Taxonomy & naming
The French ichthyologist Henri Émile Sauvage described Rheocles sikorae in 1891, from material collected in eastern Madagascar. Eschmeyer's Catalog of Fishes records the valid name with a parenthetical authority, (Sauvage, 1891) — the parentheses meaning the species was first placed in a different genus and later moved to Rheocles. It sits in the family Bedotiidae, the Madagascar rainbowfishes: a small endemic radiation within the order Atheriniformes (the silversides and their allies), confined entirely to the island.
The genus name comes from the Greek rheos, a stream or current — a fitting label for fishes typical of Madagascar's flowing eastern rivers. The epithet sikorae honours Franz Sikora (1863–1902), an Austrian explorer and collector who worked on the island in the 1890s and whose specimens reached European museums in that period.
Bedotiidae holds just two genera, Bedotia and Rheocles, and the family carries real biogeographic interest as part of Madagascar's relictual freshwater fauna. Rheocles has been revised more than once, and several of its species are poorly known; R. sikorae is among the least-studied. The published record is thin enough that much of its relationships and variation remains unresolved.
Morphology
This is a small, elongate, laterally compressed rainbowfish. FishBase reports a maximum of about 4.5 in total length from early CLOFFA-era material; most fish are smaller. The build is typically bedotiid — a slender, silvery, stream-adapted body with two separate dorsal fins, the standard atheriniform arrangement shared with the silversides.
Colour and meristic descriptions are sparse in the accessible literature, and FishBase holds no photograph, so this account stops short of asserting fine pattern details that can't be traced to a source. Rheocles as a genus run far more subdued than the brightly hued Melanotaenia of New Guinea: silvery fishes, sometimes with darker scale margins or a faint lateral stripe, the males usually showing a little more colour and fin development than the females.
With no species-specific morphological treatment at hand, the safest summary is that R. sikorae is a small (to roughly 4.5 in) silvery stream rainbowfish of the eastern Madagascan type. For precise fin-ray or scale counts, go to Sauvage's original description and any later generic revision rather than a generalised account like this one.
Habitat
The species is endemic to eastern Madagascar, in the high valleys and streams of the east coast drainage. FishBase records it from the Mangoro stream system in the high valley of the east coast and from the Manambola River near Anosibe; the IUCN assessment references the Manambolo River near Anosibe An'ala (Stiassny 1990). These are flowing, often clear upland and mid-elevation waters of the humid eastern slope — the classic habitat of the stream-dwelling Bedotiidae.
It is logged as a freshwater, pelagic, tropical fish at roughly 64–72 °F. That is cooler than the lowland waters many aquarium rainbowfishes come from, in line with its upland, running-water range. Other water-chemistry values aren't well documented, and rather than guess at pH or hardness this account leaves them open; the streams it occupies are typically soft, well-oxygenated, and flowing.
The restricted, river-system endemism is the defining ecological fact here, as it is for most Madagascan freshwater fishes. The IUCN assessment finds it in forested streams in good condition, but poorly known and rarely collected — part of why its status stays Data Deficient rather than something higher.
Feeding
No dietary study of Rheocles sikorae appears in the readily accessible literature, and FishBase gives no diet. Its estimated trophic level of around 3.0 fits a small carnivore or invertebrate feeder rather than a herbivore.
By analogy with other stream-dwelling Bedotiidae and the silverside order, R. sikorae is most plausibly a micro-predator of small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and surface-falling insects taken from the flowing water column. A pelagic, open-water habit suits a fish that intercepts drifting and surface prey, not one that grazes algae off the substrate.
With no direct feeding observations at hand, treat all of that as inference from its biology and relatives, not a documented diet. The feeding ecology of R. sikorae is simply poorly known.
Mating
Mating behaviour has not been documented for Rheocles sikorae — one of the genuinely blank spots in the record. No courtship descriptions, spawning observations, or seasonal-timing records exist in the accessible sources.
The better-studied members of Bedotiidae, notably species of Bedotia, are egg-depositing fishes: males court females with fin and colour displays, and the adhesive eggs hang from vegetation or fine substrate by short filaments, the general atheriniform pattern. R. sikorae probably follows a broadly similar plan, with males displaying in flowing water, but that is an extrapolation from the family, not a recorded fact for this species.
With no direct evidence, this account makes no claims about pair formation, display, or breeding season in R. sikorae.
Breeding
No published captive- or wild-breeding account for Rheocles sikorae exists in the accessible record. The species is effectively absent from the aquarium hobby, so the breeders' observations that pile up around popular rainbowfishes have no counterpart here.
Bedotiid rainbowfishes are generally egg-scattering or plant-attaching spawners. They produce adhesive eggs hung from fine-leaved vegetation by short filaments and give no parental care, the standard mode across the atheriniform rainbowfishes. If R. sikorae fits the family pattern, eggs would be laid among aquatic plants or fine substrate in its stream and left to develop on their own.
That is inference, not record. Clutch size, incubation period, fry development, and any seasonal spawning trigger go undocumented in the sources consulted, and this account invents none of them. Its reproductive biology awaits study.
In the aquarium
For practical purposes, this is not an aquarium fish. No reliable hobby care or breeding profile for it was located; it turns up only generically in Madagascar-biotope material, not as a trade or hobby species. The Madagascar rainbowfishes that do reach specialist collections are mostly Bedotia, kept by a handful of dedicated aquarists. Rheocles are rarer still in captivity.
If you infer conditions from the habitat, they point to a flowing-water set-up: well-oxygenated, gently to moderately moving water, on the cooler side at roughly 64–72 °F to match its upland eastern-Madagascan range, soft and clean, with a long swimming lane for an active pelagic stream fish. Keep a group, not a single specimen — it shoals — and any tankmates would have to share its taste for cooler, moving water.
That is inference, not a tested regime; there is no real hobby experience with this species to lean on. R. sikorae is a wild Madagascan endemic of conservation interest, not a candidate for the general trade, and any keeping belongs in the hands of specialists working with proper provenance.
Conservation
The IUCN Red List lists Rheocles sikorae as Data Deficient (DD), assessed on 23 June 2016. The category means there isn't enough information on its distribution, population, and threats to pin down a more specific status — a common verdict among Madagascar's poorly studied freshwater fishes.
The assessment calls it poorly known and rarely collected, recorded from the Manambolo River near Anosibe An'ala in forested streams in good condition. It also flags taxonomic confusion between R. sikorae and Rheocles wrightae, the two possibly synonymous. A poorly known, rarely collected fish with unresolved species limits lands at Data Deficient rather than a threatened category, even as deforestation across its range is named as a concern.
The wider picture matters. Madagascar's endemic freshwater fauna, Bedotiidae included, ranks among the most threatened anywhere, hit by deforestation, sedimentation, watershed degradation, and introduced fishes across the island's rivers. A narrowly endemic stream fish has little buffer if those pressures reach its catchments. Read Data Deficient not as reassurance but as a call for survey work — the gap in knowledge is itself the conservation problem.