A small catfish with a relatively short first dorsal fin and the classic tandan feature of conjoined caudal and anal fins. Maximum size ~240 mm; usually less than 150 mm. The skin is smooth, with no scales. The body is slender with a roundly pointed tail. The head and nape profile is concave and the eyes are close to the snout. The longish nasal barbel extends to, or behind back of head, with other barbels reaching to or beyond the base of the pectoral fin.
Colour ranges from light grey to almost black, sometimes mottled, or pale yellow-brown, generally with a golden sheen. A white form is also known. The individuals collected from the Basin differ slightly morphologically to other populations, and it is possible that genetic research may show that there are cryptic species within P. rendahli.
Virtually nothing is known of this tandan’s ecology in the Basin, but elsewhere it is a benthic feeder, consuming relatively small items such as insect larvae, (predominantly chironomids and mayflies), microcrustaceans (ostracods or cladocerans) and detritus. In the Alligator Rivers region of the Northern Territory males mature at approximately 100 mm total length and females at 110 mm, and breeding occurs early in the wet season (~November–December).
In this locality spawning occurs in muddy lowland lagoons. Average fecundity is about 900 eggs, but up to 3465 (in a 209 mm TL fish) has been recorded. Mean egg diameter is 1.3 mm.
Little is known of movement requirements other than that there is a migration into lowland lagoons to spawn, and an upstream movement to refuge habitats early in the dry season. In the Alligator Rivers region, Rendahl’s tandan was recorded at surface water temperatures of from 26–38°C. It is found in both riverine and off-channel habitats but is commonly recorded in floodplain lagoons and wetlands. A benthic species, this tandan is found in greatest abundance in slow-flowing areas with dense submerged vegetation, indicating that it is tolerant of low dissolved oxygen levels.
The species is eaten by piscivorous waterbirds such as the Darter (around 30% of the diet in one study) and Whiskered tern (Rendahls tandan and Mogurnda mogurnda comprising 64% of dry weight of fish in stomachs in another study).
The species has only been recognised from the Basin since the early 2000s where it is only known from the Condamine (Dogwood, Charley’s, Nudley, Branch, Coolamalah creeks; Caliguel and Karreel lagoons) and Balonne (Undulla Creek and Beardmore Dam) and recently the Warrego (7 Mile Waterhole) valleys in southern Qld. There is a single record from NSW in 2022 in the Narran River ~ 30 km northwest of Lightning Ridge.
Although recorded during sampling for the Pilot Sustainable Rivers Audit (SRA) in 2003, it was not recorded at all during the full SRA (2004–2013) and only once during the MDB Fish Survey (2021/22), indicating its patchiness and rarity. The majority of Rendahl’s tandan captured in the Basin to date have been captured in fyke nets. It is more commonly encountered in offstream lagoon habitats and tributaries of the Condamine-Balonne than in the main river. Rendahls tandan has been recorded to co-occur with Hyrtl’s tandan in the Condamine catchment, but Rendahls is usually at much lower abundances.
Outside the Basin it has a wide but patchy distribution in northern Australia in the Kimberleys, Cape York, the Burdekin and coastal streams of the NT. Its northern Australian distribution indicates that is a warm water species and is unlikely to be widely distributed in the Basin.
Almost nothing is known of this species in the Basin, so it is difficult to identify threats. However, wetland degradation (including by feral pigs) and alienation, and loss of macrophytes may be issues to consider. Aquatic weed removal in floodplain lagoons of the Burdekin River resulted in improved fish community structure, including Rendahls tandan.
Allen et al. 2002; Bishop et al. 2001; Dostine & Morton 1989a,b; Larson & Martin 1989; MDBC 2004b; Perna et al. 2012; Pusey et al. 2004, 2017, 2020; Waltham & Schaffer 2018.
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