Taxonomy & naming
Bedotia albomarginata was described by John S. Sparks and Lukas M. R. Rush in 2005, in the journal Zootaxa, in a paper titled 'A new rainbowfish (Teleostei: Melanotaenioidei: Bedotiidae) from the southeastern highlands of Madagascar, with comments on the biogeography of Bedotia.' The type material came from the Mananara and Rienana River drainages of southeastern Madagascar. Eschmeyer's Catalog of Fishes records the valid name as Bedotia albomarginata Sparks & Rush, 2005; because the species was described in Bedotia, where it remains, the authority is written without parentheses.
The genus Bedotia is the larger of the two genera in the family Bedotiidae, the Madagascar rainbowfishes, an old endemic radiation confined to the freshwaters of that island. Bedotiids sit within the Atheriniformes (the silversides and their allies) and are placed close to the true rainbowfishes of New Guinea and Australia, a relationship reflected in the suborder name Melanotaenioidei used in the original description. Many Bedotia are narrow-range endemics tied to particular river systems, and the 2005 paper that named B. albomarginata also took up the biogeography of the genus — Madagascar's river drainages have each tended to harbour their own distinct lineages.
The genus name honours Maurice Bedot (1859–1927), a Swiss systematist of hydrozoans born in Geneva. The specific epithet albomarginata is Latin, from 'albo' (white) and 'marginatus' (edged or bordered), and refers to the white marginal stripe on the male's second dorsal fin — and, in some individuals, the anal fin — a feature that helps distinguish this species from its congeners.
Morphology
Bedotia albomarginata is a small, elongate, laterally compressed fish reaching about 3.5 in in standard length. The snout is somewhat elongate, the eye is large, and the mouth is large and oblique — a build typical of an active, sight-feeding midwater fish. Like other bedotiids it carries two separate dorsal fins: a small anterior dorsal of four to six feeble, unbranched rays, followed by a longer second dorsal of eleven to fourteen rays (fifteen to twenty soft rays across the two fins combined, with no true spines). The anal fin is long, with sixteen to nineteen soft rays. Counts recorded in the original description include thirty-three to thirty-six vertebrae, thirty-seven to thirty-eight scales in the longitudinal series, and nineteen to twenty predorsal scales.
In colour the fish is dark olive to grayish-brown above the lateral midline and white to pearlescent below. The most conspicuous mark is a broad, continuous black midlateral stripe running the length of the body, set off in places by iridescent golden scales. Both sexes carry a bright, blotchy anal fin in yellow to orange or orangish-red. Sexual differences are concentrated in the unpaired fins: males develop the white margin on the second dorsal fin that gives the species its name, a courtship-display character rather than a feature obvious at every moment.
Many of the other Bedotia are superficially similar slim, striped, highland-stream rainbowfishes. The readiest way to pick out B. albomarginata is the combination of heavy black flank stripe, coloured anal fin in both sexes, and the male's white-edged second dorsal.
Habitat
The species is endemic to the Mananara and Rienana River drainages of the southeastern highlands of Madagascar. Within those drainages it is restricted to inland, generally forested highland habitats; the original description notes that it is not found in the deforested lower reaches of either the Mananara or the Rienana. It occurs in small rivers, shallow streams, and swamps — habitats that, in this region, are likely subject to periodic drying or strongly reduced flow.
This is a pelagic freshwater fish of tropical highland waters, and like many Madagascar rainbowfishes it is bound to intact forest. Clear the surrounding trees and the habitat seems to turn unsuitable; the fish persists in the upper, still-wooded reaches but drops out where the catchment has been cleared. Precise field measurements of temperature, pH, and hardness specific to B. albomarginata are not well documented in the cached sources, so habitat water chemistry below is drawn cautiously from genus-level information for highland Bedotia rather than from species-specific readings.
The narrow distribution — two adjacent drainages, and within them only the forested portions — is the central fact of the species' situation. There is no large outlying population in some other basin to fall back on, which leaves the fish vulnerable to anything that degrades its small range.
Feeding
Bedotia albomarginata is a micropredatory omnivore. FishBase gives a modeled trophic level of about 2.6, consistent with a diet built largely from small invertebrates — insect larvae, tiny crustaceans, zooplankton, and insects taken at or near the surface — supplemented by other small food items in the water column. The large, oblique mouth and large eye fit a fish that hunts small, moving prey by sight in flowing or still shallow water.
This is typical of the genus and of stream-dwelling rainbowfishes generally — opportunistic surface- and midwater-feeders that pick off drifting and swimming invertebrates rather than grazing heavily on plant material or sifting substrate. Detailed gut-content studies specific to B. albomarginata are not available in the cached sources, so the diet is described here at the level the data support.
Mating
Direct observations of courtship in Bedotia albomarginata are sparse, but the species' breeding biology can be inferred from its close relatives in the genus, which are well known to aquarists. Madagascar rainbowfishes are egg-scatterers that lay adhesive eggs among fine-leaved plants. Males display to females using their fins and colours; in this species the white-margined second dorsal fin is a male display character, and the bright anal fin presumably also figures in courtship signalling.
As with rainbowfishes broadly, spawning is most likely an extended affair rather than a single event, with a receptive female depositing eggs among vegetation over a prolonged period in response to male display. Because species-specific spawning observations for B. albomarginata are not documented in the cached material, this account stays close to the general Bedotia pattern and avoids asserting precise behaviours that have not been recorded for this fish.
Breeding
Breeding in Bedotia follows the rainbowfish template: the fish are egg-scatterers that lay small adhesive eggs among fine-leaved plants or spawning mops over an extended spawning period rather than in a single large clutch. For the genus, the eggs are reported to hatch in roughly one to two weeks. There is no parental care; the adhesive eggs are simply attached to the spawning medium and left.
Species-specific captive-breeding accounts for B. albomarginata are sparse, because the fish is rare in the aquarium trade and most published husbandry covers the more familiar Bedotia. Breeders working with Madagascar rainbowfishes typically condition a group, allow spawning into mops or fine plants over several days, and then transfer the egg-laden medium to a separate rearing container to protect the eggs and fry from the adults. The very small fry require correspondingly small first foods. The figures given here are genus-level and should be treated cautiously as a guide rather than as measured values for this particular species.
In the aquarium
Bedotia albomarginata is only rarely seen in the hobby, and species-specific aquarium guidance is correspondingly thin; the practical husbandry that exists is genus-level advice for Madagascar rainbowfishes. As a group, Bedotia are peaceful, active schooling fishes that do best kept in groups, where males colour up and display against one another. A tank with open swimming space and some fine-leaved planting suits them, both for the fish's comfort and to provide a spawning medium.
General Bedotia care suggests a temperature in the range of roughly 68–79 °F (68–79 °F), a near-neutral pH around 6.5–7.5, and moderately hard water — figures that should be read as a starting point rather than as measured requirements for B. albomarginata specifically. Their diet in captivity matches the wild micropredator profile: small live and frozen invertebrates, supplemented with good prepared foods, keep them in condition and colour.
Keep in mind that this is an Endangered, narrowly endemic species rarely available as captive-bred stock. Anyone who manages to get hold of it is working with a conservation-sensitive fish; the priority should be maintaining a healthy group and, ideally, breeding it to take pressure off wild collection.
Conservation
Bedotia albomarginata is assessed as Endangered (EN) on the IUCN Red List, under criterion B1ab(iii), an assessment dated 21 June 2016. The criterion points to a small and fragmented range with a continuing decline in the quality of its habitat — exactly the situation the species' biology implies. Confined to the forested reaches of just two river drainages, and absent from the deforested lower portions of those same rivers, the fish has very little geographic buffer against habitat loss.
The principal threat is the clearance and degradation of the highland forest that the species depends on. Deforestation in southeastern Madagascar alters stream flow, water quality, and sedimentation in ways that the original description links directly to the fish's disappearance from cleared reaches. Because the species occupies habitats that may already be subject to periodic drying, any additional stress on flow and water quality compounds the risk.
The fish is not listed on CITES or CMS and is harmless to humans. As with many Malagasy freshwater endemics, its long-term prospects are tied to the fate of Madagascar's forests and watersheds, and to whether protected forested catchments can keep its two small drainages intact.