Taxonomy & naming
Telmatherina wahjui was described by the Swiss ichthyologist Maurice Kottelat in 1991 as part of his work on the freshwater fishes of Sulawesi. It belongs to the family Telmatherinidae, the sailfin silversides, within the order Atheriniformes — the same broad lineage as the marine silversides and the rainbowfishes, though the telmatherinids are a distinct, largely freshwater family of the Indo-Australian region. Telmatherina is the namesake genus, and the dominant one in the endemic silverside radiation of the Malili Lakes of central Sulawesi.
The genus name combines the Greek telma, meaning swamp or pool, with atherina, the classical Greek name for the smelt or sand-smelt — a reference to the silverside affinities of these fishes. The specific epithet wahjui is a patronym honouring Beni Nurtjahja Wahju (1934–2012), an Indonesian conservationist who was a founding member of The Nature Conservancy's Indonesia Advisory Board and founding chairman of the Indonesian Natural Heritage Foundation (Yayasan Pusaka Alam Nusantara). Eschmeyer's Catalog of Fishes governs the valid name; because the species was described in Telmatherina and remains there, the authority is written without parentheses as Kottelat, 1991. The principal reference work covering the species is Kottelat, Whitten, Kartikasari & Wirjoatmodjo's 1993 Freshwater Fishes of Western Indonesia and Sulawesi.
Morphology
Telmatherina wahjui is a small fish, reported to reach about 2 in in standard length (Kottelat et al. 1993). Like other members of its genus it is slender and laterally compressed, with the silvery, semi-translucent body and relatively large eye typical of the silverside body plan, and the two separate dorsal fins that characterise the atheriniforms.
Outside the original taxonomic literature, species-specific descriptions of coloration and fin morphology for T. wahjui are thin, and this profile does not reconstruct them from better-known congeners. Within the Lake Matano radiation, males of several Telmatherina species develop elongated dorsal and anal fin rays and bright nuptial colours for display, while females stay plainer — but the precise pattern for this species belongs to Kottelat's 1991 description, not to inference. What is safe to say: this is a small, streamlined, current-adapted silverside, not a deep-bodied or heavily ornamented one.
Habitat
Telmatherina wahjui is endemic to Lake Matano, the deepest and oldest of the Malili Lakes system in central Sulawesi, Indonesia. The IUCN records it from only three known localities, all associated with the outlet region where Lake Matano drains toward the Petea (Patea) River. It is a specialist of a narrow and uncommon habitat type within the lake: shallow, fast-flowing surge and outlet water with medium-to-strong current over coarse gravel and rock substrate, down to roughly 5 ft depth (Miesen et al. 2016).
High-energy, gravel-bottomed outflow is scarce in an otherwise large, deep, clear ancient lake, which goes a long way toward explaining why the species is so localized. The broader Malili Lakes — Matano, Mahalona, Towuti and their connecting rivers — are a hotspot of freshwater endemism in Wallacea, and the sailfin silversides there are a textbook lacustrine species flock. Telmatherina generally favour steep sites with gravel or rock bottoms; T. wahjui sits at the strongly rheophilic, outlet-adapted end of that spectrum, pinned to the transition zone between the still lake and the river that drains it.
Feeding
A small midwater and surface-oriented silverside, T. wahjui has an estimated trophic level around 3.1 (FishBase), which points to a diet dominated by small invertebrates. Like the other sailfin silversides of the Malili Lakes, it most likely takes small aquatic and drifting invertebrates — insect larvae, micro-crustaceans, zooplankton and drift carried past by the current — plus whatever else the fast water delivers.
Species-specific diet data are limited, and gut-content studies of this particular fish are not in the cached sources, so no finer claims about prey selection are made here. The habitat itself is the best clue: drift-feeding on invertebrates swept past in flowing water is the simplest reading of a small, slender, current-adapted silverside living where the lake spills into a river.
Mating
Detailed courtship and mating behaviour specific to Telmatherina wahjui is not documented in the cached sources, and is not invented here. Across the Lake Matano radiation, though, sailfin silversides run visually driven mating systems: males display to females with their fins and nuptial coloration, and male reproductive behaviour — even sperm traits — has been studied as a driver of the lake's rapid speciation.
As a member of this radiation, T. wahjui is an egg-laying silverside that breeds in its gravelly, flowing outlet habitat. Its confinement to a narrow, current-swept zone suggests breeding is tied to that microhabitat — but without direct observations, the specifics of pairing and spawning stay data sparse here rather than borrowed from congeners.
Breeding
No captive-breeding record exists for Telmatherina wahjui in the cached material, and the species is not established in the aquarium hobby, so there is no body of hobbyist spawning experience to draw on. Sailfin silversides as a group are egg-scattering or substrate-associated spawners that attach adhesive eggs among gravel, vegetation or other surfaces, with no parental care, and T. wahjui is presumed to follow the general telmatherinid pattern.
Beyond that general expectation, the breeding biology — clutch size, egg size, incubation period, spawning triggers — is undocumented in the available sources. Rather than transfer figures from better-studied relatives, this profile leaves them sparse. Any future husbandry would have to rebuild breeding conditions from the wild habitat: shallow, well-oxygenated, fast-flowing water over coarse gravel at tropical temperatures.
In the aquarium
Telmatherina wahjui is effectively absent from the aquarium trade. It is a narrowly endemic, Endangered silverside of a single ancient lake, and no dedicated hobby care information for the species could be located, and it is rarely if ever kept. So this section offers no stocking, tankmate, or breeding recommendations — they would be guesswork for a fish that is not in cultivation.
If it were ever maintained, say in a conservation or research setting, its needs would have to come from the wild habitat rather than from generic rainbowfish guidance: warm, clear, well-oxygenated freshwater with brisk flow over coarse gravel substrate, matching the shallow surge and outlet conditions of Lake Matano. Given its conservation status and the pressures on its habitat, wild collection for the trade is not appropriate, and the cached sources give no basis for treating it as an aquarium fish.
Conservation
Telmatherina wahjui is assessed as Endangered (EN) on the IUCN Red List under criteria B1ab(i,ii,v)+2ab(i,ii,v), in an assessment dated 13 June 2018 and published in 2019 (assessor D. Lumbantobing). Its population trend is recorded as decreasing. The listing reflects a tiny, fragmented distribution — only three known localities clustered around the Lake Matano outlet to the Petea River — combined with active threats to exactly that habitat. Recent surveys turn it up only rarely in the outlet area, a sign of decline.
The most direct threat is hydrological modification of the outflow: cascading sluice gates along the Lake Matano-to-Petea outflow alter the current-and-gravel conditions that the three known localities depend on. The wider Malili Lakes system faces a familiar suite of pressures: nickel mining and its sedimentation, logging, urbanisation, hydropower-driven waterway modification, sewage and industrial waste, and introduced non-native fishes. Predatory introduced cichlids are a particular worry — 'Flowerhorn'-type cichlids have turned up in the system with telmatherinid remains in their guts. Confined to a single lake and a single specialized habitat within it, T. wahjui has essentially no outlying population to buffer these impacts, which is the core reason for its Endangered status.