Taxonomy & naming
The tailbar cichlid was described by Edward C. Taylor and Robert Rush Miller in 1980 as Cichlasoma hartwegi, from material in the upper Grijalva (Río Grande de Chiapa) basin of Chiapas, Mexico. The epithet honours Dr Norman Edouard 'Kibe' Hartweg (1904–1964), an American herpetologist whose speciality was the distribution and taxonomy of turtles. As with the other Middle American 'Cichlasoma', the species was later moved through Paraneetroplus and into Vieja, where the Catalog of Fishes lists it as valid (Vieja hartwegi). Because the original description rested on 45 juvenile and sub-adult specimens, the species' full range of adult form was poorly understood for decades; Gómez-González and colleagues redescribed it in 2018 (Zootaxa), documenting its extensive variation and formally recognising a distinct rheophilic (fast-water) morph alongside the typical lake-and-slow-water form. It sits in the Neotropical subfamily Cichlinae among the heroine cichlids, and within Vieja it is the genus's specialist of upland, flowing water.
Morphology
Vieja hartwegi reaches about 8 in in standard length according to FishBase, with captive males said to grow to roughly 10 in and females to about 7 in. It is a moderately deep-bodied, compressed cichlid, but its form is unusually plastic: the redescription distinguishes a tall-bodied morph typical of lakes and calmer water from a slimmer, more streamlined rheophilic morph found in strong current. The diagnostic mark is the one in its common name — a conspicuous dark vertical bar crossing the caudal peduncle at the base of the tail. The ground colour ranges from silvery and olive in subadults to a striking pearly turquoise-green or blue-green in conditioned adults, often overlaid with darker mid-body blotching or barring and a scatter of iridescent spots; spawning fish intensify markedly. Sexual dimorphism follows the genus pattern: males grow larger and develop a steeper forehead and the beginnings of a nuchal hump, while ripe females are deeper-bodied; the species' wide individual and ecophenotypic variation makes single specimens hard to pin down without locality data.
Habitat
The tailbar cichlid is a narrow-range endemic of the upper Grijalva (Río Grande de Chiapa) basin on the Atlantic slope of Chiapas, Mexico, in the leeward uplands of the Sierra Madre del Sur at altitudes from roughly 870 to 1,1608 ft. Its IUCN assessment describes a fish of flowing streams in clear to murky water with moderate to strong currents — except in backwaters where juveniles congregate — over substrates of boulders, marl, sand, brush piles and some mud, with vegetation usually sparse and limited to Potamogeton and green algae. Depths run from about 2–3 ft in the streams (which are typically 10–20 ft wide) to as much as 10 ft in the main Río Grande de Chiapa, which is roughly 197 ft wide near Tuxtla Gutiérrez; adults were taken in the deeper river, juveniles in the shallows. Recorded in-situ chemistry is warm to cool and hard: water temperatures of 68–84 °F from January to April with a mean around 64 °F in the higher-altitude habitats, pH 7.0–8.0, well-oxygenated water (about 10 mg/l dissolved oxygen) and a high carbonate hardness (FishBase lists dH from 33 upward). The species has also colonised the reservoirs built across its range.
Feeding
Vieja hartwegi is a benthic omnivore. FishBase records adults feeding on benthic detritus, aquatic plants and invertebrates over the rocks, sand, silt and mud of its stream and lake habitats, and the IUCN account adds zoobenthos and phytobenthos to the menu of fish living in the reservoirs. Its trophic level is estimated at about 2.5 — squarely that of an omnivore with a plant-and-detritus base supplemented by small animal prey rather than a predator. In its flowing-water home it works the bottom and the surfaces of boulders and submerged brush for algae, plant material and the invertebrates among them, a generalist foraging role typical of the genus but pursued in faster, clearer water than most Vieja occupy.
Mating
Like its congeners the tailbar cichlid is a monogamous, biparental substrate spawner that becomes territorial around breeding. Detailed wild courtship observations are scarce, but the species follows the heroine-cichlid pattern: a pair forms, claims a territory centred on a hard spawning surface, and both partners defend it, with the usual lateral displays, jaw-locking and joint cleaning of the chosen site preceding the spawn. In captivity it is reported as a comparatively willing breeder once a compatible pair settles, though — as throughout the genus — pairing can be combative and a male may harass an unready female, so matched, mutually willing fish in adequate space are important. The intensification of body colour as fish ripen accompanies this pre-spawning phase.
Breeding
Vieja hartwegi is a biparental, open substrate spawner. The pair cleans a flat hard surface — a rock or similar — and the female deposits her eggs on it in the open, after which both parents guard and fan the clutch and, once the wrigglers hatch, move and shepherd the fry, defending them vigorously against intruders. Specific wild fecundity figures are not well documented, but the genus typically produces several hundred fry per spawning, and the IUCN assessment notes that in the absence of impacts the population would take less than 15 months to double, based on preliminary estimates of fertility and age at first maturity — indicating a productive, fast-maturing fish. In aquaria the species spawns in the standard biparental manner given a compatible pair, a flat spawning site and a digging substrate; the brood is grown on conventional small foods. As with the rest of Vieja, the practical challenge is managing the pair's spawning aggression rather than any difficulty in getting eggs.
In the aquarium
The tailbar cichlid is an attractive, moderately sized Vieja that is kept by Central American cichlid enthusiasts but is far less common in the trade than its showier relatives. At an adult size around 8–10 in it needs a large tank: a footprint on the order of 150 × 19.5 in (about 75 US gal) is a reasonable minimum for a single fish or a settled pair, with more space strongly advised given the species' restless, active temperament. Unusually for the genus, it comes from clear, cool-to-warm, well-oxygenated flowing water, and it appreciates conditions that reflect this: a temperature toward the middle of its 68–84 °F range, hard alkaline water (pH about 7.5–8.0, high carbonate hardness), brisk filtration and strong oxygenation, and — in contrast to the lowland Vieja — a degree of current it actively tolerates.
Use a substrate of sand or fine gravel and aquascape with rounded boulders, cobble and roots that recall its rocky stream bed and break up sightlines; the fish dig and rearrange, so décor must be stable. As with all Vieja, rooted plants are largely a lost cause, and any greenery is best regarded as food or restricted to tough species. Feeding should respect the genus's omnivorous, plant-leaning diet: green and spirulina-based foods and quality pellets as the base, with frozen or live invertebrates (bloodworm, brine shrimp) as supplements; the wild fish are detritus-and-algae grazers as much as invertebrate feeders.
Temperament is robust but manageable. Outside breeding the tailbar is feisty rather than exceptionally aggressive and can share a large tank with other sturdy Central American cichlids and comparably sized, current-tolerant tankmates; small or delicate fish are unsuitable, both as potential prey and as targets. Spawning sharply raises aggression, and a guarding pair will dominate the tank, so the usual safeguards — ample space, broken lines of sight and a contingency plan to separate fish — apply. Keepers most often go wrong by treating it like a placid lowland Vieja: it wants cleaner, more oxygenated, more current-rich water than its relatives, and stagnant, under-filtered conditions suit it poorly.
Conservation
The IUCN Red List assesses Vieja hartwegi as Endangered (criterion B2ab(iii); assessed 23 October 2017 by Schmitter-Soto, Snoeks & Lalèyè, published 2019), an uplisting from the Vulnerable status it held in 2009. The assessment cites a very small area of occupancy (about 50 mi²) across an estimated four locations, with a continuing decline in habitat quality. The dominant threat is damming: a series of large dams (including the Malpaso hydroelectric scheme) has converted long reaches of the species' flowing-water habitat into reservoirs unsuitable for it, severely fragmenting the surviving populations, drawing development and pollution to the area and aiding the spread of introduced tilapia (Oreochromis mossambicus and Tilapia rendalli), with which it competes. Widespread deforestation of the Grijalva headwaters compounds the problem through erosion, sedimentation and declining water quality, and the species is naturally uncommon and sensitive to environmental change. It is fished locally where large individuals occur, and SEMARNAT lists it as a species of special concern under Mexican law (NOM-059). Of the species treated here it is by some distance the most imperilled, and its fate is tied directly to how the upper Grijalva's rivers are managed.