A small, strongly laterally-compressed fish, the adults of which have a relatively deep body. Maximum length is 90 mm, commonly <70 mm. The eyes are large and positioned towards the top of the head, and the mouth is moderately large, oblique and upturned, with several rows of small, conical teeth. There are two dorsal fins separated by a small gap, with the first short-based and the second long-based. There is a long-based anal fin and the tail is moderately forked. There is no lateral line and the scales are relatively large. Colouration varies with sex, age and habitat, but is generally silvery with a greenish iridescence and whitish on the lower head and belly. Younger individuals are translucent. The majority of scales have a brownish margin. There is a pink to-reddish spot on the operculum. Males have red spots on the dorsal, caudal and anal fins, and the fins of breeding males have a blackish margin. Females and immatures have clear fins.
Rainbowfishes are a tropical to sub-tropical family, and Murray–Darling rainbowfish is the southern-most species in the family. The species is generally restricted to the lowland parts of the southern Basin, and prefers slow-flowing rivers, wetlands and billabongs. In the northern Basin the species is more widespread and may also be found in more upland reaches (e.g. above Warwick on the Condamine; and Mingoola on the Dumeresq). In the Lower Murray in SA, the species was more abundant in permanent (rather than ephemeral) wetlands, and less abundant in small shallow wetlands. During low flows in the Lower Murray, the species presence in the main channel has been associated with fine woody debris and submerged macrophytes (e.g. Potamogeton). However, experimental aquaria trials show that adults will preferentially use open sandy substrates compared to submerged macrophytes in the absence of predators. It is a schooling species—schools of 30 or more are commonly seen swimming just below the water surface. Breeding is seasonal, generally spring-summer when water temperature exceeds 20°C, and males perform an elaborate courtship display. Fecundity is low (average 130 eggs, range 35–333) with females laying 5–20 eggs per batch, in 3–4 batches per day for several days. The eggs sink and lodge amongst aquatic plants, where they attach via adhesive threads. Eggs are round, 1.3–1.8 mm diameter, and hatch after about 1 week, with the larvae ~2.0–3.7 mm long. Individuals mature at 10–12 months old.
Until recently the species was considered non-migratory, but individuals as small as 21 mm have been recorded moving through a fishway on the Murrumbidgee River, most commonly in the afternoon and dusk. The species has also been recorded in low num- bers undertaking lateral movements between the main channel and perennial wetlands in spring in the lower Murray River. As such, both longitudinal and lateral movements are likely important for this species, but the precise purpose of these movements remains unknown.
The species is carnivorous, consuming aquatic invertebrates as well as terrestrial invertebrates that fall on the water surface. Some filamentous algae is also consumed.
Formerly widespread across the Basin, the Murray–Darling rainbowfish has declined in the Murray region, but is still patchily recorded from the middle and lower sections of the Murray, Murrumbidgee and Macquarie rivers, and parts of the McIntyre, Gwydir, Namoi and Bogan rivers in NSW. It is abundant in the reach of the Murray River below Yarrawonga, but is rare in the Barmah-Millewa Forest. In Vic, cold winter temperatures limit it to tributaries such as the Broken, Goulburn and Loddon rivers. The species is not present in the relatively upland ACT, and is generally absent from the Lower Lakes in SA. It is still common but localised/patchy in wetlands and vegetated edges of the main channel and anabranch habitats of the Lower Murray River in SA and the lower Victorian section below the Darling junction. In Qld it appears to be common in the mid and upper sections of the Condamine River and its tributaries, but patchy elsewhere. In the Darling River it often forms hybrids with Desert rainbowfish which is the only rainbowfish present in the Paroo and Warrego rivers. Similarly in in the Condamine and downstream sections of the Balonne rivers it appears to have formed a natural hybrid zone with Crimson-spotted rainbowfish (M. duboulayi). 2,451 individuals were caught in the Sustainable Rivers Audit (2004–2013) with the species captured in 12 river valleys, with 74% captured in lowland zones (0–200 m elevation) and 24% in slopes zones (200–400 m elevation). 3,274 individuals from 12 river valleys were caught in the MDB Fish Survey (2014/15–2021/22).
Predation of adults by Redfin perch and larvae by Eastern gambusia are considered potential threats, as are loss of aquatic vegetation (spawning sites and cover) and cold-water pollution. Recent laboratory research has shown the susceptibility of Murray–Darling Rainbowfish to endocrine disrupting chemicals (EDC) (gender benders) commonly found in wastewater treatment plants, industrial detergents, or in fire-fighting foams. Such chemicals mimic oestrogen which can lead to feminisation, skewed sex ratios, and gonad abnormalities, as well as impaired anti-antioxidant capacity that can increase susceptibility to diseases.
Backhouse & Frusher 1980; Baumgartner 2003; Baumgartner et al. 2008; Cadwallader & Backhouse 1983; Bice et al. 2014; Conallin et al. 2011; Humphries et al. 2002; Hutchison et al. 2020; King et al. 2007; Koehn & O’Connor 1990; Lloyd & Walker 1986; McGuigan et al. 2000; Milton & Arthington 1984; Miranda et al. 2020; Moffat & Voller 2002; Smith et al. 2009; Wedderburn & Hammer 2003; Wedderburn & Suitor 2012; P. Unmack unpublished data.
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