Taxonomy & naming
The Belgian ichthyologist Pierre (Pieter) Gosse described Amphilophus lyonsi in 1966, placing it originally in Cichlasoma from material tied to a Belgian collecting expedition to lower Central America. Like most Middle American cichlids, it was later transferred out of that catch-all genus; the Catalog of Fishes and FishBase now list it as valid as Amphilophus lyonsi (Gosse, 1966). It is a heroine cichlid of subfamily Cichlinae, and it sits with the Costa Rican and Panamanian Amphilophus of the genus's southern wing rather than with the famous Midas and red-devil cichlids of Nicaragua. William A. Bussing reviewed, keyed and figured the species in his standard reference, 'Peces de las aguas continentales de Costa Rica' (1998). The epithet lyonsi honours a man named Lyons connected with the species' collection in the region — one of several lower-Central-American cichlids named for the naturalists and patrons behind mid-twentieth-century survey work in Costa Rica and Panama.
Morphology
Amphilophus lyonsi is a small cichlid, reaching about 6 in standard length (FishBase) — a compact species rather than one of the genus's giants. Its body is laterally compressed and moderately deep in the Amphilophus mould, and the mouth is fairly small, matching a diet of detritus and invertebrates. The colour is understated, as befits a forest-stream fish: a brownish, greyish or tan ground, often crossed by a dark mid-lateral band or a row of blotches and marked with the usual spot at the caudal-fin base, the pattern darkening under stress and in breeding. Dominant males can carry a slightly heavier head profile, but the dramatic nuchal hump of the big Midas relatives never develops on a fish this small. Bussing's Costa Rican treatment documents the detailed diagnostic characters and keys the species against its regional congeners. Sexual dimorphism follows the genus norm — larger males, rounder ripe females — though it stays modest at this size.
Habitat
Lyons' cichlid lives in the Pacific-slope streams and rivers of southern Costa Rica and adjacent western Panama. FishBase records it from the Pacific drainage of Costa Rica — the Coto, or Río Coto Colorado, region — and from the Dupí drainage of Panama, in the warm, lowland-to-foothill forest rivers of the far south of Central America, close to where the region's cichlid fauna gives way at the Panamanian land bridge. This is clear, flowing, warm water, with FishBase giving a temperature range of roughly 79–84 °F. The setting is the small, often forested Pacific streams of the Costa Rica–Panama border country, a high-rainfall area of rich but localised freshwater fish communities. That the habitat is fragmented — scattered across a number of separate small drainages rather than gathered into one large basin — bears directly on the species' threatened status.
Feeding
Amphilophus lyonsi is a low-trophic-level omnivore-detritivore. FishBase records a diet of seeds, aquatic insects and bottom detritus and places the species low on the food web, at a trophic level of about 2.4 — well below the predatory big Amphilophus, and in keeping with a small fish that works the stream bed for soft, mixed food. It grazes and picks over the substrate for insect larvae and other small invertebrates, and takes the fallen seeds and the plant and detrital matter that collect on forest-stream bottoms. The small mouth and modest size suit this bottom-oriented, generalist feeding rather than active predation, and in the stream community the fish acts as a small benthic invertivore-detritivore, not a top predator.
Mating
Like other Amphilophus, Lyons' cichlid is a biparental, pair-bonding substrate spawner. A male and female pair off and together stake out and defend a territory around a chosen nest site on the stream bed. Courtship runs through the genus-typical sequence — lateral displays, fin-spreading, quivering, and cooperative cleaning of the spawning surface — with both fish deepening their colour as they come into breeding condition. The bold, assertive temperament aquarists note despite the fish's small size is most obvious here, at the nest, where the pair defends its patch vigorously. Direct field observations of mating in this little-studied species are scarce, but its behaviour can be read confidently from the strongly conserved biparental pattern of the genus.
Breeding
Amphilophus lyonsi breeds as an open or semi-open biparental substrate spawner in the genus pattern. The adhesive eggs go down on a cleaned hard surface — a stone, a root, or a cleared depression in the substrate — and both parents tend them, the female fanning and guarding the clutch while the male holds the territory. Once the eggs hatch, the wrigglers are moved to a pit, and after they become free-swimming the fry are herded and guarded as a school by both adults. Specific clutch counts are not well documented for the species; for a small Amphilophus of this size, broods are likely on the order of a hundred to a few hundred eggs, with prolonged biparental care through the egg, larval and fry stages. Spawning is expected to track the favourable season, when stream conditions suit raising fry, and in the aquarium the fish is reported to breed willingly once a compatible pair has settled.
In the aquarium
Uncommon but well liked among specialists in lower-Central-American cichlids, Lyons' cichlid earns its keep with a manageable size and a notably bold personality. As one of the smaller Amphilophus, it is far less demanding than the big red-devil relatives — but it is still a territorial substrate-spawner, and it behaves like one.
Tank: A pair does well in a moderately sized aquarium, on the order of 40 US gal or more, with sand or fine gravel plus rocks and wood for cover, territory markers and spawning sites. Leave some open swimming room and keep the flow gentle to moderate, in keeping with its stream origins.
Water: Warm and on the softer-to-moderate, near-neutral side its forest-stream habitat suggests — around 79–84 °F and pH roughly 6.5–7.5 — with good filtration and regular water changes.
Feeding: Straightforward for a small omnivore-detritivore. Quality sinking pellets and small frozen and live foods, with some vegetable matter to match its mixed, detritus-leaning natural diet, taken mostly from the bottom.
Tankmates: Despite the size, expect a feisty fish, especially when breeding, so choose tankmates of similar temperament and give a pair the room to hold a territory.
Given its restricted range and Endangered status in the wild, captive-bred stock is the responsible source, and the species makes a rewarding subject for anyone interested in keeping and propagating a threatened cichlid.
Conservation
The IUCN Red List assesses Amphilophus lyonsi as Endangered (assessed 7 November 2019). The driver is its small, fragmented range: the fish is confined to a limited set of Pacific-slope streams along the southern Costa Rica–Panama border, where freshwater habitat is naturally patchy and increasingly squeezed. The documented threats to lower-Central-American Pacific streams apply directly — deforestation and land-use change in the catchments, pollution from agriculture (including the region's banana and oil-palm plantations) and domestic wastewater, water abstraction and altered stream flow, and the loss of the clear forest-stream conditions the fish depends on. A species restricted to a handful of separate drainages has little buffer against losing any one of them, which is what puts it in the Endangered category despite Costa Rica's relatively strong conservation framework. It carries no CITES listing, and keeping its forested stream catchments intact is the key to its persistence.