The scales on top of the head are large and irregular in shape. The back is silver to golden, and the ventral surface is always paler with a silvery sheen. There is a silvery-black stripe midlaterally, and the opercula are bright silver. The scale margins on the back are darker, giving a reticulated appearance.
Biology and Habitat
Found around the margins of lakes, wetlands, backwaters and billabongs, the Murray hardyhead prefers open water, shallow, slowflowing or still habitats, with sand or silt substrates, but can also be found in deeper habitats with dense aquatic vegetation. The species appears to thrive in ephemeral deflation basin lakes and can survive in highly saline environments. Mature males have been recorded at 27–34 mm caudal fork length and females at 41–43 mm, with fish maturing in their first year of life. Spawning occurs over an extended breeding season (September–April) but breeding usually occurs in late spring–early summer. The species is a batch spawner, with ovarian eggs at various stages of development. Details of the reproductive ecology and spawning site are currently being studied and are probably similar to other hardyhead species, which lay adhesive eggs amongst aquatic vegetation. The Murray hardyhead is considered to be a largely annual species, although some individuals survive into their second year. It is omnivorous, eating primarily microcrustaceans but also some aquatic insects and algae. It is usually found in schools of distinct size classes—juveniles found throughout lakes (in open waters and associated with edge structure) and adults observed over shallow habitats including open sand banks and in association with emergent macrophytes. Little is known of its movements.
Distribution and Abundance
The Murray hardyhead is found only in the lowland areas of the southern Basin: in the mid to lower Murray as far upstream as Yarrawonga, and lower to mid Murrumbidgee drainages. Formerly abundant, it has suffered a significant reduction in distribution and is now a nationally threatened species. It is probably now extinct in the Murrumbidgee in NSW, and since 2000 only a single individual has been collected in NSW despite extensive surveys. Intermittently abundant at some locations, it is still present in a number of lakes near Swan Hill and Mildura in Vic, a few wetlands/saline basins along the Lower Murray, and the Lower Lakes in SA.
Potential Threats
The precise reasons for its dramatic decline are not known, but suspected to include increased salinisation, habitat degradation, altered flow regimes (decreasing connectivity with floodplain lakes), and impacts of alien species such as Eastern gambusia. The complete drying of some wetlands due to flow modification, coupled with the loss of connectivity with mainstream sites, has recently caused the extinction of two of the five remaining populations in Victoria.