Taxonomy & naming
Ptychochromis ernestmagnusi was described in 2010 by the American ichthyologists John S. Sparks and Melanie L. J. Stiassny, both of the American Museum of Natural History, in the journal Zootaxa (No. 2341, pp. 33–51). The paper did more than name a new fish: it presented an updated molecular and morphological phylogeny of the whole genus and a revised generic diagnosis. The holotype (AMNH 249490) and its paratype series were collected from the Mananara du Nord River at Antanambaobe village, at roughly 16°16′S, 49°40′E and about 394 ft elevation, in northeastern Madagascar. The genus Ptychochromis was erected by Franz Steindachner in 1880 and sits in the subfamily Ptychochrominae (often written Ptychochromatinae), one of the endemic Malagasy cichlid lineages whose deep divergence from mainland African and Neotropical cichlids has made the family a textbook case in Gondwanan biogeography. Within the genus, Sparks and Stiassny recognised an 'eastern' species group defined by palatine morphology; ernestmagnusi belongs to this group and is diagnosed by an anterior displacement of the first supraneural bone so that it overlies the rear margin of the supraoccipital, a feature it shares only with Ptychochromis grandidieri, Ptychochromis loisellei, P. makira and P. curvidens. Its closest relative appears to be the geographically neighbouring P. makira: the two share supraneurals with a distinctively flattened dorsal profile, interpreted as a shared derived character. No synonyms exist — the species has been considered valid since its description.
Morphology
This is a moderately large, fusiform riverine cichlid. The type series reaches about 6 in in standard length, though as with most Ptychochromis wild adults in undisturbed habitat probably grow larger, and captive congeners of similar build commonly approach 8–10 in. The fin counts are typical of the genus: twelve to thirteen dorsal spines followed by eleven to thirteen soft rays, three anal spines and seven to nine anal soft rays, over twenty-seven or twenty-eight vertebrae. The body is fairly deep and laterally compressed with a steep forehead. Its base coloration is a dusky greyish-green, and the common name 'black-finned' reflects the dark median fins. The most useful field marks are a series of strong, paired lateral flank bars oriented vertically — distinguishing it from the sister species P. makira, in which those bars are V-shaped — together with conspicuous iridescent spangling scattered across the flank and along the base of the dorsal fin. Sparks and Stiassny noted further internal diagnostics separating it from P. makira: four lachrymal laterosensory foramina rather than three, and seven infraorbital bones rather than six. Detailed observations of breeding coloration and sexual dimorphism specific to this species have not been published; in its close relatives the sexes are similar in pattern, males tend to grow somewhat larger with more produced finnage, and both sexes darken dramatically and flush with gold during spawning.
Habitat
Ptychochromis ernestmagnusi is known only from the middle to lower reaches of the Mananara du Nord River in the SAVA region of northeastern Madagascar — a wet, low-elevation landscape that the describers characterised as humid lowland rainforest with a steep overall topographic profile. The type locality lies inland at about 394 ft elevation. Within the river the species is associated with a generally rocky to sandy substrate, the kind of moderately flowing, structured habitat favoured across the eastern Ptychochromis group. Because the genus as a whole is notably euryhaline — several congeners penetrate brackish estuarine water and tolerate a wide swing in chemistry — it is reasonable to expect this species also ranges into the lower, tidally influenced river, though that has not been documented for ernestmagnusi specifically. No in-situ water chemistry was reported in the original description. By the standards of Madagascar's eastern rivers the water is warm, soft to moderately mineralised and near-neutral; husbandry of related species clusters around pH 7 to 8 and roughly 72–82 °F, and those figures are the best available proxy. The species' full distribution is genuinely unknown: it may well occur in neighbouring drainages but has yet to be recorded outside the Mananara.
Feeding
No dietary study has been carried out on Ptychochromis ernestmagnusi, and the original description offers no gut-content data. FishBase places it at an estimated trophic level near 3.4, in line with the genus's reputation as a generalist, opportunistic feeder rather than a specialist. Ptychochromis are robust-jawed cichlids that take a broad omnivorous diet in the wild — aquatic insects and their larvae, small crustaceans and other benthic invertebrates, plant and detrital material, and whatever else the substrate yields. Several congeners are described sifting mouthfuls of sand and expelling it through the gills in the manner of South American eartheaters, working a rocky-to-sandy bottom for invertebrate prey, which fits the substrate this species is associated with. In aquaria the related eastern Ptychochromis accept frozen and live foods (bloodworm, mysis, brine shrimp, river shrimp, earthworms) alongside good-quality prepared cichlid foods without difficulty, and ernestmagnusi would be expected to behave the same.
Mating
The reproductive behaviour of Ptychochromis ernestmagnusi has not been observed or recorded — it has not, as far as published reports show, been kept or bred in captivity, and no field observations of spawning exist. What can be said is drawn from its close relatives, which are consistent enough across the genus to be informative. Eastern Ptychochromis form monogamous pairs and are biparental substrate spawners. Pair formation is preceded by a period of restless site inspection, with the prospective parents examining logs, rock faces, caves and crevices before settling on a site. As spawning nears, both partners undergo a striking colour change — darkening to near-black while the dorsal fin and the region around the eye flush with intense gold. The pair then defends a territory around the chosen surface with real determination, driving off tankmates much as Central American cichlids do, though typically with less jaw-locking. Intraspecific aggression can be severe, and unpaired or poorly matched fish may be harassed, which is one reason keepers of related species maintain them as groups in large tanks so a pair can sort itself out without casualties.
Breeding
Breeding has never been documented for Ptychochromis ernestmagnusi, in the wild or in captivity, so this account is necessarily genus-typical and clearly labelled as such. Eastern Ptychochromis are biparental substrate (open- and cave-) spawners. The female lays a sheet of small cream-coloured eggs on a cleaned hard surface — a log, a rock face, or the inside of a cave or tube — and fans them assiduously while the male patrols and defends the perimeter; clutches in closely related congeners run to roughly several hundred eggs. After hatching, the wrigglers are commonly transferred to a pit dug near the spawning site and are guarded by both parents until free-swimming, typically within a week to ten days of spawning depending on temperature. A recurring difficulty noted by breeders of Malagasy cichlids is parental cannibalism of first clutches, which keepers often manage by removing the eggs to a separate hatching vessel. None of this has been confirmed for ernestmagnusi itself; documenting its reproduction in a conservation-breeding setting would be a genuine contribution, given how little is known.
In the aquarium
Ptychochromis ernestmagnusi is essentially absent from the aquarium hobby. There is no established trade in the species, no published captive-breeding record, and it does not appear in the rotation of Malagasy cichlids that occasionally reach dedicated keepers and public aquaria. Anyone encountering it would almost certainly do so through a conservation-breeding program rather than the ornamental trade, and that scarcity should temper expectations: this is not a fish to seek out casually.
What husbandry guidance exists comes from its close relatives, which are kept successfully by experienced cichlid keepers and by institutions such as the Citizen Conservation network and several zoos. The eastern Ptychochromis are described as easy to keep but capable of marked intraspecific aggression, so they are best maintained as a group — roughly eight or more adults — in a large tank. A footprint of at least 5 ft in length and around 130 US gal is the standard recommendation for a group of a 5.5–10 in species, and bigger is better; a single ill-matched pair in a cramped tank is a recipe for injury. The substrate should be sand or fine gravel that the fish can sift, with rockwork, driftwood, tubes and caves providing both spawning surfaces and sightline breaks that defuse aggression. Strong, well-maintained filtration with high turnover and high water quality matters more than hitting precise chemistry, as these fish are tolerant of a wide range; figures that have worked for the congener Ptychochromis loisellei are about 75–77 °F, pH ~7.5, carbonate hardness around 3 °dH, general hardness ~5 and conductivity ~230 µS, while another eastern form has been bred at pH 7.7 and 79 °F. Feeding is undemanding: a varied menu of flake and quality cichlid pellets supplemented with frozen and live foods (mosquito larvae, mysis, brine shrimp, river shrimp, earthworms) keeps them in condition and helps bring a pair into breeding readiness.
Tankmates should be robust enough to coexist with a territorial substrate spawner — other Malagasy cichlids such as Paratilapia and Paretroplus, or sturdy catfish — rather than small or timid fish that will be bullied during spawning. The honest summary is that ernestmagnusi is a fish for the serious conservation aquarist: were stock ever to become available, its value would lie in establishing an ex-situ population, not in casual display.
Conservation
The IUCN Red List assesses Ptychochromis ernestmagnusi as Data Deficient (DD), assessed on 14 May 2016 by the FishBase team (RMCA) together with J.S. Sparks. The Data Deficient listing is itself the headline: the species is known from a single river system, its full range is unmapped, and no population data, ecological study or trend information exists — there is simply not enough information to judge its risk of extinction. That classification should not be read as reassuring. Madagascar's exclusively endemic freshwater fish fauna is one of the most threatened vertebrate assemblages anywhere; across the island's native freshwater fishes a large fraction are formally Endangered, Critically Endangered or already gone, and the genus Ptychochromis includes some of the most imperilled cichlids on Earth (the Mangarahara cichlid, P. insolitus, was for a time considered the rarest fish in the world). The drivers are relentless: catastrophic deforestation and the soil erosion and siltation that follow it, river pollution from mining and gold-panning, agricultural runoff, and the spread of introduced fishes that prey on and outcompete the natives. A single-river endemic in northeastern Madagascar is, by definition, highly exposed to all of these. It carries no CITES listing. The most useful conservation steps would be basic survey work to establish its true distribution and abundance, and the kind of cooperative ex-situ breeding networks that have kept other Malagasy cichlids from disappearing entirely.