmy Leatherjacket and Taylor's Pygmy Leatherjacket, Brachaluteres taylori which is recorded in tropical waters from northern to southern Queensland.

Habitat requirements

The Southern Pygmy Leatherjacket can be found in a range of habitats including shallow seagrass beds and areas of kelp in coastal bays.

The image at right shows a Southern Pygmy Leatherjacket in a kelp bed at a depth of 5 m at Fly Point, Nelson Bay, New South Wales.

Structural, physiological and behavioural adaptations

A Southern Pygmy Leatherjacket with its first dorsal spine erected.

Dorsal fin spine

The short, stout, first dorsal spine of the Southern Pygmy Leatherjacket is located above the eyes. It can be laid down into a groove along the back of the fish, or raised vertically. A small second dorsal spine is embedded in the body tissue and acts as a trigger which locks the first dorsal fin into an erect position. This is achieved by a ball and socket mechanism. The trigger must be released before the first spine can be lowered. This feature gave rise to the common name of the related triggerfishesfishes (family Balistidae).

When alarmed or attacked, some species of leatherjackets will wedge themselves tightly into a crack in the substrate by erecting the first dorsal spine and the dewlap. This makes it very difficult for a predator to dislodge them.

First dorsal fin spines come in quite a range of sizes, position and structure, from the small, spine of the Blue-finned Leatherjacket to the large anteriorly-placed spine of the Rhinoceros Leatherjacket, Pseudaluteres nasicornis.

Body shape

The Southern Pygmy Leatherjacket has a very compressed body, which is almost circular. The lower surface is made up of a large ventral flap (called a dewlap) which can be raised and lowered. The abdomen is also expandable by inflating. Like all the leatherjackets, the Southern Pygmy Leatherjacket lacks pelvic fins. Instead, they all have a shaft-like pelvic bone, the end of which is covered with specialised scales. This species looks very different to the Bearded Leatherjacket.

Colouration

Southern Pygmy Leatherjacket - camouflage

The colouration of the Southern Pygmy Leatherjacket is highly variable, ranging from a pale yellow-brown to dark green with small spots or ocelli.

This variable colouration helps to camouflage the fish and often makes it very difficult to see. The images on this page show just some of the amazing range of colour patters displayed by the Southern Pygmy Leatherjacket.

The colouration of this fish can change during courtship. View the Southern Pygmy Leatherjacket courtship page.

Others species of leatherjackets, such as the Prickly Leatherjacket use a combination of colouration and body filaments to fool potential predators.

Swimming mode

The Southern Pygmy Leatherjacket is not a strong swimmer. It has small pectoral fins, a large rounded caudal fin and long-based dorsal and anal fins. The species avoids predators by hiding in seagrass and kelp beds.

It may not be fast but it is very manouverable. When moving slowly between blades of seagrass or fronds of kelp, it swims by undulations of the opposing dorsal and anal fins. When swimming at higher speed, for example when avoiding a predator, the fish beats the caudal fin.

Leathery Skin

The common name of the leatherjackets comes from their leathery skin. Anglers have long known that these fishes do not need scaling. The tough skin can be peeled off the fish like a glove.

The skin of the leatherjackets is smooth and velvety in some species, such as the Southern Pygmy Leatherjacket, whereas in others such as the Rough Leatherjacket it has the texture of course sandpaper.

The texture of the skin comes from the scales, each of which bears one to many tiny spines known as spinules.