Taxonomy & naming
Philypnodon macrostomus was described by Donald Hoese and Scott Reader in 2006, splitting it from the larger Philypnodon grandiceps with which it had previously been confused. The genus Philypnodon was established earlier for the flathead gudgeons — a small group of Australian eleotrids with strongly flattened heads and wide mouths — and the Catalog of Fishes (Eschmeyer, CAS) places the species in the family Eleotridae. The type locality is Glenreagh, New South Wales.
Like all eleotrids, Philypnodon macrostomus is not a true goby despite the hobby often lumping all small benthic gobiiforms together. The true gobies of Gobiidae and Gobionellidae bear fused pelvic fins forming a sucker disc; the sleepers and gudgeons of Eleotridae keep their pelvic fins separate. This is easily confirmed in the hand, and distinguishes Philypnodon from the gobiid gobies that live alongside it in some estuaries. No old synonymous combinations are widely cited for this species, as it is relatively newly described; material formerly assigned to it was generally referred to P. grandiceps.
Morphology
The dwarf flathead gudgeon is a small, dorsoventrally flattened fish, reaching around 2 in in length in most individuals (often 1.5 in), with males reaching up to approximately 1.5 in standard length in some records. The head is distinctly flattened and broad, typical of the genus, and the mouth is notably large — in males it extends posteriorly beyond the eye, the character that gives the species its name. Gill openings are restricted, another useful identification feature. The coloration is dark brown above, becoming lighter brown to pale orange ventrally; two dark lines radiate posteriorly from behind the eye, and the dorsal fins carry orange striping while the caudal fin shows dark banding.
The pelvic fins are separate from each other, as in all eleotrids — never fused into a sucker disc. Sexual dimorphism is present but subtle; the large-mouth character is pronounced in males. The species is most easily confused with P. grandiceps, which grows considerably larger; P. macrostomus also has restricted gill openings that help separate it.
Habitat
Philypnodon macrostomus inhabits slow, sheltered fresh and brackish waters across eastern Australia. It is found in off-channel wetlands, small anabranches, billabongs, and calm reaches of streams and rivers, preferring low-energy environments over muddy, rocky or weedy substrates with abundant submerged vegetation and other cover. It also penetrates upper estuaries and can tolerate brackish water up to full-strength seawater in the uppermost tidal reaches, though it is primarily a freshwater fish throughout most of its inland range.
The species ranges from Baffle Creek in central Queensland southward through coastal and near-coastal drainages of New South Wales and Victoria to Wilsons Promontory, with an inland extension into the Murray-Darling Basin reaching eastern South Australia. It is patchily distributed within the Murray-Darling, and becomes more restricted to estuarine conditions in the southern extremity of its range. It occurs alongside the larger P. grandiceps in parts of its range. Water temperatures in the breeding season are typically cool, with breeding recorded at 66–72 °F.
Feeding
Philypnodon macrostomus is a micro-predator, feeding on small invertebrates associated with the substrate and aquatic vegetation. Its natural diet consists primarily of insects and minute crustaceans, which it picks from among the benthos and plant stems in the calm, weedy habitats it favours. The large mouth for its body size is an advantage in capturing relatively sizable prey items.
In the aquarium, it should be offered small live and frozen foods — bloodworm, brine shrimp, daphnia, small copepods and similar invertebrate-based items. Like most gudgeons and sleepers, it typically shows little interest in dry flake and should not be maintained on it long-term. Feeding a varied diet of appropriately sized meaty foods is the key to keeping it in good condition.
Mating
Philypnodon macrostomus is a resident freshwater and brackish-water spawner — unlike the amphidromous stream gobies of the genus Stiphodon or Sicyopterus, it does not produce planktonic larvae that must develop at sea, and its entire reproductive cycle takes place within its home waters. This is the reason the species can in principle be bred in captivity, where amphidromous gobies cannot.
Males defend a territory and a suitable spawning site — a cavity, crevice, rock overhang or dense plant shelter — and court ripe females. The species matures in approximately one year. Conditioning pairs on live and frozen foods ahead of spawning will encourage breeding behaviour.
Breeding
Breeding has been recorded in Australian populations at water temperatures of 66–72 °F, the cooler end of the temperate-to-subtropical range the species inhabits. The female deposits adhesive eggs at the chosen spawning site, and the male guards them through incubation — a genuine parental-care strategy typical of the resident gudgeons. This is not open-water egg-scattering and not mouthbrooding.
In the aquarium, providing caves, pipes or crevices alongside aquatic planting and a calm, clean environment will give a conditioned pair the best chance. A slight dip in temperature to the lower end of their tolerance, combined with live-food conditioning, may trigger spawning. Because the species is rarely kept by hobbyists, detailed captive breeding accounts are sparse, but the biology points to the same cave-spawner-with-male-guard pattern seen in other resident Australian gudgeons such as Mogurnda and Hypseleotris.
In the aquarium
The dwarf flathead gudgeon is an uncommon aquarium fish despite its modest size and tolerance of a wide range of conditions. A small, species-appropriate tank with a sandy or fine gravel substrate, plenty of submerged vegetation and rock or root cover to provide retreat sites and territory breaks suits it well. A tightly fitting lid is advisable, as small gobiiforms are capable jumpers. The species tolerates fresh water and low-end brackish without issue, so a small addition of marine salt is optional for estuarine collections but not required for most aquarists.
Water should be kept cool to moderate — the breeding temperature record of 66–72 °F suggests it is not a warmth-demanding tropical fish; a cooler subtropical temperature range is likely ideal for long-term health. Live and frozen foods should be the diet staple. Males can be territorial with each other, so a single male with one or more females, given adequate cover, avoids unnecessary aggression. It is a quiet, cryptic, bottom-associated fish rather than an active display species, but its patterning and large-mouthed flathead shape give it genuine character.
Conservation
Philypnodon macrostomus is assessed as Least Concern on the IUCN Red List (2019 assessment), reflecting its wide distribution across eastern Australia and its tolerance of varied freshwater and estuarine conditions. It ranges across multiple river systems from central Queensland to eastern South Australia, including portions of the large Murray-Darling Basin, giving it a broad enough footprint that no immediate range-wide threat has been identified.
Nonetheless, the species faces the same suite of pressures affecting freshwater fish across south-eastern Australia: habitat modification, altered hydrology from agriculture and water extraction, degradation of off-channel wetlands and anabranches, and the spread of invasive species. Within the Murray-Darling Basin it is patchily distributed and its populations there should be monitored. The species is not known to be traded significantly in the aquarium hobby, so collection pressure is not a documented concern. Its small size and cryptic habits mean it can be overlooked in surveys; continued distribution recording across its range will improve understanding of its population status.