Taxonomy & naming
The arroyo cichlid was described by the German-British ichthyologist Albert Günther in 1862 as Theraps irregularis, the species on which he founded the genus Theraps. The epithet irregularis — Latin for 'irregular' — refers to the rather irregular, broken transverse blackish bands on the sides. For most of the twentieth century the broad assemblage of elongate Mexican river cichlids was lumped into Cichlasoma, and Theraps was treated as a subgenus or a loose grouping that, at its largest, contained a dozen or more species. By the 1990s the working concept of the genus had narrowed to three rheophilic species sharing the irregularis type: T. irregularis itself, T. lentiginosus, and T. coeruleus — the last described by Stawikowski and Werner in 1987 from the Río Mizol Há, a tributary of the Río Tulija (in turn feeding the Río Grijalva) about 19 mi south of Palenque, Mexico, on characteristics the authors used to define Theraps as distinct from other cichlasomine genera. The molecular revision by McMahan, Matamoros, Piller & Chakrabarty (2015, Zootaxa 3999) pared Theraps down further, and the comprehensive phylogeny of Říčan, Piálek, Dragová & Novák (2016) then showed that even the surviving handful were not all close relatives of the type species: godmanni and intermedium were moved to the resurrected genus Chuco, nourissati to the new Wajpamheros, and the coeruleus–lentiginosus pair — the two species that had shared the genus with irregularis for decades — to Rheoheros. What remains in Theraps is its type species alone — irregularis — making the genus, today, effectively monotypic in both the Catalog of Fishes and FishBase. The species is the nomenclatural anchor of the larger 'Theraps–Paraneetroplus' lineage of slim, flowing-water Atlantic-slope cichlids.
Morphology
Theraps irregularis is a medium-sized, strikingly elongate and laterally compressed cichlid — the streamlined, almost torpedo-like body of a dedicated current-dweller, reaching about 7.5 in standard length per FishBase. The ground colour is silvery to pale olive, crossed by a series of irregular, broken dark vertical bars — the markings the name records — that vary with mood and breeding state, often over a faint bluish-green iridescence. The head is pointed and the mouth carries the comb-like teeth of an algae-scraper. The slim caudal peduncle and low-slung body are classic adaptations for holding station and feeding in fast water. Sexual dimorphism is modest: males grow somewhat larger and develop slightly more produced fin tips and a steeper forehead, while ripe females are deeper-bellied and drop a visible ovipositor when spawning. As with most narrow-range Mexican river cichlids, much of the detailed morphological record rests on preserved museum material rather than systematic study of living fish.
Habitat
The species occurs on the Atlantic (Gulf of Mexico) slope of Mexico and Guatemala, in the Usumacinta, Sarstún and Río Dulce drainages, reaching into Belize. It is a fish of fast-moving, rocky upper-river valleys — clear, well-oxygenated montane and foothill streams over rock and gravel — rather than sluggish lowland water. FishBase classes it as freshwater and benthopelagic and gives a preferred temperature band of roughly 79–86 °F, with pH from about 7.0 upward and hardness of 10–20 dH, consistent with the warm, mineral-rich clear streams it favours. The Usumacinta is one of the most fish-rich and most heavily endemic drainages of Middle America and is recognised as its own Freshwater Ecoregion of the World. Field surveys of these drainages record it sharing the water, depending on locality, with a substantial community of other Middle American cichlids and fishes — Chuco intermedius, Herichthys pearsi, Nandopsis friedrichsthalii, N. salvini, Theraps lentiginosus, Thorichthys helleri, T. meeki, T. pasionis, Vieja argentea, and V. heterospilus, alongside Astyanax mexicanus, several Poecilia livebearers, Xiphophorus helleri, and the catfish Rhamdia guatemalensis — a snapshot of the crowded, diverse fish fauna typical of Usumacinta-system streams. Quantitative in-situ chemistry and a published depth range specific to this species are sparse; the figures here lean on FishBase's environmental envelope and the rheophilic character of the fish rather than a dedicated field-limnology study.
Feeding
Theraps irregularis is a grazer relatively low on the food web. FishBase gives it a trophic level near 2.6 and records its diet as aufwuchs — the carpet of algae, biofilm and associated micro-invertebrates that coats submerged rock in flowing water. The elongate, current-adapted body and the comb-like teeth suit a fish that works hard surfaces in the riffles and runs, scraping and combing the algal turf and taking the small invertebrates that come with it. This places the arroyo cichlid squarely among the rock-scraping grazers of fast Central American rivers. Because so much of its energy comes from bulky, fibre-rich grazed material, it has the long gut and the patient, near-continuous feeding style of a herbivore rather than the lunge-and-gulp habit of a predator — a point that matters in captivity, where a meaty, protein-heavy diet does not suit it.
Mating
As with most heroine cichlids, reproduction in Theraps irregularis is organised around a monogamous pair that bonds and then jointly holds and defends a territory. Detailed field observations of courtship in this species have not been published, so the account is built from the genus's biology and from 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 flowing-water cichlid of this build a defensible hard surface in or beside the current — a rock face, a crevice, an undercut — typically anchors the territory, and intraspecific aggression rises as the territory is claimed and held. The reliable route to a compatible pair in captivity is to raise a cohort together and let a bond develop rather than to force two adults into proximity.
Breeding
Theraps irregularis 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 slab of wood, the wall of a crevice — 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 for the species; cichlids of this body size typically lay on the order of one to several hundred eggs, and a productive pair can be expected to fall in that range. 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. The species is occasionally bred by specialist keepers of riverine Central American cichlids, but systematic spawning accounts remain scarce.
In the aquarium
Theraps irregularis is a fish for the experienced keeper of riverine cichlids rather than a beginner — not because it is delicate, but because of its preference for warm, fast, very clean water and the management that an active, territorial cichlid demands. A current-loving fish of this size is best in a tank of at least 75 US gallons (roughly 75 US gal) with strong, well-oxygenated flow; a longer footprint suits its swimming style and makes spreading aggression easier. Temperature should sit in the warm part of the range it prefers (about 79–86 °F) with vigorous circulation rather than a still tank.
Water should be near-neutral to mildly alkaline (about pH 7.0–8.0) and moderately hard (the fish favours mineral-rich water, 10–20 dH); like all active cichlids it is intolerant of accumulated dissolved waste and dissolved-oxygen sag, so generous filtration, strong flow and large 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; it will rearrange light décor and uproot rooted plants, so robust or potted plants and secure rockwork are the sensible furnishings. Because the fish is naturally an aufwuchs grazer, its diet should be built around quality vegetable- and spirulina-based prepared foods supplemented sparingly with frozen or live items; an over-rich, meaty diet is a poor match for its biology and is implicated in the bloat that plagues over-fed Central American grazers.
Tankmates should be similarly sized, robust riverine cichlids and large, fast dither fish that can hold their own in current; very small or slow fish will be outcompeted or, during spawning, harassed. The species is territorial and becomes markedly more aggressive in breeding condition, so the proven approach is to grow a group of six or more juveniles together to allow a natural pair to form and to spread aggression. One published hobbyist account (Cichlids Yearbook) describes six Theraps irregularis holding a single corner of a heavily stocked 500-litre community tank without incident, alongside a group of Central and South American cichlids and a territorial tetra — a real-world data point suggesting the species can behave as a peaceable, if firmly territory-holding, community fish once it has the room to claim its own ground. Given space, warm clean flowing water, a vegetable-leaning diet and time, it is a handsome and energetic aquarium fish, though it remains uncommon outside specialist circles.
Conservation
The IUCN Red List assesses Theraps irregularis as Least Concern, in an assessment dated 1 March 2018. The Least Concern listing reflects a reasonably widespread species, present across the Usumacinta, Sarstún and Río Dulce drainages of Mexico, Guatemala and Belize, with no documented basin-wide threat severe enough to qualify it for a threatened category. It carries no CITES listing. As a flowing-water specialist of clear montane streams, however, it is exposed to anything that alters flow or warms and silts its habitat — damming, deforestation in the catchment, water extraction, pollution and introduced species are the familiar pressures on the region's rivers. None is documented as a present population-level threat to this particular cichlid, but as the type species of its genus and a characteristic component of the Usumacinta rheophilic cichlid fauna, its fortunes track the health of the fast, clean upland reaches it depends on.