Taxonomy & naming
The Swiss ichthyologist Maurice Kottelat described Telmatherina opudi in 1991, as part of his systematic work on the sailfin silversides of Sulawesi. It sits in the family Telmatherinidae within the order Atheriniformes — the same broad order that holds the marine silversides and the freshwater rainbowfishes — and in the genus Telmatherina, whose members are all endemic to the Malili Lakes of central and South Sulawesi, Indonesia. Eschmeyer's Catalog of Fishes records the valid name as Telmatherina opudi Kottelat, 1991; the species has never left its original genus, so no parenthetical authority is used.
The genus name Telmatherina joins the Greek telma, 'swamp', with atherina, the classical name for the smelt or silverside. It belongs to a radiation of sailfin silversides that have long frustrated attempts to delimit them. Meristic characters barely separate the species; they fall out instead on combinations of body proportion and colour pattern. Kottelat's diagnosis separates T. opudi from other pointed-finned Telmatherina by a brown body with bluish belly, pelvic fins, and tips of the anal and second dorsal fins; the longest anal and second-dorsal rays shorter than the body depth; a body depth of roughly 26 to 32 percent of standard length; and a snout about as wide as it is long, broader than the space between the eyes.
The epithet opudi is neither a coinage nor an eponym but a local name. Along the shores of Lake Matano the community calls these fishes 'opudi', and Kottelat took that vernacular as the name. It later turned up in the checklists and revisions of the Malili sailfin silversides — chiefly the preliminary checklist of Herder and colleagues in 2006, which mapped the colour morphs, distributions, and threats across the radiation.
Morphology
This is a small fish, reaching about 2.5 in in total length in the largest recorded males; females and most individuals run smaller. The body is slender and laterally compressed in the silverside manner, with two separated dorsal fins — the family's 'sailfin' character is most developed in some of its relatives, less so here. Body depth runs to roughly a quarter to a third of standard length, and the snout is about as wide as long, a proportion Kottelat leaned on to separate the species from its congeners.
Colour is the most useful field character. The base is brown, overlaid with a bluish wash on the belly and pelvic fins and on the tips of the anal and second dorsal fins. Within Lake Matano, males occur as two colour morphs — a yellow form and a blue-grey form. That within-population polymorphism runs through the whole Malili radiation, and it has made these fishes a model system for studying sexual selection and speciation in progress. Females are plainer than displaying males; the sexes were figured separately in the systematic literature.
For finer morphometrics, go to Kottelat's 1991 description and the later checklist work rather than a second-hand restatement. In build it is a typical open-water silverside, made for active swimming in the clear inshore shallows of its one lake.
Habitat
Telmatherina opudi is endemic to Lake Matano, the uppermost and deepest of the three principal Malili Lakes (Matano, Mahalona, and Towuti) in the highlands of South Sulawesi, Indonesia. Matano is an ancient tectonic lake, among the deepest in Southeast Asia, sitting in a Wallacean biodiversity hotspot that has thrown up endemic species flocks of fishes, snails, and shrimps. Every Telmatherina is confined to this single lake system, and T. opudi to Lake Matano alone, with an extent of occurrence on the order of 190 square kilometres.
It lives mostly in the shallow inshore zone — the clear, well-lit littoral margins — and has also been caught in the lake's outlets, in flowing water where Matano drains downstream. So its world is a permanent freshwater lake plus its immediate outflows: a 'permanent freshwater lake' and associated inland-wetland system in the IUCN's habitat scheme. Much of the range falls inside the Lake Matano Nature Park (Taman Wisata Alam) and the surrounding Protected Forest, though enforcement on the ground is reported to be thin.
Despite some confusion in popular sources, this is not a Lake Poso fish — it is restricted to the Malili system and Lake Matano, and the Poso telmatherinids are a separate set of species. Water-chemistry figures specific to its microhabitat are sparse in the accessible literature, so precise temperature and pH ranges are better read as those of a clear tropical Sulawesi highland lake than pinned to a false precision.
Feeding
An estimated trophic level of around 3.1 puts T. opudi where its body plan suggests: eating small animals, not plants. Like other sailfin silversides it reads as a micro-predator and opportunist of the clear inshore and surface waters, taking small aquatic invertebrates, zooplankton, insect larvae, and whatever else drifts past in the water column.
There are few quantitative diet studies for this species, so the feeding picture comes from its build, its place in the food web, and the general ecology of the Malili telmatherinids. Built for active mid-water swimming, it hunts by sight in the well-lit shallows.
Local fisheries leave it alone — the communities around Lake Matano prefer the endemic 'Butini' goby, Glossogobius matanensis — so harvesting pressure is slight, and what we know of its feeding comes from ecological survey work rather than fisheries data.
Mating
The Malili sailfin silversides are a well-worked system for studying mating behaviour and sexual selection, and T. opudi shares the genus's broad reproductive pattern. Males are colour-polymorphic — yellow and blue-grey forms — and male colour is a main axis of mate choice and male-male competition. Behavioural work on the radiation (Gray and McKinnon, 2006, among others) has made Telmatherina a touchstone for how colour signalling can split a population within one lake.
Males display to females and fight rival males. The dimorphism — brighter, more strongly patterned males against plainer females — tracks the weight these visual displays carry, and the clear, well-lit inshore shallows it favours are exactly where colour-based signalling pays off.
The fine details of its own courtship sequence are not well separated from the genus literature, so read the account above as that of the Matano radiation generally: intense, vision-mediated mate choice among colour morphs in shallow clear water, rather than a species-specific record.
Breeding
Telmatherina are substrate spawners that provide no parental care. The eggs are adhesive and attach to aquatic plants and stone substrates in the shallows, where they are left to develop unattended; this mode has been documented across the Malili sailfin silversides in field and laboratory studies (Gray and McKinnon, 2006; Nasution, 2017). Telmatherina opudi follows this pattern — a plant- and rock-attaching egg layer of the littoral zone rather than a nest-builder or egg-tender.
This reproductive style ties the species' breeding success directly to the condition of the lake's shallow substrate. Eggs deposited on plants and stones in the inshore zone are vulnerable to anything that degrades that environment, and sedimentation from logging and surface mining around the Soroako area — which raises turbidity and smothers substrates — has been identified as a specific threat to egg hatching and recruitment in Lake Matano's telmatherinids.
Species-specific data on clutch size, egg number, and incubation time for T. opudi are sparse in the accessible literature, so those figures are deliberately left unstated here rather than guessed. What is firmly established is the mode — adhesive eggs on plants and stones, no parental care — and the sensitivity of that strategy to declining water quality.
In the aquarium
Telmatherina opudi is a wild-lake endemic, not an established aquarium fish, and it is far less commonly kept than the rainbowfishes it superficially resembles. Sailfin silversides from the Malili Lakes do occasionally reach specialist hobbyists, usually under the broader 'Celebes' or 'sailfin silverside' banner, and the relevant husbandry lessons come from keeping the genus rather than from a body of dedicated T. opudi aquarium experience.
In keeping with its natural home — the clear, stable inshore waters of an ancient Sulawesi lake — these fishes do best in clean, well-filtered, hard and alkaline water with good oxygenation and gentle to moderate flow, in a tank with open swimming space befitting an active, schooling, mid-water silverside. They are small, peaceful, and social, and need to be kept in groups for the males to display and show their colour morphs; a group with several males under bright light is where you actually see the behaviour. Precise temperature and pH targets are not well documented for this species, so the safe approach is to reproduce the clear, neutral-to-alkaline, mineral-rich conditions characteristic of its lake rather than to chase exact numbers.
Treat this as a conservation-relevant endemic first and an aquarium subject a distant second. Anyone keeping Malili sailfin silversides should source stock responsibly and remember that the wild populations are under pressure. The species' real value in the hobby is as a window onto one of the world's great ancient-lake radiations.
Conservation
Telmatherina opudi is assessed as Near Threatened (NT) on the IUCN Red List, under criteria relating to its extremely restricted range (B1b(iii,iv,v)+2b(iii,iv,v)). It was assessed in 2018 and published in 2019; an earlier 1996 assessment had listed it as Vulnerable. The population trend is decreasing, with continuing declines in mature individuals and subpopulations reported, even though surveys in 2018 still found the fish fairly abundant at several sites. Its extent of occurrence is only about 190 square kilometres and its area of occupancy around 180 square kilometres — the footprint of a single lake.
The principal threats are biological and industrial. A widespread invasive 'flowerhorn'-type hybrid cichlid (derived from Cichlasoma trimaculatum and its hybrids) preys on the native Telmatherina, and Lake Matano now hosts on the order of fourteen invasive fish species that disrupt the endemic community. Layered on top of predation is habitat degradation: sedimentation from logging and surface mining around Soroako lowers water quality and harms egg hatching in the shallows where the species spawns, while domestic sewage and hydropower water-management add further stress. The fish is not itself harvested locally, since communities prefer the endemic Butini goby, so direct fishing pressure is minimal.
Much of the range lies within the Lake Matano Nature Park and surrounding Protected Forest, which offers some formal protection, but enforcement is limited and does nothing on its own to blunt the invasive-predator and sedimentation threats. As a single-lake endemic, T. opudi has no outlying population to buffer local declines — a textbook case of a narrowly endemic ancient-lake fish whose fate is bound up entirely with the management of one body of water.