Taxonomy & naming
The German-British ichthyologist Albert Günther described this fish in 1867 as Heros erythraeus, working from material collected in Lake Managua (Xolotlán), Nicaragua; the holotype is held at the Natural History Museum in London (BMNH 1865.7.20.33). Its history since then is tangled. For much of the twentieth century the name was treated as a junior synonym of the red devil — first under Cichlasoma labiatum, later under Amphilophus labiatus — and FishBase still lists Heros erythraeus among the synonyms of Amphilophus labiatus. Eschmeyer's Catalog of Fishes, however, currently records the species as valid as Amphilophus erythraeus (Günther 1867), reflecting work — including Stauffer & McKaye's treatments of the Nicaraguan Midas cichlids — that separates it from Amphilophus labiatus. So this is a genuine but contested taxon: a valid species in the Catalog of Fishes that some authorities continue to merge into the red devil. It sits within the Amphilophus citrinellus species complex, the 'Midas cichlid' radiation of the Nicaraguan lakes, among the heroine cichlids of subfamily Cichlinae. The epithet erythraeus comes from the Greek erythros, 'red,' for the colour the gold and red morphs of this group can show.
Morphology
Amphilophus erythraeus is a large, deep-bodied and powerfully built cichlid of the red-devil/Midas type, reaching roughly 9.5 in SL. Because the name has long been entangled with Amphilophus labiatus and Amphilophus citrinellus, most of its described morphology is shared with them: a thick, laterally compressed body, a steep forehead that swells in dominant breeding males into the pronounced nuchal hump typical of the complex, and strong jaws. Like the rest of the Midas group it comes in two colour forms — a cryptic grey-to-olive 'normal' morph, often with darker barring, and a striking amelanistic 'gold' or red morph in which the dark pigment is lost to reveal orange-to-red colour, the source of the epithet erythraeus ('red'). Telling erythraeus apart from Amphilophus labiatus and Amphilophus citrinellus in the field is genuinely hard, which is part of why its status has been disputed: the differences cited by those who recognise it lie in body proportions, lip and jaw form and meristics rather than in any single obvious mark. Sexual dimorphism follows the standard Amphilophus pattern — males larger, with the heavier head and hump; ripe females smaller and rounder.
Habitat
The species was described from Lake Managua (Xolotlán), the more northerly of Nicaragua's two great lakes, and as a member of the Nicaraguan Midas complex it belongs to the warm, hard, alkaline lowland waters of the Pacific-side rift that also hold Lake Nicaragua (Cocibolca) and the associated crater lakes. The Midas cichlids of these lakes live along shallow shorelines, often rocky or a mix of rock and sand, where they hold feeding and breeding territories. Because erythraeus has so often been merged with Amphilophus labiatus, a habitat account for it alone is hard to separate from that of the wider complex, but the setting is the same: warm water (roughly 77–86 °F), moderately alkaline and hard, over the shallow lake-margin substrates the Midas group depends on. The Nicaraguan great lakes drain ultimately to the Caribbean via the Río San Juan, and the whole system falls within the San Juan freshwater ecoregion (FEOW 205).
Feeding
As a member of the Midas/red-devil complex, Amphilophus erythraeus is a robust omnivore-to-carnivore of the lake margins. Midas cichlids are known for trophic versatility: their famously adaptable pharyngeal jaws let the same lineage crush hard-shelled prey such as snails and also work softer foods, while red-devil-type fish in particular take invertebrates, smaller fishes and other animal matter alongside plant and detrital material. The thick lips and strong jaws of the labiatus-like members are associated with picking prey from rock crevices. No diet study is published for erythraeus specifically, so its feeding is best read as that of a large, generalist Midas-group cichlid working the lake bottom and rocky margins — a higher-trophic-level feeder than the small sand-sifting cichlids that share its waters.
Mating
Like all Midas-complex cichlids, Amphilophus erythraeus is a biparental, pair-bonding substrate spawner. A male and female form a pair, establish and defend a territory, and prepare a nest site together. Courtship runs through the genus-typical repertoire — lateral displays, fin-spreading, quivering, lip-locking trials of strength and cooperative substrate-cleaning — with the pair intensifying in breeding colour; in the gold and red morphs, the vivid red that names the species can be especially striking in breeding condition. Mate choice in the Nicaraguan Midas flock has been a major research focus, because colour morph and body form influence pairing and are part of how members of the complex stay distinct, and the same assortative tendencies apply here. Territorial aggression is pronounced, particularly around the nest, in keeping with the red devil's reputation.
Breeding
Amphilophus erythraeus is a biparental substrate spawner, like the rest of the Midas group. It lays adhesive eggs on a cleaned hard surface — a rock face, a cleared patch of substrate, or the wall of a sheltered cavity — and both parents guard them. In the usual division of labour the female fans and tends the eggs while the male defends the perimeter; after hatching, the wrigglers are moved and the free-swimming fry are herded and protected as a school by both adults. Clutch sizes for erythraeus are not separately published, but large Midas/red-devil cichlids commonly produce several hundred to over a thousand eggs per spawn, with extended, vigorous parental care. As with other lake cichlids of the Nicaraguan lowlands, breeding is seasonal, intensifying under favourable conditions, and a pair in good condition will spawn repeatedly. Because the fish's identity is bound up with the red devil, the detailed breeding accounts for Amphilophus labiatus are the best practical guide to what erythraeus does.
In the aquarium
Amphilophus erythraeus is not traded under its own name in any real way; what reaches the hobby as 'red devil' is referred to Amphilophus labiatus and the closely allied Amphilophus citrinellus, and since erythraeus has long been merged with Amphilophus labiatus, the practical husbandry is effectively that of the red devil — a large, intelligent, intensely territorial Central American cichlid with demanding needs.
Tank: a single adult needs a large tank on the order of 65–105 US gal or more, with a long footprint; a pair or any attempt at a community needs considerably more. Give it robust decor (large rocks and wood) secured against a powerful digger, plus open swimming space.
Water: match the warm, hard, alkaline Nicaraguan lake chemistry — around 77–86 °F and pH roughly 7.0–8.5 — with strong filtration and generous water changes for a big, messy fish.
Feeding is easy: a varied omnivore's diet of quality pellets, frozen and live foods and some vegetable matter.
Tankmates and common mistakes: temperament is the defining challenge. Red-devil-type cichlids are notoriously aggressive, and many are best kept singly or as an established pair, with tankmates chosen only with caution and plenty of space. The usual keeper mistakes are under-sizing the tank, under-filtering, and underestimating a fish that will dominate or kill unsuitable companions.
Conservation
Amphilophus erythraeus has not been assessed under its own name by the IUCN Red List, a direct consequence of its disputed taxonomy: for assessment it has effectively been carried within Amphilophus labiatus, which the IUCN does list, and the wider Nicaraguan Midas-cichlid complex of the great lakes is not at present considered globally threatened. The real conservation context is that of the Nicaraguan great lakes themselves — Lakes Managua and Nicaragua — which face well-documented pressures: pollution and nutrient loading from agriculture and from domestic and urban wastewater (Lake Managua/Xolotlán is historically among the more polluted of the two), introduced non-native species, and the general stresses of large, heavily used lowland lakes. None of these is documented as a population-level threat to this particular fish, but the unresolved status of erythraeus means its conservation needs cannot be judged in isolation until the taxonomy is settled. It carries no CITES listing.