Taxonomy & naming
Amphilophus flaveolus was described in 2008 by Jay R. Stauffer Jr., Jeffrey K. McCrary and Kara E. Black, in 'Three new species of cichlid fishes (Teleostei: Cichlidae) from Lake Apoyo, Nicaragua' (Proceedings of the Biological Society of Washington, volume 121) — the same paper that named Amphilophus chancho and Amphilophus astorquii. The holotype (PSU 4515.1) came from Otro Lado on the shore of Lake Apoyo. The species is one of the endemic Amphilophus of Apoyo, all of which descend from a single Midas-cichlid colonisation of the crater lake and belong to the Amphilophus citrinellus species complex within the heroine cichlids of subfamily Cichlinae. The Catalog of Fishes lists it as valid, and FishBase and the IUCN recognise it under the same name. The epithet flaveolus is Latin for 'yellowish' — the describers chose it for the yellow-toned body, the feature that most obviously sets this fish apart from its grey-green congeners in the lake.
Morphology
Amphilophus flaveolus is the smallest of the formally described Apoyo endemics, reaching about 5.5 in standard length (FishBase). It is a laterally compressed cichlid in the Midas mould, but without the bulk of Amphilophus chancho or the open-water elongation of Amphilophus zaliosus. Colour is its signature: a yellowish to pale gold body, as the name flaveolus advertises, often carrying the faint dark mid-lateral markings typical of the complex, which deepen under stress and in breeding. The describers separated the three 2008 species using body proportions, gill-raker counts and pharyngeal-jaw form rather than colour alone, but the yellow cast is the field mark a diver or aquarist notices first. As a member of the citrinellus complex it can in principle show the genus's gold (amelanistic) morph, though it is named for its more uniform yellow-toned normal colouration. Sexual dimorphism runs in the usual Amphilophus direction — males a little larger and heavier-headed, ripe females rounder — but stays modest in a fish this size.
Habitat
Like the rest of the flock, Amphilophus flaveolus is endemic to Laguna de Apoyo, the roughly 21-square-kilometre crater lake south-east of Granada, Nicaragua, and lives nowhere else on Earth. Apoyo is a deep (>574 ft), warm, clear, hard and alkaline maar lake, cut off from the surrounding drainages and colonised relatively recently by a single Midas-cichlid lineage that then diversified in place. Within the lake, Amphilophus flaveolus keeps to a different microhabitat from its rock-nesting relative Amphilophus chancho: the describers recorded it nesting over sandy or muddy substrate in shallow water, generally less than 7 ft deep. That shallow soft-bottom niche is part of the ecological partitioning that let the Apoyo endemics diverge and coexist. The water is warm year-round (roughly 26-30 degrees C) and alkaline (Apoyo runs around pH 7.5-9), reflecting the lake's volcanic chemistry, and the whole range sits within the San Juan freshwater ecoregion (FEOW 205).
Feeding
Amphilophus flaveolus is a benthic omnivore of the shallow, soft-bottomed lake margins. FishBase places it at trophic level about 3.2 — an estimate drawn from body size and relatives rather than a dedicated diet study — consistent with feeding on small benthic invertebrates alongside plant and detrital material sifted from the sand and mud where it lives and breeds. Feeding divergence is part of what drove the Apoyo radiation: while some endemics, notably the slender Amphilophus zaliosus, shifted toward a more limnetic, open-water mode, deeper-bodied shallow-bottom species like Amphilophus flaveolus work the benthos. Its smaller size and shallow soft-substrate habit fit a fish foraging over sand and mud rather than excavating rock or chasing prey in open water. As across the Midas complex, the pharyngeal jaws are flexible, and the flock spans a range of jaw forms tuned to different food hardnesses.
Mating
Amphilophus flaveolus is a biparental, pair-bonding substrate spawner, and, as with every Apoyo endemic, assortative mating sits at the heart of its biology. The describers stressed that the lake's species pair preferentially with their own kind, and it is this mate choice that keeps the flock reproductively isolated despite sharing one small lake. A bonded male and female form a pair, choose and clean a nest site on the shallow sandy or muddy bottom, and defend the surrounding territory together. Courtship is the standard Amphilophus repertoire — lateral displays, fin-spreading, quivering and cooperative substrate-cleaning — with the breeding fish deepening in colour. Because Amphilophus flaveolus nests on open soft substrate rather than in a rock hole, its spawning territory centres on a cleared patch of sand or mud rather than a defended cavity.
Breeding
Amphilophus flaveolus is an open-substrate biparental spawner, and its choice of nesting habitat is one of the clearest behavioural differences from its lake-mates. The describers recorded it nesting in sandy or muddy substrates in water less than 7 ft deep — in contrast to the rock-hole nesting of Amphilophus chancho in the same lake. The adhesive eggs are laid on the cleaned substrate and tended by both parents in the genus-typical division of labour: the female fans and guards the clutch while the male patrols the territory, and once the fry are free-swimming both adults herd and defend them as a school. Clutch counts for wild Amphilophus flaveolus are not published; for a Midas-complex cichlid of this modest size, broods are likely on the order of a few hundred eggs, with prolonged biparental care of eggs, larvae and fry. Spawning is tied to favourable conditions, and a settled pair will breed repeatedly.
In the aquarium
Amphilophus flaveolus is of scientific rather than commercial interest and is essentially absent from the ornamental trade; any husbandry guidance is inferred from the broader Midas group and from the lake's measured conditions, not from established aquarium experience. It is a fairly small Central American cichlid by the standards of its genus, so its demands are more moderate than the big red-devil relatives, but it is still a territorial substrate-spawner that needs space.
Tank: give a pair a generous footprint of open sand. Because the fish naturally nests on a shallow soft bottom, fine sand rather than gravel suits both its feeding and its breeding; add some rocks or wood as territory markers and cover while leaving the open sandy areas it favours.
Water: match Apoyo's warm, hard, alkaline chemistry — temperature around 26-30 degrees C and pH roughly 7.5-8.5 — with good filtration and regular water changes.
Feeding: straightforward for an omnivore, with sinking pellets supplemented by frozen and live invertebrates and some vegetable matter.
Tankmates: expect heightened territoriality around spawning, so choose robust tankmates of comparable temperament and give a breeding pair room to hold its ground. In practice the species' rarity and Critically Endangered status make it a specialist's or conservation-oriented subject rather than a general aquarium fish.
Conservation
Amphilophus flaveolus is assessed by the IUCN Red List as Critically Endangered (assessed 16 July 2019). As with the other Apoyo endemics, the driver is its extreme range restriction: the entire global population sits in a single crater lake of about 21 square kilometres, with no refuge population anywhere else, so any lake-wide threat falls on the whole species at once. The documented pressures on Apoyo's cichlids include introduced non-native fishes — notably tilapia, which compete with and prey on the natives — along with nutrient pollution and habitat degradation from shoreline development and tourism, and water abstraction. The shallow, near-shore sandy and muddy habitat that Amphilophus flaveolus needs for both feeding and breeding is exactly the zone most exposed to shoreline disturbance and nutrient loading, which sharpens its vulnerability. The species carries no CITES listing. Protecting the Apoyo flock — several of whose members share the Critically Endangered status — comes down to safeguarding the water quality and native fish community of this one lake.