Taxonomy & naming
Chilatherina sentaniensis was first described by the Dutch zoologist Max Weber in 1907 from material collected at Lake Sentani during early scientific expeditions to what was then Dutch New Guinea. Weber did not originally place it in Chilatherina as the genus is now circumscribed, so the authority sits in parentheses — (Weber, 1907). Eschmeyer's Catalog of Fishes recognises the valid name as Chilatherina sentaniensis, and FishBase and the IUCN both treat it under this combination.
Chilatherina is one of several genera in the family Melanotaeniidae, the true rainbowfishes, a group that radiates across New Guinea and northern and eastern Australia. It is centred on northern New Guinea and separated from the larger genus Melanotaenia mainly by features of the mouth and dentition; body form overlaps broadly across the family. The epithet sentaniensis is geographic, and pointedly so — Lake Sentani is the only place this fish has ever been found.
Morphology
Chilatherina sentaniensis is a moderately elongate rainbowfish that grows to about 4 in in standard length; aquarium and field reports describe males reaching roughly 4.5 in in total length, with females usually staying under 4 in. As in rainbowfishes generally, the body deepens and grows more laterally compressed with age — most pronounced in dominant males, which take on the almost slab-sided profile typical of the family. The dorsal fin is divided into a short anterior portion and a longer posterior one; the published meristics give five to seven dorsal spines and nine to twelve soft rays, with a long anal fin of one spine and twenty-one to twenty-six soft rays.
The fish is fundamentally silvery, a bluish to greenish sheen across the upper back fading laterally toward orange and red on the lower body. A diffuse blue-green band runs along the mid-flank, overlaid by fine silver to pale-blue stripes that follow the scale rows. Mature males are the more strongly marked sex, showing roughly six to eight reddish-brown bars along the lower sides between the pectoral base and the middle of the anal fin, and intensifying their colours during display. Females and juveniles are plainer, the usual pattern of sexual dimorphism in rainbowfishes.
Habitat
Chilatherina sentaniensis is endemic to Lake Sentani, an irregularly shaped freshwater lake lying roughly 6 mi west of Jayapura in West Papua, Indonesia. It is dotted with around sixteen islands and walled off by hills to the south and the Cyclops Mountains to the north — an enclosure that goes some way to explaining how a fish tied to one body of water came to exist there at all. The species is a freshwater, pelagic, schooling fish of the open and vegetated water, recorded at low tropical latitudes around 2 to 3 degrees south.
FishBase gives a temperature range of roughly 24 to 28 degrees Celsius for the lake. Beyond temperature, field water chemistry is sparse in the accessible record, and the figures in aquarium care notes are best read as husbandry guidance rather than measured habitat values. The central fact about this fish doesn't change: its entire world is a single lake, with no outlying population to fall back on, so any shift in Sentani's water is an existential matter.
Feeding
Like rainbowfishes in general, the Sentani rainbowfish is an opportunistic omnivore, and its low trophic level — about 2.9 in FishBase's estimate — reflects a diet that mixes animal and plant material. In the wild it takes algae, aquatic insects and their larvae, small crustaceans, and other plant and invertebrate matter from the water column and from vegetation, feeding chiefly in the middle and upper levels of the lake.
In the aquarium it is undemanding and takes a broad range of foods. Hobby sources report it does well on good-quality flake and small pellets supplemented with live and frozen items such as brine shrimp and Daphnia. A varied diet with some live or frozen food improves both colour and condition, and frequent small feedings suit a constantly active grazer better than one large meal.
Mating
The Sentani rainbowfish is an egg-depositing, plant-spawning species with no parental care. As described for the genus and in hobby accounts, a male courts a female by leading her over fine-leaved plants or similar cover, intensifying his colours as he displays. It is a continual spawner, releasing eggs in small batches over an extended period rather than in a single seasonal clutch.
Males display most strongly in the presence of rivals, so the fish are at their most colourful and most reproductively inclined when several are kept together. Spawning behaviour centres on the male guiding a receptive female to a spawning site among vegetation, where a few large adhesive eggs are deposited at a time and left unattended.
Breeding
In captivity the Sentani rainbowfish breeds in the way typical of the family. A courting male leads the female over fine-leaved plants such as Java moss, or over synthetic spawning mops, where she deposits a few comparatively large eggs at a time, the eggs attaching to the vegetation by fine filaments. Spawning is spread over many days or weeks, with small numbers of eggs released continually rather than a single large batch — the egg-laying behaviour described in an Australian government risk assessment as occurring on aquatic vegetation or debris, without any damage to the habitat and without parental care.
Adults offer no protection to eggs or fry and will cannibalise their own spawn, so breeders typically move the eggs or the spawning medium to a separate rearing container. Hobby accounts report the eggs hatch in roughly six to nine days depending on temperature, and that the tiny fry need very small first foods — infusoria and the like — before they can manage newly hatched brine shrimp. Growth is slow, and the fuller male colours arrive only with maturity.
In the aquarium
Chilatherina sentaniensis is a peaceful, active shoaling fish that occupies the middle and upper levels of the tank and is best kept in a group of six or more, where males display against one another and hold better colour. Hobby sources rate it as suitable for beginner-to-intermediate aquarists and note a lifespan of up to about ten years, long-lived for a fish of its size. As a restless swimmer it needs open swimming space and benefits from a tank long enough to let a school move freely.
Reported aquarium parameters run warm and neutral-to-alkaline: pH about 7.0 to 8.0, general hardness of 5 to 20 dGH, and temperatures of roughly 24 to 29 degrees Celsius. These come from care guidance rather than measured habitat chemistry, but they sit well within what the genus tolerates. A planted tank with an open central lane, some surface movement, and good lighting shows the fish to best effect. Pick tankmates that are similarly sized, peaceful, and won't outcompete it at feeding time.
Conservation
Chilatherina sentaniensis is assessed as Critically Endangered on the IUCN Red List, under criterion B1ab(iii), in an assessment dated 5 December 2019. The listing reflects the species' extreme geographic confinement — a single lake — combined with mounting and well-documented pressures on that habitat. Deforestation in the catchment, water pollution, and the introduction of non-native fishes into Lake Sentani have all been cited as threats, and once-abundant populations are reported to have declined sharply, with the fish possibly gone from parts of its former range.
A narrow endemic in one lake basin has essentially no resilience to this: there is no large population anywhere else to buffer local losses, so degradation of Sentani translates directly into decline of the species. Its place in the aquarium hobby offers some insurance, since it breeds in captivity and can be held outside the wild — but a captive line is no substitute for a working lake. In the end the survival of the Sentani rainbowfish is tied to the fate of the single body of water that bears its name.