Taxonomy & naming
Barton's cichlid was described by the American ichthyologist Tarleton Hoffman Bean in 1892 as Heros bartoni, on material from the springs of the Río Verde valley in San Luis Potosí, Mexico. The species then passed through the long nomenclatural churn that has dogged the Middle American cichlids, shuffled in turn through Cichlasoma, Herichthys and the catch-all Nandopsis before Robert Rush Miller and others settled it in Herichthys. In 2015 Mauricio De la Maza-Benignos and colleagues, in a phylogeographic analysis of Herichthys published in Hydrobiologia, split a clade of molar-toothed, snail-eating cichlids of the southern Pánuco — the 'labridens species group' — into the new genus Nosferatu, with bartoni among them; the genus is named for the dark vertical bar these fish raise in breeding colour, which the authors likened to the shadowy figure of F. W. Murnau's 1922 silent-film vampire. The split is not universally accepted: FishBase and Eschmeyer's Catalog of Fishes continue to file the fish as Herichthys bartoni, and a later phylogeographic reassessment argued the Nosferatu species are better retained within Herichthys. Herichthys bartoni and Herichthys bartoni therefore denote the same animal. Within the group, bartoni is one of the two molar-toothed snail-crushers that co-occur in the Media Luna system alongside the namesake Nosferatu labridens, and the pair are textbook examples of sympatric trophic partitioning in a confined spring.
Morphology
Herichthys bartoni is a robust, moderately deep-bodied cichlid reaching about 7 in in standard length, with females staying somewhat smaller than males. The body is laterally compressed with a steep forehead and a smallish, slightly subterminal mouth. As in its relatives, the diagnostic hardware is internal: a broad, stout lower pharyngeal plate paved with rounded, flattened molariform teeth, the mill of a fish built to crush hard-shelled molluscs. Outside of breeding it is a fairly plain brassy-tan to grey-green animal, often marked with a row of dark mid-lateral blotches and a scatter of small dots, and frequently showing the dark spot at the caudal base typical of the genus. Breeding colour transforms it: the ground colour pales and the fish raises a bold dark vertical bar (or paired dark fields) across the flank, the trait the genus Nosferatu was named for, set against lighter, sometimes pearly or yellowish areas. Sexual dimorphism is modest — males grow larger and develop a heavier nuchal profile with age — and the most reliable way to sex the fish is by venting, the male's genital papilla being narrower and more pointed than the female's.
Habitat
Barton's cichlid is a narrow-range endemic of the Río Verde valley in San Luis Potosí, on Mexico's Atlantic slope, where it occurs in the upper Río Verde and the warm karstic spring-lakes of the Laguna de la Media Luna system within the Pánuco basin. These are clear, well-oxygenated thermal waters that run roughly 70–86 °F, hard and strongly alkaline (FishBase gives pH 7.0–8.5 with very high hardness), fringed with water lilies and submerged macrophytes over sand, rock and travertine. La Media Luna itself is a deep, crystalline crescent-shaped spring popular with divers precisely because of its clarity and warmth; bartoni is generally a shallow-water, benthopelagic fish of the upper few metres around vegetation and structure, though it ranges into deeper water in the spring basins. It shares the system with the molluscivore Nosferatu labridens, the goodeid and characin fauna of the springs, and — increasingly — translocated cichlids introduced into the basin. Beyond these few spring-fed waters and their outflows the species occurs nowhere else.
Feeding
Herichthys bartoni is a hard-prey specialist with a broad omnivorous fringe. Its molariform pharyngeal mill is built to crush the abundant aquatic snails of the Río Verde springs, and FishBase summarises its diet as smaller fishes, aquatic insects, algae and other invertebrates, placing it around trophic level 3.4. In the spring community it occupies the snail-crushing guild alongside its relative Nosferatu labridens, the two partitioning the hard-shelled gastropod resource that few other fishes in the system can exploit — a much-cited case of sympatric divergence in feeding morphology within a single confined habitat. Where snails are scarce the fish readily takes insect larvae, small crustaceans, plant material and detritus, so it is best described as a molluscivore with generalist tendencies rather than an obligate snail-eater.
Mating
Barton's cichlid is a monogamous, pair-bonding substrate spawner whose pairs become strongly territorial as they come into condition. Courtship and the switch into breeding livery go together: a ripe pair claims a patch of shallow water at the base of a rock or other cover and defends it against conspecifics and other cichlids, the dark bar of the breeding pattern serving as both courtship and threat signal. Because the Media Luna springs hold near-constant temperature and chemistry through the year, pairs can form and spawn in essentially any month rather than on a seasonal cue. As in most members of the tribe Herichthyini, both members of the pair hold the territory together and the bond is reinforced through the spawning cycle, with the female taking the larger share of direct egg and larval care and the male patrolling the perimeter.
Breeding
Herichthys bartoni is a biparental substrate spawner. The pair cleans a firm surface — a rock face, a flat stone, or the wall of an excavated pit at the base of cover — and the female lays an adhesive clutch of ovoid eggs directly onto it, after which both parents guard and fan the developing eggs and then shepherd the free-swimming fry. Clutch sizes in the genus run to several hundred eggs (its close relative Nosferatu labridens averages roughly 300–1,000 per spawning), and the constant-temperature springs allow reproduction to proceed year-round rather than in a defined season. The female tends mainly to the eggs and larvae while the male defends the wider territory; the fry are led in a tight school over the substrate and the parents continue to guard them well after they become mobile. The species is an egg-layer that has been bred in captivity, though it is not regarded as an easy or especially prolific aquarium spawner.
In the aquarium
Barton's cichlid is a specialist's fish, kept in the Central American hobby for its breeding colour and its spring-endemic provenance rather than for ease of care. It is a substantial, territorial cichlid: a single pair needs an aquarium of at least roughly 100–105 US gal, longer rather than taller, with enough footprint that a pair — or a fish being introduced to a prospective mate — can hold and retreat from territory. Décor should echo the spring biotope: a fine sand bottom that the fish can sift and work, bogwood, and rockwork arranged into caves and broken sightlines, which both suits the animal and tempers aggression.
Water chemistry is the part keepers most often get wrong. Unlike the soft-water cichlids of South America, this is a hard-water, alkaline species from mineral-rich thermal springs, and it does best at pH above 7.5 with high hardness; soft, acidic water is a mistake. A keeping temperature around 75–82 °F suits it well (the wild springs run warmer, into the low 30s °C, but cooler aquarium temperatures favour long-term health), with strong filtration and good oxygenation to mirror the clear, well-oxygenated spring water. The species accepts standard aquarium foods readily, but its natural diet should be respected: offer a protein-rich, largely carnivorous menu and include snails, which are an ideal and natural item that also exercises the pharyngeal mill. Tankmates should be robust, similarly sized Central Americans able to stand up to a spawning pair; small or timid fish will be bullied, and two adults in cramped quarters can turn lethal. It is neither a community fish nor a beginner's fish — a beautiful, demanding cichlid that rewards space, hard alkaline water and patience.
Conservation
Herichthys bartoni is assessed by the IUCN Red List as Endangered (criterion B1ab(iii); assessed 5 February 2019), on the basis of its very small range — confined to the Río Verde valley springs and the upper Río Verde of San Luis Potosí — and a declining, increasingly fragmented area of suitable habitat. In Mexico it is additionally covered by the official norm NOM-059-SEMARNAT. The pressures the assessors identify are the classic ones for a single-valley spring endemic: tourism and recreation at the springs (La Media Luna is a heavily used dive and swimming destination), dams and water diversion for agriculture (the flagship spring's natural outflow to the Río Viejo has been blocked), invasive and translocated fishes introduced into the Media Luna system, and pollution from domestic wastewater and agricultural runoff. As one of only a handful of cichlids restricted to this single thermal-spring system, bartoni is genuinely vulnerable to any degradation of its tiny range, and its long-term security depends on the protection of the Media Luna springs themselves.