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Riffle galaxias

Other common name(s): 
Scientific name: 
Galaxias arcanus
Raadik, 2014
Tarmo A. Raadik
Threatened but recovering

A newly described species. The majority of the following information is from Raadik (2014). Previously referred to as Galaxias sp. 2.


A moderate-size galaxiid (max. recorded length 98 mm Length to Caudal Fork; LCF) commonly 60–75 mm LCF). The Riffle galaxias differs from all other species in the Mountain galaxias complex in the Basin by having a shallow body with a straight ventral profile; long and shallow caudal peduncle (which is longer than the caudal fin); and, a distinctive snout extending forward from the thick and fleshy-lipped upper jaw. The lower jaw is noticeably shorter than the upper jaw and the head is wide and shallow.

The mouth is subterminal and extends to under about the middle of the eye. It has large, ventrally oriented pectoral and pelvic fins and the anal fin commences < 0.4 of the distance along dorsal fin base. The tail is weakly forked.

The body is olive-brown to beige, with dark grey, dark brown or almost black, irregularly shaped blotches or bands on the back. The head and snout are usually darker than the body, often dark grey. It lacks the black bars along the lateral line often found in other galaxiids. The base of the caudal fin usually has a narrow, vertical, dark to relatively pale grey band.  

Biology and Habitat

Previously confused with Mountain galaxias (Galaxias olidus) the Riffle galaxias was only described as a separate species in 2014, and so many aspects of its ecology are unknown. It has been recorded from cold to relatively cool, clear water in flowing creeks to large rivers usually in shallow, fast-flowing and riffles and runs. It has a primary preference for cobble-bottomed substrates, with a secondary preference for flowing water habitats during both day and night.

Fish are usually found amongst or under rocks on the stream bed, or amongst small and large timber debris. It is usually not associated with aquatic vegetation. Laboratory trials have shown Riffle galaxias prefer complex substrate (e.g. cobbles) and avoid trout if possible when cover is limited (e.g. over sandy substrates), but don’t avoid trout in cobble habitats.  

The spawning period is probably spring to summer (October–December, possibly extending into January), although some ripe or almost ripe females have been recorded in May/June. Males generally mature earlier than females, with ripe individuals recorded in May and June.  

Adults are usually solitary, with juveniles and younger adults observed in loose shoals of 50+ fish, often moving upstream through pools between riffles. Fecundity is unknown, as is spawning site, but spawning possibly occurs on the underside of cobbles in riffles (as it does for several other galaxias). Spawning cues and time to hatching are unknown.

Movement requirements are also unknown, but the species is thought to be non-migratory and relatively sedentary, although juveniles (38–67 mm LCF) have been observed moving upstream in May-June. If similar to other members of the Mountain galaxias complex, home-range of adults is likely to be limited and less than 100m. The diet is unknown, but is likely to consist of drifting and benthic aquatic invertebrates taken in pools. Like many species in the Mountain galaxias complex, it has been recorded with small grey to black cysts in the flesh and fins, which are probably the metacercariae of trematode parasites.

Distribution and Abundance

This species is restricted to a thin band on the north of the Great Dividing Range in north-eastern Victoria, including the upper Murray River from near its headwaters and extending westward to the Goulburn River system. It is currently not recorded from the Murray River downstream from about Albury, and may occur in tributaries of the upper Murray River in NSW. The species has been recorded from 150–880 m elevation, and may be restricted in range to steeper-gradient streams in foothill to upland reaches by specific habitat requirements, such as diverse substrates and fast-flows.

The species is found with other members of the Mountain galaxias complex, primarily with Mountain galaxias and Obscure galaxias. Also found with Flat-headed galaxias at the lower elevations within its range, and with Climbing galaxias which has been translocated into the upper Murray and tributaries.

452 individuals were caught in the Sustainable Rivers Audit (2004–2013) with the species captured in 6 river valleys (Upper Murray, Broken, Goulburn, Kiewa, Mitta Mitta, Ovens) all in Vic, with 80% from the Ovens and Kiewa. Almost all individuals were collected from slopes, upland or montane elevational zones, with only 5 from lowlands. 459 individuals from 3 river valleys (Ovens, Goulburn, Kiewa) were caught in the MDB Fish Survey (2014/15–2021/22).

Potential Threats

Unlike other Galaxiids, interaction with trout is not a major threat to this species, possibly because of its ability to reside in shallow fast-flowing habitats and the coarse substrates of riffles and runs which provides predation refuges from trout, and where trout are less common. Riparian and catchment degradation can result in increased sedimentation of streams which can reduce the availability of cobble refugia.

Flow regulation, which can lead to changed flow regimes for riffles may be a key threat by reducing habitat availability and allowing predators to gain access when riffles are drowned out.  

General References

Raadik 2011, 2014; Sowersby 2007; Sowersby et al. 2016.

This species account is an extract from Fishes of the Murray-Darling Basin (second edition) and should be cited as "Lintermans, M. 2023, Fishes of the Murray–Darling Basin, Australian River Restoration Centre, Canberra."

Other Fish in this family

Front book cover of Fishes of the Murray–Darling Basin

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