Taxonomy & naming
Paratherina cyanea was described by Aurich in 1935 and is placed in the family Telmatherinidae, the sailfin silversides, within the order Atheriniformes. Eschmeyer's Catalog of Fishes treats the name as valid in the genus Paratherina. The genus is one of three — alongside Telmatherina and Tominanga — that together make up the endemic sailfin-silverside flock of the Malili Lakes system in central Sulawesi, Indonesia.
Phylogenetic work on the Malili Lakes radiation (Stelbrink et al. 2014) recovers Paratherina as a morphologically and genetically distinct lineage within the system, separable from Telmatherina and Tominanga on both nuclear DNA and morphology. Molecular dating placed the deeper sailfin-silverside divergences at several million years, with the most recent common ancestor of Paratherina and Telmatherina estimated at roughly 3 to 10 million years, consistent with a long in-situ history in these ancient lakes.
The genus name Paratherina pairs the Greek para, beside or near, with atherina, the classical Greek name for the silverside or smelt — apt for a near-relative of the true silversides. The epithet cyanea comes from the Greek kyanos, dark blue, for the fish's bluish coloration.
Morphology
An elongate, laterally compressed silverside, Paratherina cyanea reaches a maximum recorded length of about 8 in total length — one of the larger members of the Malili Lakes sailfin-silverside assemblage. The body is slender and streamlined in the way typical of pelagic atheriniforms: two separated dorsal fins, the front one of a few spines and a longer soft-rayed posterior dorsal, and an anal fin set well back. That arrangement gives the group its silvery, schooling look.
As the epithet cyanea suggests, the fish carries a bluish cast. The sexes differ across the Malili sailfin silversides, with males the more strongly coloured and ornamented and females plainer. Published meristic and coloration data for this particular species are thin, and the fine distinctions among Paratherina forms rest on the specialist taxonomic and phylogenetic literature rather than on widely available accounts.
Habitat
Paratherina cyanea is endemic to the Malili Lakes of central Sulawesi, occurring specifically in Lakes Towuti and Mahalona within the Malili system (the chain also includes Lake Matano, linked to the others by rivers). FishBase records it as a freshwater, pelagic, non-migratory tropical species, with a latitudinal range of roughly 2 to 3 degrees south.
The Malili Lakes are an ancient, tectonically formed lake chain in Wallacea — a freshwater biodiversity hotspot with endemic species flocks of fishes, snails, shrimps, and crabs. Lake Towuti is the largest, Lake Mahalona a smaller upstream basin; short rivers tie the lakes together, which both shapes the distribution of their endemic fishes and allows some exchange between basins. Being pelagic, P. cyanea works the open water rather than narrow marginal habitats, schooling in the upper water column as silversides do.
Feeding
Specific dietary studies of Paratherina cyanea are sparse, but its ecology can be read from its body plan and from what is known of the sailfin silversides generally. FishBase assigns it a modeled trophic level of about 3.2, indicating a carnivorous to omnivorous diet weighted toward animal prey rather than plant material.
A slender, open-water schooler fits a planktivorous, small-invertebrate habit — zooplankton and small invertebrates taken from the water column. Gut-content and feeding-behaviour data for this particular species are not in the accessible literature, so finer claims about its diet are best left unstated.
Mating
The Malili sailfin silversides are sexually dimorphic: males of many forms develop brighter coloration and carry the ornament that gives the group its 'sailfin' name. Mate choice and male coloration come up again and again in the study of this radiation, and sexual selection is thought to be one of the drivers of its diversification.
Species-level courtship and pairing behaviour for Paratherina cyanea is poorly documented in accessible sources. FishBase classes it as a batch spawner — a fish that releases eggs in multiple smaller batches over time rather than in a single annual clutch — which points to an extended, repeated spawning pattern rather than one brief breeding event. Beyond that, accounts of its display or pairing are sparse.
Breeding
FishBase characterises the reproduction of Paratherina cyanea as that of a nonguarder and open-water/substratum egg scatterer, with no parental care. The fish broadcasts its eggs into open water or over the substrate and does nothing to guard or tend eggs or young — the egg-scattering strategy common among silversides.
It is recorded as a batch spawner, releasing eggs in successive batches over a spawning period rather than all at once. As with the rest of its biology, the fine breeding details — clutch sizes, egg characteristics, incubation times, fry development — are not well documented for this species, and are better flagged as data-sparse than guessed at.
In the aquarium
This is a fish of scientific rather than hobby interest. It is not a standard aquarium-trade species the way most rainbowfishes are, and reliable, species-specific husbandry accounts simply aren't out there. The few sailfin silversides that reach specialist keepers are reckoned fishes for the experienced aquarist, wanting roomy tanks to suit an active, open-water schooler.
What can be said with confidence follows from where it lives. A large pelagic schooler from clear, warm tropical lakes will need a long tank with open swimming space, stable warm water, and good filtration and oxygenation. A wild-type fish from a narrowly endemic, Near Threatened population is not one to source casually from wild collection. Beyond those general principles, care parameters specific to P. cyanea are sparse, and aren't asserted here.
Conservation
Paratherina cyanea is assessed as Near Threatened (NT) on the IUCN Red List under criteria B1b(iii)+2b(iii), assessed on 29 June 2018. The Near Threatened listing reflects the species' restricted range within the Malili Lakes — confined to Lakes Towuti and Mahalona — combined with documented threats to the quality of that habitat.
The Malili Lakes face the pressures common to small, heavily endemic lake systems: catchment disturbance, regional mining, sedimentation, shifting water quality, and introduced non-native fishes, all of which bear on the endemic fauna. A species limited to two basins of one lake chain has little geographic buffer, which is why a fish not yet sliding toward extinction is still flagged as one to watch. It has not been evaluated under CITES or CMS. Its fate is tied to that of the Malili Lakes as a whole — a Wallacean freshwater hotspot whose endemic species flocks make it a priority for protection.