Taxonomy & naming
Telmatherina sarasinorum was described by the Swiss ichthyologist Maurice Kottelat in 1991, in the journal Ichthyological Exploration of Freshwaters (1: 321). It belongs to the family Telmatherinidae, the sailfin silversides, a small group within the order Atheriniformes that is centred on the Malili lake system of central Sulawesi. Eschmeyer's Catalog of Fishes records the valid name as Telmatherina sarasinorum Kottelat, 1991; the species was described in Telmatherina and remains there, so no parenthetical authority is used.
The genus name Telmatherina combines the Greek telma, 'swamp', with atherina, the classical Greek name for the smelt or silverside, marking the group as silverside-like fishes of standing waters. The specific epithet sarasinorum is a patronym in the genitive plural honouring two people: the Swiss cousins Paul Benedikt Sarasin (1856–1929) and Karl Friedrich 'Fritz' Sarasin (1859–1942), naturalists whose expeditions to Sulawesi brought Lake Matano to scientific attention and yielded the first collections of Telmatherina. The genus is the namesake of the wider 'Telmatherina species flock', a cluster of closely related and partly hybridising forms whose rapid diversification within the Malili lakes has made them a model system for studies of speciation.
Morphology
Telmatherina sarasinorum is a small, slender, silvery fish reaching about 2.5 in in standard length, with males larger and more ornamented than females. Its most distinctive feature is its finnage. Like other sailfin silversides it carries two dorsal fins and an enlarged anal fin, but in this species both are large and sharply pointed. In large males they extend so far that when folded back they reach into the posterior half of the caudal-fin region, the longest rays exceeding the depth of the body. That finnage is the readiest way to tell adult males from their congeners in Matano.
Colour is the other diagnostic character. The species is set apart from the pointed-finned members of the Matano flock by a yellow or bluish cast to the body and fins, the exact hue varying between individuals and with mood and condition. As in many members of this rapidly radiating group, body shape and meristics overlap broadly with related forms, so colour and fin proportions — rather than counts of fin rays or scales — carry most of the diagnostic weight. The slim, fusiform body and upturned mouth reflect a fish that feeds and courts in open midwater rather than on the bottom.
Habitat
The species is endemic to Lake Matano in the Malili lake system of South Sulawesi, Indonesia — a single-lake endemic found nowhere else on Earth. Lake Matano is an ancient, deep tectonic lake, one of the oldest lakes in Southeast Asia, and its long isolation has driven the in-situ radiation of the Telmatherina flock to which this fish belongs. FishBase classifies it as a freshwater, pelagic, tropical species, occupying the open and near-shore water column rather than burrowing cover.
Because its entire global distribution is one lake, the physical setting of Matano effectively defines the species' habitat: clear, warm, mineral-rich tropical water over rocky and sandy substrates, with the fish concentrating in the upper, well-lit layers where it courts and feeds. Detailed published figures for the water chemistry experienced by this particular species are sparse, and rather than invent precise values the safest statement is that it lives in the warm, stable, relatively hard waters characteristic of this ancient Sulawesi lake. The narrowness of that range is the single most important fact of its biology and the reason its conservation status is watched closely.
Feeding
Telmatherina sarasinorum is a small midwater carnivore with a notably specialised diet. A detailed stomach-content study on Lake Matano (Rasuna et al., 2022), based on more than two thousand individuals sampled across a full year, found that its diet is dominated by two items: aquatic and terrestrial insects, and fish eggs. By the Index of Preponderance, insects made up the great majority of the diet in both sexes (around 71% in males and 95% in females), while fish eggs formed a substantial second component — and made up roughly a quarter of the diet in males (about 26%) against only a few percent in females.
That sex difference points straight to the species' signature behaviour: males are habitual egg predators, raiding the spawning events of other sailfin silversides — and of their own kind — to consume freshly laid eggs. The study reported a low niche breadth, marking the fish as a dietary specialist rather than a broad opportunist, with high dietary overlap between the sexes. An estimated trophic level near 3.1 puts it as a small predator feeding chiefly on invertebrates and eggs rather than plant or detrital material. In the aquarium it would accordingly be expected to take small live and frozen invertebrate foods readily, though this species is rarely if ever kept by hobbyists.
Mating
Courtship and mating in Telmatherina sarasinorum are bound up with the egg-eating behaviour that makes the species famous. Males display to females in open water, and pairs spawn in the substrate or among structure, but the spawning arena doubles as a feeding ground: rival males — and even the courting males themselves — frequently break off to snatch and eat the eggs a spawning pair releases. This conspecific and heterospecific egg predation is well documented in the Matano sailfin silversides and has been studied as a case of sexual conflict and 'sneaking' overlapping with predation.
The dietary data underline how routine this is: with fish eggs forming about a quarter of the male diet, egg-raiding is not an occasional aberration but a regular part of how males make their living. The result is a mating system in which the same individuals that compete to fertilise eggs also profit by consuming them, a dynamic that has drawn research interest for what it reveals about reproductive strategy and food competition in a young, fast-diversifying species flock. Beyond these behavioural observations, fine details of the species' specific spawning periodicity and site selection in the wild are not well documented in the sources at hand.
Breeding
Like other atheriniform silversides, Telmatherina sarasinorum is an egg-layer rather than a livebearer, depositing adhesive eggs rather than guarding a clutch. The species is not a typical aquarium-bred fish, and detailed captive-breeding accounts — incubation times, fry foods, and rearing protocols — are essentially absent from the available literature, so any specific figures would be guesswork and are better left unstated than fabricated.
What is clear from field study is that reproduction is a continual, ecologically important event in Matano: spawning pairs are common enough that the eggs they release form a reliable food source for egg-raiding males throughout the year, with the year-round sampling in the 2022 diet study turning up fish eggs as a consistent dietary item. That implies an extended or non-strongly-seasonal spawning pattern rather than a single brief breeding season. The combination of open-water courtship, substrate or structure spawning, and heavy egg predation by conspecifics means that, in the wild, only a fraction of eggs laid are likely to survive to hatch — selective pressure that may help explain the intensity of male competition in this flock.
In the aquarium
Telmatherina sarasinorum is essentially a fish of scientific rather than hobby interest, and it is rarely encountered in the aquarium trade. There is little established husbandry literature for it, so the notes here are general inferences from its biology rather than tested aquarium practice, and a keeper should treat them as such.
As a warm-water, open-swimming midwater species from an ancient hard-water lake, it would in principle need a spacious, well-filtered tank with stable warm temperatures, good water movement, and the harder, more alkaline water chemistry typical of the Malili lakes rather than soft acidic conditions. Its specialised, partly egg-based diet and its habit of preying on the eggs of tankmates would make it a poor community fish around other spawning species, and a species-focused setup would be more appropriate. Given its restricted wild range and conservation status, any specimens in the hobby should be captive-bred or responsibly sourced; this is not a fish to take casually from the wild. For most aquarists, the more readily available Melanotaenia and related rainbowfishes are far better choices; T. sarasinorum is best appreciated as a product of evolution shut inside one Sulawesi lake.
Conservation
Telmatherina sarasinorum is assessed as Near Threatened (NT) on the IUCN Red List, under range-based criteria reflecting its restriction to a single lake (the assessment cites a small extent of occurrence together with ongoing or projected declines in habitat quality and area). The fundamental vulnerability is the same one that makes the species scientifically interesting: it exists only in Lake Matano, with no outlying populations to buffer it against local change.
The principal threats to Matano's endemic fishes are habitat alteration and the introduction of non-native species. Invasive fishes — including introduced predators and competitors that have spread through the Malili lakes — pose a particular risk to small endemic silversides, both directly through predation and indirectly through competition and disruption of the spawning dynamics on which this egg-feeding species depends. As a narrow endemic of an ancient lake, T. sarasinorum is part of a fragile, irreplaceable radiation, and its long-term security rests on protecting the integrity of Lake Matano itself. CITES and CMS list it as Not Evaluated; its safeguard is the conservation of its one home water.