Taxonomy & naming
The curve-bar cichlid was described by the French ichthyologist Jacques Pellegrin in 1903 as Heros (Cichlasoma) labridens, from two large syntypes (about 222 and 8 in total length) that the University of Guanajuato naturalist Alfredo Dugès had collected in 1889 and sent to the Muséum national d'Histoire naturelle in Paris (MNHN 1889-0019 and 1889-0020). Pellegrin noted that these types shared an origin with those of Herichthys bartoni, which fixes the type locality in the Río Verde valley of the Huasteca Potosina, San Luis Potosí. The species then passed through the long nomenclatural churn that has plagued the Middle American cichlids: it was shuffled through Cichlasoma, Cichlosoma, Parapetenia, and Nandopsis, and was placed in Herichthys by Kullander in 1996. In 2015 Mauricio De la Maza-Benignos and colleagues, in a phylogeographic analysis of Herichthys published in Hydrobiologia, split a clade of northeastern-Mexican (Pánuco–Tamesí and Tamiahua) cichlids into the new genus Nosferatu, with labridens 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 silent-film vampire Nosferatu (the on-screen count is actually named Orlok). The split is not universally accepted — a 2024 phylogeographic reassessment argued the Nosferatu species are better retained within Herichthys, and FishBase and the Catalog of Fishes still file the fish under Herichthys labridens — so Herichthys labridens and Herichthys labridens denote the same animal. Within the group, labridens is the namesake of the 'labridens species group' of yellow-and-black, molar-toothed snail-eaters of the southern Pánuco.
Morphology
Herichthys labridens is a robust, moderately deep-bodied cichlid with a slightly downturned mouth and a projecting upper jaw. Wild males reach roughly 8 in and females stay smaller, to about 6 in; FishBase records a maximum of 10 in SL, and captive fish on rich diets can exceed wild dimensions. The most quoted external feature is the one Pellegrin named it for — a conspicuous tooth visible in the upper lip even in living fish (labridens, literally 'lip-tooth'). The diagnostic hardware, though, is internal: a broad, stout lower pharyngeal plate paved with rounded, flattened molariform teeth, which Pellegrin already recognised in 1903 as the apparatus of an animal built to crush hard-shelled molluscs. Outside of breeding the fish is a fairly plain brassy-tan to grey with a scatter of small dark dots on the head and forehead that do not extend onto the body and a magenta blotch in the pectoral axil. Breeding colour is what makes it unmistakable: the body turns canary yellow and develops two velvety black areas — one over the lower head and chest reaching back into the front of the dorsal fin and including the pectorals, the second running from the base of the anal fin to the caudal base but leaving the anal fin itself yellow. Sexual dimorphism is otherwise weak; the sexes are most reliably told apart by venting, the male's genital papilla being more pointed and the female's shorter and blunt.
Habitat
The species is a narrow-range endemic of the Río Verde valley in San Luis Potosí, in the upper Pánuco basin on Mexico's Atlantic slope. It exists in two ecological forms. A lacustrine form lives in the valley's warm karstic spring-lakes and their pools — La Media Luna, Los Anteojitos, Manga Larga, Poza Azul, Los Peroles, Laguna del Cedral, Laguna de las Tablas, Laguna de San Bartolo, Puerta del Río and a few others — clear, well-oxygenated thermal water that runs roughly 77–90 °F year-round and is fringed with water lilies (Nymphaea) and the submerged macrophyte Najas marina. A riverine form occurs in the Ríos Verde and Santa María up to about 1,0 ft elevation, over sand and rock in stronger current at a cooler 72–82 °F. Across both, the water is hard and alkaline, with pH typically above 7.5 (FishBase gives a pH range of 7.0–8.5 and high hardness). The springs are shallow to moderately deep — La Media Luna is a popular dive site precisely because of its clear, warm, vegetated basin — and the fish is generally a shallow-water, benthopelagic inhabitant of the upper few metres around vegetation and structure. The flagship spring formerly drained into the Río Viejo, but that outflow has been blocked and diverted for agriculture, and the system is now shared with native Herichthys bartoni and a translocated Herichthys cf. carpintis.
Feeding
Herichthys labridens is, above all, a molluscivore — its common Spanish name, mojarra caracolera (the 'snail mojarra'), records the habit directly. The molariform pharyngeal mill lets it crack the abundant aquatic snails of the Río Verde springs, and gut analyses by Taylor and Miller (1983) confirmed snails plus detritus as the staple of the lacustrine form. FishBase places it around trophic level 3.5, consistent with a hard-prey specialist that also takes softer fare. The riverine form is more catholic, recorded by Díaz-Pardo and colleagues (2002) taking crustaceans, insects, seeds and plant material as well, so the species is best described as a molluscivore with broader omnivorous tendencies rather than an obligate snail-eater. In the spring community it occupies the snail-crushing niche alongside its molar-toothed relative Herichthys bartoni, the two partitioning a resource — hard-shelled gastropods — that few other fishes in the system can exploit.
Mating
The curve-bar cichlid is a monogamous, pair-bonding substrate spawner, and pairs become strongly territorial as they come into breeding condition. Courtship and the switch into the canary-and-black 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. Because the Río Verde springs hold near-constant temperature and conditions throughout the year, lacustrine pairs can form and spawn in essentially any month; in the seasonally variable rivers, Artigas Azas observed breeding pairs only from roughly March to October. As with most Herichthyini, both members of the pair hold the territory together, the bond reinforced through the spawning cycle, and the brilliant bicolour pattern doubles as a courtship and threat signal during this phase.
Breeding
Herichthys labridens is a biparental substrate spawner. The pair excavates a small cave or pit at the base of a rock, or adopts a suitable enclosed crevice, and the female lays an adhesive clutch of ovoid, yellowish eggs roughly 0.5 in on the long axis directly onto the cleaned surface. Reported clutches are substantial — Artigas Azas gives an average of about 300 to 1,000 eggs per spawning. Both parents tend the brood: they guard and fan the eggs, and once the young are mobile the parents continue to defend the school. A distinctive trait of the species is the way the pair provisions the fry, agitating the substrate with the belly and pectoral fins to stir up food for them — an unusual contact-feeding behaviour that has drawn scientific interest. In the constant-temperature springs reproduction proceeds year-round; in the rivers it is seasonal. FishBase flags the species as 'not easy to raise in captivity', and getting pairs to spawn and rear young reliably remains a challenge for hobbyists despite the relatively large clutches.
In the aquarium
The curve-bar cichlid is a prized but demanding fish in the specialist Central American hobby, kept far more for its spectacular breeding colour than for ease of care. It is a substantial, territorial cichlid: a single pair needs an aquarium of at least roughly 105 US gal, longer rather than taller, with plenty of footprint so a pair — or a fish being introduced to a prospective mate — can hold and retreat from territory. Décor should reproduce its spring biotope: a fine sand bottom (the fish sifts and works the substrate, and its breeding ritual involves stirring sand for the fry), 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 — it comes from mineral-rich thermal springs — and it does best at pH above 7.5 with high hardness; soft, acidic water is a mistake. A temperature of about 75–82 °F is a good keeping range (the wild springs run warmer, into the low 30s °C, but cooler keeping temperatures suit 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 — snails are an ideal and natural item that also exercises the pharyngeal mill — and include carotenoid-rich foods to bring out the breeding yellows. Tankmates should be robust, similarly sized Central Americans able to stand up to a spawning pair; small or timid fish will be bullied or eaten, and two adults in cramped quarters can turn lethal. It is not a community fish and not a beginner's fish, and the honest reputation in the hobby is of a beautiful cichlid that rewards space, hard alkaline water, and patience.
Conservation
Herichthys labridens is assessed by the IUCN Red List as Endangered (criterion B1ab(iii); assessed 18 October 2018 by Soto Galera, Mejía Guerrero and Pérez-Miranda, published 2019), on the basis of its tiny range — confined to the Río Verde valley springs and rivers of San Luis Potosí — and a declining, increasingly fragmented area of suitable habitat. The population trend is decreasing. In Mexico it is additionally listed under the official norm NOM-059-SEMARNAT (historically as a species in danger of extinction). The pressures the assessors identify are the classic ones for a single-valley spring endemic: tourism and recreation development at the springs (La Media Luna is a heavily used dive and swimming destination), recreational disturbance, dams and water diversion for agriculture (the flagship spring's natural outflow to the Río Viejo has been cut), invasive and translocated fishes — including a Herichthys cf. carpintis introduced into the Media Luna system — and pollution from domestic wastewater and agricultural runoff. The curator of the Cichlid Room Companion, who has worked the habitat for years, regards the species as genuinely vulnerable even if perhaps not on the brink, and points to its narrow range and the steady human pressure on the springs as the real long-term concern.