Taxonomy & naming
The French ichthyologist Jacques Pellegrin described Rheocles alaotrensis in 1914. It has since been moved into the genus Rheocles from the one it was first placed in, so the authority is cited in parentheses as (Pellegrin, 1914) — the convention recorded in Eschmeyer's Catalog of Fishes, which governs the valid name. It sits in the family Bedotiidae, the Madagascar rainbowfishes, within the order Atheriniformes, the silversides. Despite the shared common name, that makes it only a distant relative of the true rainbowfishes (Melanotaeniidae) of New Guinea and Australia.
The Bedotiidae are a small family endemic to Madagascar, holding just the genera Bedotia and Rheocles — one of the many ancient, isolated freshwater lineages that make the island's fish fauna both distinctive and vulnerable. The genus name Rheocles comes from the Greek 'rheos', a stream or current, a nod to the flowing-water habitats several members of the genus favour. The epithet 'alaotrensis' is geographic, tying the species to Lake Alaotra, the type region from which Pellegrin's material came.
'Katrana' is the Malagasy name used for the fish around the lake. As with most narrowly endemic Malagasy fishes, little additional common-name nomenclature has built up in the international literature, and it is most reliably referred to by its scientific name.
Morphology
This is a small, elongate, laterally compressed fish reaching a maximum recorded 5.5 in total length, though most individuals run smaller. The build is typical silversides — slender and streamlined, suited to open and flowing water — not the deep, diamond-shaped profile of the New Guinea rainbowfishes it shares an English name with.
Detailed morphometric and colour data are sparse in the accessible literature. Rather than guess at fin counts or coloration that cannot be traced to a source, it is more honest to say documentation is thin. Published length-weight parameters (a Bayesian estimate of a = 0.01122, b = 3.04) confirm the slender, near-isometric proportions expected of a small pelagic silversides. As a bedotiid it can be expected to show the family pattern — a divided dorsal fin and a silvery body — but diagnostic and live-colour details for R. alaotrensis are poorly represented in the sources consulted.
Habitat
The species is endemic to central-east Madagascar, with its core range in the Lake Alaotra basin and the Maningory River system that drains the lake. There are additional records from Anjozorobe and from tributaries of the Betsiboka on the western slope, but the centre of gravity is the Alaotra wetland complex on the central-east plateau. FishBase characterises it as a freshwater, pelagic, tropical species — a fish of open water rather than dense cover.
Lake Alaotra, Madagascar's largest, is a shallow lake-and-marsh complex ringed by extensive reedbeds and floodplain — broad, shallow, and seasonally dynamic, with water temperature singled out in the conservation literature as a defining feature. It supports a number of endemic species and is recognised as a wetland of international importance. It is also one of the most heavily modified freshwater environments in Madagascar.
Those pressures sit at the centre of the species' biology. The Alaotra complex has been reshaped by invasive water hyacinth (Eichhornia crassipes), by introduced non-native fishes, by sedimentation from deforestation in the catchment, and by the spread of rice agriculture around the lake margins. For a fish whose entire global range is essentially this one basin, the condition of the wetland is the condition of the species.
Feeding
No dietary studies of R. alaotrensis turn up in the sources consulted, so its feeding ecology has to be inferred — cautiously — from its body form and from the general habits of the Bedotiidae and other small atheriniforms. FishBase places it at a trophic level of roughly 3.0, consistent with a small predator of invertebrates rather than a herbivore or piscivore.
A slender, pelagic silversides of this kind would take small aquatic and terrestrial invertebrates: insect larvae, micro-crustaceans, and insects picked off the surface and out of the water column. Beyond that, prey records for this particular species are too thin to name proportions or specific items without guessing. What can be said with confidence is that it works an insectivorous, invertebrate-feeding niche in the open waters of the Alaotra system.
Mating
Courtship and mating behaviour specific to R. alaotrensis is not documented in the accessible literature. Like several other facets of its biology, it is poorly studied next to the popular aquarium rainbowfishes, and reliable observations of wild reproduction are scarce.
Across the Bedotiidae, the Madagascar rainbowfishes lay eggs that attach to vegetation or substrate by fine filaments — a mode shared widely among atheriniforms. R. alaotrensis can reasonably be expected to follow it. But the specifics — pair formation, display, spawning season, site selection — aren't in the sources at hand, and are flagged here as a genuine gap rather than filled with conjecture.
Breeding
Captive-breeding and wild-reproduction data are limited. R. alaotrensis never became an aquarium staple the way the New Guinea rainbowfishes did, and the published record carries little on clutch size, egg development, or fry rearing.
By analogy with related Bedotiidae and other small atheriniforms, it is likely a continuous or seasonally extended spawner that lays adhesive eggs onto fine-leaved plants, anchored by short filaments, with fry hanging near vegetation after hatching. Those are family-level expectations, not species observations. The honest summary, given a standard of saying less over guessing, is that the breeding biology of R. alaotrensis is data-sparse — any precise egg counts or incubation times would be invented, not sourced.
In the aquarium
This is not a common aquarium fish. It does not appear in the ornamental trade at anything like the volume of the Melanotaenia rainbowfishes, and FishBase carries no aquarium-maintenance fact sheet for it. As an Endangered, narrowly endemic species, it matters far more as a conservation subject than as a hobby fish — and wild collection for the trade is no proper use of a threatened single-basin endemic.
Where it is kept at all, it is most likely held in conservation-oriented or specialist breeding programmes rather than the general hobby. Care would follow the broad needs of a small, active, pelagic silversides from warm, shallow lake and stream waters: open swimming space, stable warm-tropical temperatures, good water quality, a varied invertebrate-based diet. No species-specific husbandry protocol for R. alaotrensis is documented in the sources consulted, so that remains a general picture. The responsible posture is to treat it as a fish whose survival matters more than its display value, and to source any captive stock through legitimate conservation channels rather than wild capture.
Conservation
R. alaotrensis is assessed as Endangered (EN) on the IUCN Red List under criteria B1ab(i,iii,v). The listing reflects its extremely restricted range — essentially the Lake Alaotra basin and the Maningory drainage — together with documented declines in the extent and quality of its habitat and in its population. A fish confined to one lake system has no outlying populations to fall back on, so degradation of the Alaotra wetland translates straight into risk for the species.
The pressures are severe and well documented. Invasive water hyacinth (Eichhornia crassipes) chokes open water and alters the wetland; introduced non-native fishes prey on or compete with the natives; deforestation in the catchment drives sedimentation; and rice agriculture keeps expanding around the lake. The same wetland is famous as the only home of the Critically Endangered Alaotra gentle lemur (Hapalemur alaotrensis) — and the threats to lemur and rainbowfish trace back to one transformation of the lake.
Saving R. alaotrensis is therefore inseparable from saving Lake Alaotra. Protecting the fish means protecting and restoring the wetland — controlling invasive plants and fishes, cutting sedimentation, managing the lake's use sustainably — not any measure aimed at the species in isolation. It is one more reminder of how much of Madagascar's freshwater biodiversity hangs on the fate of a handful of irreplaceable water bodies.