Taxonomy & naming
Rheocles derhami was described by Melanie L. J. Stiassny and Damaris M. Rodriguez in 2001, in Ichthyological Exploration of Freshwaters (12(2): 97–104), from material collected in the Amboaboa (Ambomboa) River drainage of northeastern Madagascar. Eschmeyer's Catalog of Fishes records the valid name as Rheocles derhami Stiassny & Rodriguez, 2001; because it was described in Rheocles, where it remains, no parenthetical authority is used.
The genus Rheocles belongs to the family Bedotiidae, the Malagasy rainbowfishes, an endemic radiation of atheriniform fishes confined to the freshwaters of Madagascar. Rheocles is distinguished from the family's other genus, Bedotia, by a suite of morphological characters, and within the genus R. derhami is separated from its congeners by a short upper jaw (32–41% of head length, versus 46–58% in R. sikorae and R. wrightae), the absence of black pigment on the genital papilla, a higher count of gill rakers on the lower limb of the first arch (9–10, versus 6–7 in R. pellegrini), a shorter caudal peduncle than R. lateralis, and the absence of chest and belly scales and paired predorsal scale rows that characterise R. alaotrensis.
The generic name Rheocles derives from the Greek rheos, a stream or current, in reference to the lotic habitats these fishes favour. The specific epithet derhami is a patronym honouring Patrick de Rham, the Swiss ichthyologist and aquarist who served as director of the Aquatic Conservation Network and worked extensively on Madagascar's threatened freshwater fishes. No widely used common name is recorded for the species; like its relatives it may be referred to generically as a Madagascar rainbowfish.
Morphology
Rheocles derhami is a small rainbowfish, reaching about 2 in in standard length in the largest recorded specimens. The body is fusiform and of normal proportions for the genus — an elongate, silverside-like rainbowfish rather than the deep-bodied form seen in the Australasian Melanotaeniidae. Meristic counts from the original description give two dorsal spines and 14–19 dorsal soft rays, one anal spine and 15–21 anal soft rays, and 35–38 vertebrae.
In life the sexes differ in colour. Males are light grey over the body with bright, silvery-blue second dorsal, anal, and caudal fins and a distinctive orange-red flush over the throat region; females are also light grey but lack the bright fin and throat colours. The species is most reliably separated from its congeners not by colour but by hard characters — chiefly the short upper jaw, the gill-raker count, the caudal-peduncle proportions, and the scalation features noted above. Its reported trophic level is around 3.2, consistent with a small invertebrate-feeding rainbowfish. The fish is harmless to humans.
Habitat
Rheocles derhami is endemic to Madagascar and known only from the Amboaboa (Ambomboa) and Mangarahara rivers in the Sofia drainage of the northeastern interior — the type locality lies in the Amboaboa River near Marotandrano. These are upland, flowing-water (lotic) habitats: permanent rivers, streams, and creeks of the upper catchment, where the water was historically relatively cool and clear. The species is a freshwater, pelagic fish of tropical streams.
The condition of that habitat is the whole story here. The upper Mangarahara–Amboaboa system once held clear, pristine river reaches; deforestation and land conversion across the catchment have since driven heavy sedimentation, turning long stretches of the main river into shallow pools and runs. It shares this drainage with the famously imperilled Mangarahara cichlid (Ptychochromis insolitus), and both fishes face the same threat: the degradation of a single, increasingly fragmented river system. The known extent of occurrence is tiny, on the order of 31 mi², and the species now effectively occupies a single location.
Feeding
No diet study specific to Rheocles derhami has been published, and feeding information for the species is data-sparse. Its reported trophic level of about 3.2 and its body form indicate a small carnivore or omnivore that takes invertebrate prey, consistent with the general ecology of stream-dwelling rainbowfishes.
By analogy with other Rheocles and with the family Bedotiidae more broadly, the natural diet is expected to consist mainly of small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken from the surface and the water column — supplemented opportunistically with other small food items. In the absence of direct dietary observations for this species, no more precise account can be given without risk of fabrication.
Mating
Reproductive behaviour has not been documented in the wild for Rheocles derhami, and specific information is sparse. The original description records the sexual dichromatism — males with bright silvery-blue median fins and an orange-red throat, females plainer and not brightly coloured — which strongly implies that males use colour in courtship display, as is typical of rainbowfishes generally.
In bedotiid rainbowfishes the broad pattern is one of substrate- and plant-associated spawning, with males displaying to females and pairs releasing eggs among fine-leaved vegetation. No courtship sequence, spawning site, or seasonal timing has been described specifically for R. derhami, so any detailed account would be speculative. The marked colour difference between the sexes does point to visual male display as part of the mating system.
Breeding
There is no published account of breeding in Rheocles derhami, in the wild or in captivity, and this section is necessarily data-sparse. The species is essentially absent from the aquarium trade and has not, as far as the available sources show, been the subject of a documented captive-breeding effort.
The general reproductive mode across the Bedotiidae is egg-scattering over fine-leaved plants, with adhesive eggs that attach by fine filaments to the spawning substrate and are laid over an extended period rather than in a single large clutch — the pattern seen in better-studied Rheocles and Bedotia kept by specialist aquarists. It is reasonable to expect R. derhami to follow this family pattern, but no clutch size, incubation period, or fry-rearing data exist for the species itself, and none should be invented. Given its Critically Endangered status, any future ex-situ breeding programme would be of direct conservation value.
In the aquarium
Rheocles derhami is not an aquarium fish in any practical sense. It is Critically Endangered, known from a single remote location in the Malagasy interior, and absent from the ornamental-fish trade; no dedicated hobby care profile for the species exists, and the genus Rheocles as a whole is only rarely kept, mostly by specialist conservation-minded aquarists.
What husbandry context exists is generic to the Bedotiidae rather than specific to this fish. Malagasy rainbowfishes are typically maintained as active, schooling stream fishes that appreciate clean, well-oxygenated, moving water, ample swimming space, and a varied diet of small live and frozen invertebrates; they spawn by scattering adhesive eggs over fine-leaved plants or mops. None of it is validated for R. derhami in particular; honestly, the species' aquarium requirements are undocumented. The most useful thing an aquarist can do for this fish is support in-situ habitat protection and any sanctioned conservation breeding of Madagascar's threatened bedotiids.
Conservation
Rheocles derhami is assessed as Critically Endangered on the IUCN Red List (criteria A2ac; B1ab(iii,v)), in an assessment dated 21 June 2016. The population trend is decreasing, the range is severely fragmented, and a continuing decline of mature individuals is documented. The justification infers a population reduction of more than 90% over the preceding ten years, with a linear decline since 1992, and the extent of occurrence is estimated at only about 31 mi² within what is now treated as a single location.
The species is known only from the Amboaboa and Mangarahara rivers of the Sofia drainage in northeastern Madagascar, and recent field surveys (2013–2015) recovered only a single specimen, from the Amboaboa near Marotandrano village, with no other subpopulations located. The principal driver of decline is habitat loss: sedimentation from deforestation and land conversion in the catchment has degraded the once clear, cool upper-river habitat, transforming the main river into shallow pools and runs and driving rapid ecosystem degradation. CITES has not evaluated the species.
R. derhami shares its drainage — and its plight — with the critically endangered Mangarahara cichlid, which makes the upper Mangarahara–Amboaboa system a focal point for Malagasy freshwater conservation. A range that small, a declining and fragmented population, and a wild record reduced to a handful of fish put it among the most threatened freshwater fishes of Madagascar. Its persistence depends on protecting and restoring the integrity of its single river system.