An attractive, small, robust fish with a rounded head, small mouth, and gape that extends to the anterior margin of the eye. Maximum size 152 mm; commonly 60–120 mm. The tail is rounded, and there are two dorsal fins, the first short-based and lower than the longer and taller second dorsal. The back is dark brownish to yellowish brown (but can be iridescent blue in general with a series of blue blotches towards the tail), fading to lighter brown or cream on the belly. A row of darkish blotches are present on the sides from the start of the second dorsal fin to the start of the caudal fin, surrounded by numerous red and white spots.
Males have 3–4 brown-to-purple facial stripes extending from behind the eye to the back of the operculum. Females generally have two stripes, which are less prominent. There are numerous red spots on the dorsal, anal and caudal fins.
The Southern purple-spotted gudgeon is a slow-moving ambush predator, consuming small fishes and aquatic macroinvertebrates (shrimps, freshwater prawns, dragonfly larvae, beetles) and also worms and tadpoles. It obtains its food from the benthos, the water column, and the water surface. It is generally a benthic species, usually associated with dense cover such as aquatic vegetation, cobble and rocks. It is found in slow moving or still waters of creeks, rivers, wetlands and billabongs, and prefers slower flowing habitats. They are typically found very close to the banks, often in surprisingly shallow water a few cm deep (especially juveniles).
The species is tolerant of a broad range of temperatures and pH, relatively high salinity, and low levels of dissolved oxygen. Studies on coastal and northern populations have shown that movement is very restricted, with movements by males (but not females) between pools common, but individuals rarely move more than 1–2 km. Males mature at 45 mm and females at 49 mm.
The male has an elaborate courtship display and pairing, and spawning occurs in summer when water temperature exceeds 20°C. Females can spawn several times during a spawning season. The eggs are adhesive and 280–1300 are deposited in a single batch on a rock, log or aquatic plants. The eggs are elongate, pointed at both ends, transparent and 1.0–1.3 mm wide and 2.0–3.8 mm long. The male stays to guard and fan the eggs, which hatch after 3–9 days depending on water temperature. Newly hatched larvae are approximately 4 mm long.
This species contains at least two cryptic taxa and several genetic management units, with the MDB populations a distinct lineage. Historically this gudgeon was present throughout the Murray-Darling Basin as well as in coastal streams of NSW from the Clarence River north. Today the species remains quite common in Qld coastal basins, but in the Murray-Darling Basin it has undergone a significant decline. It was presumed extinct in Vic from the 1940s until rediscovered in 1995, lost again by 2001. and then rediscovered in 2019 in the Kerang Lakes. In SA it had not been recorded for 30 years until 2002 when a population was discovered in Jury Swamp in the lower Murray River. This wild population was subsequently extirpated as the swamp dried in the Millennium Drought, but a number of rescued individuals were then bred in captivity and surrogate dams before being reintroduced in 2014 following the end of the drought. Low numbers have since been detected at the site.
It is patchily distributed and rare in tributaries of the Darling River in northern NSW, where it occurs as a number of small, isolated populations in catchments of the Macquarie (Wuuluman and Buckinbah creeks), Gwydir (Halls and Keera creeks), Macintryre (parts of the Dumaresq, Severn and Deepwater rivers, Tenterfield Creek). It is patchy, but sometimes locally common in a few upper tributaries of the Condamine River around Toowoomba and Warwick, although many of these sites are quite degraded due to extreme riparian vegetation removal. Records in the mid 1990s from the Cardross Lakes near Mildura were the first from Vic in more than 50 years, but intensive follow-up surveys of this lake system failed to relocate the species. 31 individuals were caught In the Sustainable Rivers Audit from 2004–2013 from 3 river valleys (Border Rivers, Condamine, Gwydir) with 25 coming from the Border Rivers montane, slopes and uplands zones. Only 7 have been recorded in the MDB Fish Survey (2014/15–2021/22), all from the Border Rivers.
The species has been recently reintroduced into sites in the Castlereagh, Macquarie, Lachlan and Murrumbidgee catchments in NSW, but while survival of stocked fish has been determined in some sites, no recruitment has been detected. It has also been released near Bendigo, Kerang, Mildura and Deniliquin with breeding detected at several sites. Rescues from the wild into captivity were conducted in response to the severe 2017–19 drought in NSW and also the 2019–20 bushfires. Some populations had habitat severely impacted by post-fire sedimentation.
The precise reasons for the decline of this species are unknown, but interactions with alien species (particularly Eastern gambusia and Redfin perch), habitat degradation, particularly the loss of aquatic plants, and fluctuations in water levels with river regulation, are thought to be significant. Individual populations in the MDB have low genetic diversity as a result of their highly fragmented distribution, plus their low dispersal ability. This makes populations susceptible to random or chance catastrophes (drought, floods, pollutants, habitat loss, introductions of alien species) that may cause local extinctions.
Bice et al. 2019c; Boxall et al. 2002; Cadwallader & Backhouse 1983; Faulks et al. 2008; Hammer et al. 2012, 2013, 2015; Larson & Hoese 1996a; Llewellyn 2006; Merrick & Schmida 1984; Miles 2013; Moffat & Voller 2002; NSW FSC 2008a; Raadik & Harrington 1996. Sasaki et al. 2016; Shipham et al. 2013; Stoessel et al. 2022; Whiterod et al. 2019; Zukowski et al. 2021.
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