Taxonomy & naming
The northern checkmark cichlid was described by the German-British ichthyologist Albert Günther in 1862. The epithet intermedium ('intermediate') reflects Günther's sense that the fish sat between other species he was describing — in his framing, intermediate between forms then placed in Heros. For most of the twentieth century it was lodged in the wastebasket genus Cichlasoma and later placed in Theraps as that genus was broadly circumscribed. The molecular revision by McMahan, Matamoros, Piller & Chakrabarty (2015) kept it briefly in Theraps, but the comprehensive phylogeny of Říčan, Piálek, Dragová & Novák (2016) showed that intermedium and its sister godmanni are not close relatives of the Theraps type species, Theraps irregularis, and resurrected the genus Chuco — Günther's own subgeneric name, from a Guatemalan vernacular word for cichlids — to hold the pair. Chuco today comprises intermedium, godmanni and the closely allied 'macracanthus' form, all elongate 'lotic' (flowing-water) cichlids of the Atlantic slope; intermedium is the northern, more widely distributed member of the checkmark pair.
Morphology
Chuco intermedium is a medium-sized, elongate and laterally compressed cichlid of the streamlined riverine type, reaching about 8 in standard length per FishBase — somewhat smaller than its southern sister Chuco godmanni. The body is silvery to olive with a greenish iridescent cast, and the diagnostic feature is the dark 'checkmark' on the rear of the body: a roughly check- or hook-shaped figure formed by the mid-lateral and caudal-peduncle markings, set off by broken bars or blotches along the flank that strengthen with mood and breeding state. The head is fairly pointed and the mouth carries the comb-like teeth of a grazer. Sexual dimorphism follows the heroine pattern: males grow larger, develop a steeper forehead and, in old dominant fish, a modest nuchal hump, while ripe females are deeper-bellied and drop a visible ovipositor when spawning. As with most Central American river cichlids the detailed morphological record rests largely on preserved museum material.
Habitat
The species occurs on the Atlantic (Gulf of Mexico) slope of Mexico, Guatemala and Belize, in the Grijalva and Usumacinta river drainages — the great fish-rich system spanning the Chiapas highlands and the Petén lowlands. It is a fish of moderate-to-fast flowing water, often found in groups near rock formations in clear, well-oxygenated reaches over rock and gravel. FishBase classes it as freshwater and benthopelagic and gives a preferred temperature band of roughly 79–86 °F with pH around 7.5, the warm, alkaline envelope of mineral-rich rivers. The Grijalva–Usumacinta is recognised as its own Freshwater Ecoregion of the World, noted for an exceptionally rich and partly endemic cichlid fauna. 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
FishBase places Chuco intermedium at a trophic level near 3.3, reflecting an omnivorous habit inferred from its size and closest relatives. The genus works hard surfaces and the substrate in flowing water with its comb-like teeth, scraping algae and aufwuchs and combing the bottom for the small invertebrates that come with it, supplementing with fresh vegetation. The mix of plant, algal, detrital and small-animal food places the northern checkmark cichlid among the bottom-working omnivores of the Grijalva–Usumacinta rivers rather than among dedicated piscivores. As with its relatives, a large share of its wild intake is bulky, low-energy grazed material, which shapes both its gut and its patient, near-continuous feeding style; observers note it often forages in loose groups around rock.
Mating
As with most heroine cichlids, reproduction in Chuco intermedium 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 typically anchors the pair's territory, and intraspecific aggression rises as the territory is claimed and held. The reliable route to a compatible pair in captivity is to grow a cohort together and let a bond form rather than to force two adults into proximity.
Breeding
Chuco intermedium 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; a cichlid of this body size can lay several hundred eggs in a productive spawn. 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. It is occasionally bred by specialist keepers of Central American riverine cichlids, but systematic spawning accounts remain scarce.
In the aquarium
Chuco intermedium is a fish for the experienced keeper of riverine cichlids rather than a beginner — current-loving, active and territorial, it needs space and well-oxygenated flow. A pair is best in a tank of at least 75 US gallons (roughly 75 US gal) with strong circulation and a long footprint that suits its swimming and helps spread aggression. Temperature should sit in the warm part of its range (about 79–86 °F) with vigorous flow rather than a still tank.
Water should be near-neutral to mildly alkaline (about pH 7.5–8.0) and moderately hard, matching the mineral-rich Grijalva–Usumacinta rivers; like all active cichlids it is intolerant of accumulated dissolved waste and 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 that create broken sightlines and defensible territories are the sensible furnishings. Because the fish is naturally a bottom-working omnivore that grazes a great deal of aufwuchs, its diet should be built around quality vegetable- and spirulina-based prepared foods supplemented with frozen or live items in moderation; an over-rich, exclusively meaty diet is a poor match for its biology.
Tankmates should be robust, similarly sized riverine cichlids and large, fast dither fish that can hold their own in current; 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. It is an attractive, characterful fish but remains uncommon outside specialist circles.
Conservation
The IUCN Red List assesses Chuco intermedium as Least Concern, in an assessment dated 5 February 2019. The Least Concern listing reflects a reasonably widespread species across the Grijalva and Usumacinta 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, rocky reaches, however, it is exposed to anything that alters flow or warms and silts its habitat — the Grijalva is one of the most heavily dammed rivers in Mexico, and deforestation, water extraction, pollution and introduced species press on the wider system. None of these is documented as a present population-level threat to this particular cichlid, but its fortunes track the health of the fast, clean reaches it depends on.