Taxonomy & naming
The species was described by the German cichlid specialists Rainer Stawikowski and Uwe Werner in 1987 as Theraps coeruleus, in a paper framed as a study of the species 'around Theraps lentiginosus.' The epithet coeruleus is Latin for 'dark blue, sky-blue,' for the bluish cast of the living fish. As with the rest of the elongate Atlantic-slope river cichlids it was placed in Theraps until the molecular revisions of McMahan, Matamoros, Piller & Chakrabarty (2015) and Říčan, Piálek, Dragová & Novák (2016), which recovered coeruleus and lentiginosus as a distinct lineage and assigned them to the new genus Rheoheros ('stream Heros'). Rheoheros today contains just these two species; coeruleus is the smaller, more narrowly distributed of the pair, and is its type locality's signature cichlid. The species' restricted range and recent description make it one of the less-studied members of the herichthyine radiation.
Morphology
Rheoheros coeruleus is the smallest member of its genus, an elongate and laterally compressed cichlid reaching only about 4.5 in standard length per FishBase — modest for a Central American river cichlid. The body is silvery to olive with the bluish iridescent sheen the name records, marked with a variable pattern of faint broken bars and scattered dark spots that strengthen with mood and breeding condition. The head is fairly pointed and the mouth carries the comb-like teeth of an aufwuchs grazer. Sexual dimorphism follows the heroine pattern: males grow somewhat larger and develop a steeper forehead and slightly more produced fin tips, while ripe females are deeper-bellied and drop a visible ovipositor when spawning. As a recently described, narrow-range species, its detailed morphological record rests largely on the original description and a small body of subsequent material.
Habitat
The species is restricted to the upper Río Tulijá, part of the Usumacinta drainage in Chiapas, southern Mexico — a montane, current-loving fish of cool, fast water. FishBase classes it as freshwater and benthopelagic and gives a preferred temperature band of roughly 73–77 °F with pH around 7.0–8.0, on the cooler side of the tropical range and consistent with a clear, well-oxygenated highland stream of high mineral content. It is described as a current-loving fish preferring montane habitats. The Usumacinta is recognised as its own Freshwater Ecoregion of the World, noted for a rich and partly endemic cichlid fauna, and coeruleus is among the genuinely narrow-range endemics of the system. Quantitative in-situ chemistry beyond the FishBase envelope and a published depth range are sparse, as is typical for a recently described, restricted-range species; the figures here lean on FishBase and the rheophilic character of the fish rather than a dedicated field-limnology study.
Feeding
Rheoheros coeruleus is a grazer low on the food web. FishBase gives it a trophic level near 2.8 and records its diet as aufwuchs and small encrusted invertebrates, primarily tiny crustaceans — the algal-and-biofilm turf that coats submerged rock in flowing water, together with the small animals living in it. The comb-like teeth and elongate, current-adapted body suit a fish that works hard surfaces in the riffles, scraping and combing the algal mat and taking the small invertebrates that come with it. This places the fish among the rock-scraping grazers of fast Central American rivers. Its small size and current-dwelling habit make it a specialist of the rocky, fast-water microhabitat of its single highland river.
Mating
As with most heroine cichlids, reproduction in Rheoheros coeruleus is organised around a monogamous pair that bonds and then jointly holds and defends a territory; Stawikowski and Werner's work on the lentiginosus group gave particular attention to feeding adaptations, social behaviour and reproduction in these fish. Detailed field courtship descriptions specific to coeruleus are limited, so the account leans on the genus's biology and aquarium experience. Pairs form within a group as fish mature; courtship runs through the familiar cichlid sequence of lateral displays, fin-spreading, jaw-locking and substrate cleaning, the female deepening in colour and dropping her ovipositor as she ripens. In a small, flowing-water cichlid of this build a defensible crevice or rock face in or beside the current typically anchors the pair's territory, and intraspecific aggression rises as the territory is claimed and held.
Breeding
Rheoheros coeruleus is a biparental substrate (open) spawner, the standard mode for the elongate Middle American river cichlids. The pair selects and meticulously cleans a hard surface — a flat rock, a crevice wall, a slab of wood — and the female lays an adhesive sheet of eggs over it while the male follows to fertilise. A precise wild clutch count has not been published; a small cichlid of this size typically lays a smaller clutch than its larger relatives, on the order of one to a few hundred eggs. Both parents share the brood: the female fans and tends the eggs while the male patrols and defends the perimeter, and after the eggs hatch the wrigglers are moved to pre-dug pits before the free-swimming fry are shepherded as a tight school by both adults. The young feed initially on fine prepared foods and the aufwuchs the adults graze. Systematic captive-breeding accounts are scarce for this narrow-range species, though its small adult size makes it more manageable than most of its genus-mates.
In the aquarium
Rheoheros coeruleus is the most aquarium-tractable member of its genus by virtue of its small size, but it remains a specialist's fish: a current-loving cichlid of cool, fast, mineral-rich water that needs the right conditions rather than just a large tank. A pair can be housed in a tank of around 40–55 US gallons (roughly 40–55 US gal) — modest by Central American cichlid standards — provided there is strong, well-oxygenated flow and plenty of rockwork. Temperature should sit toward the lower tropical range (about 73–79 °F) with vigorous circulation, reflecting its preference for cool highland streams.
Water should be near-neutral to alkaline (about pH 7.0–8.0) and moderately hard to hard — this is a fish of high-mineral mountain water, so very soft, acidic conditions do not suit it; like all current-dwelling fish it is intolerant of accumulated dissolved waste and oxygen sag, so good filtration, strong flow and regular water changes are essential. A substrate of sand or fine gravel with abundant smooth water-worn rock suits its bottom-grazing, rock-scraping habits and provides the crevices it uses as cover and spawning sites; robust or potted plants and secure rockwork that create broken sightlines are the sensible furnishings. Because the fish grazes aufwuchs and small invertebrates, its diet should be built around quality vegetable- and spirulina-based prepared foods supplemented with small frozen or live items such as cyclops and finely chopped bloodworm.
Tankmates should be robust but not overly large current-tolerant species; its modest size means it can be bullied by the bigger Central American cichlids, so company should be chosen with that in mind. The species is territorial and becomes more aggressive in breeding condition, so a group grown together in a well-structured tank works best for forming a compatible pair. It is rarely seen in the trade and is of interest mainly to keepers of rheophilic, dwarf-to-medium Central American cichlids.
Conservation
The IUCN Red List assesses Rheoheros coeruleus as Data Deficient, in an assessment dated 17 December 2018. The Data Deficient listing reflects exactly what its biology suggests: a recently described, narrow-range fish confined to the upper Río Tulijá for which there is not enough information on population size, trend or specific threats to place it confidently in a threatened or non-threatened category. It carries no CITES listing. That single-river endemism is itself the heart of the conservation concern — a species confined to one highland stream has nowhere else to go, and as a rheophilic specialist of cool, clear, high-mineral water it is especially exposed to anything that alters flow or warms and silts its habitat: deforestation in the catchment, water extraction, pollution and introduced species are the familiar pressures on the wider Usumacinta system. For a Data Deficient single-river endemic the appropriate posture is caution and a call for the basic distribution and population surveys the Red List flags as missing.