Taxonomy & naming
The Tamasopo cichlid was described in 1993 by Juan Miguel Artigas Azas — the Mexican cichlid biologist who would later found the Cichlid Room Companion — as Herichthys tamasopoensis. Despite the common name, there is no river actually called the Tamasopo: the type locality is the Río Gallinas and its tributaries, which run mainly through the Tamasopo municipality of San Luis Potosí, and the species epithet honours that county rather than a watercourse. Artigas Azas traced the place name itself to the Huastec 'Tam-Azote', meaning 'place of waterfalls' — an apt origin for a fish isolated by one. It is a true member of Herichthys and was not transferred to the genus Nosferatu erected by De la Maza-Benignos and colleagues in 2015 for the molar-toothed snail-eaters; FishBase and Eschmeyer's Catalog of Fishes both retain it as a valid Herichthys. The species belongs to the lineage of slimmer, algae-grazing Herichthys (the group that includes the Texas cichlid Herichthys cyanoguttatus and Herichthys carpintis) rather than the heavy-jawed molluscivores, which is reflected in its lighter build and dentition. Artigas Azas diagnosed the new species against its closest relative, Herichthys carpintis — a fish of the lower Río Pánuco and Río Verde that is conspicuously absent from the Río Gallinas itself — by a shallower body, a shorter snout, a proportionally longer caudal peduncle, a straight (non-concave) dorsal head profile even in large adults, a smaller adult size, and compressed, truncated outer jaw teeth. The Río Gallinas system, which the species shares with the Nosferatu snail-eaters Nosferatu pame and Nosferatu steindachneri, is cut off from the rest of the Pánuco drainage by the 105-metre Tamul waterfall, a barrier that blocks any upstream movement of fish and explains why H. carpintis never reached this stretch of river naturally.
Morphology
Herichthys tamasopoensis is a relatively slender, elongate Herichthys reaching about 7 in total length. Artigas Azas's original diagnosis quantifies just how much shallower-bodied it is than Herichthys carpintis — 43.5–44.6% of standard length versus 46.6–48.7% in carpintis — with a proportionally shorter snout (37.5–40.7% of head length versus 46.0–51.0%) and a longer caudal peduncle (its length equal to its own depth, versus only 66.7–81.8% of the depth in carpintis, which reaches a larger 8.5 in). The dorsal head profile runs essentially straight from the snout tip to the first dorsal spine even in large adults, lacking the concavity above the eye that develops in carpintis — a useful field mark alongside the smaller adult size. In the mouth, the anterior teeth of the outer row in each jaw are distinctively compressed and become truncated (flat-tipped) with age rather than staying simple and conical, while the lower pharyngeal teeth are slender and pointed rather than molariform, matching an algae-grazing rather than a snail-crushing diet. In non-breeding dress it is a pale brassy to grey-green fish, often finely flecked with iridescent blue or pearl and marked with a row of dark mid-lateral blotches and the dark caudal spot of the genus. In breeding condition, as in its relatives, the pattern sharpens into contrasting pale and dark fields — the forward body lightening while dark areas strengthen on the rear flank and the chest — making courting pairs strikingly marked. Sexual dimorphism is weak: males average a little larger and may carry slightly longer fins, and venting is the most reliable way to sex the fish.
Habitat
The Tamasopo cichlid is a narrow-range endemic of the Río Gallinas and its tributaries, a Pánuco tributary that drains the Rascón valley through the Tamasopo municipality of San Luis Potosí, on Mexico's Atlantic slope. The river runs from roughly 1476 ft down to about 984 ft above sea level through the Sierra Madre Oriental before dropping over the 105-metre Tamul waterfall into the Río Tampaón — a barrier that blocks all upstream fish movement and is the reason H. tamasopoensis evolved in isolation from its close relative H. carpintis, which is common lower in the Pánuco but never reached the Gallinas naturally. It lives in hard, clear, strongly alkaline, spring-fed water that runs fast over rock, gravel and the travertine formations characteristic of the Huasteca; Artigas Azas's own chemistry measurements along the river found a general hardness exceeding 100° DH — exceptionally hard water even by the standards of a limestone-country cichlid — alongside the pH range of 7.8–8.3 also given by FishBase. One well-documented site within the system, the oxygen-rich rocky pools of the Río Puente de Dios, supports a dense population and has furnished some of the best photographic and collection records of the species. It is a fish of flowing, well-oxygenated reaches around rocks and submerged structure, occurring syntopically with a range of other Pánuco fishes including the molar-toothed Nosferatu snail-eaters that share the system. Its distribution is sharply bounded by the geography of the Río Gallinas drainage — closed off by the Tamul waterfall at one end and the limits of the Rascón valley at the other; beyond this single small river system it occurs nowhere else, which is the root of its conservation concern.
Feeding
Herichthys tamasopoensis is principally an algae and aufwuchs grazer, with a trophic level around 3.4. Its small, somewhat ventral mouth and slender pharyngeal dentition are built for rasping the films of algae and the associated micro-invertebrates (aufwuchs) that coat the rocks and travertine of its fast streams, rather than for crushing snails as its molar-toothed neighbours do. This grazing habit places it in a different feeding guild from the sympatric Nosferatu species, and is part of how several cichlids partition the same stretch of river. It will also take small invertebrates and organic matter opportunistically, so it is best described as a herbivore-leaning omnivore that specialises on the rich algal growth of its clear, sunlit spring waters.
Mating
The Tamasopo cichlid is a monogamous, pair-bonding substrate spawner. Pairs form as fish come into condition and become strongly territorial, claiming and defending a patch of the rocky streambed, with the sharpened breeding pattern serving as a courtship and threat display. Breeding here is markedly seasonal rather than continuous: Artigas Azas documented a reproductive cycle running from mid-December to the end of May — distinctly earlier than the March-to-August season of H. carpintis in the warmer water downstream — with activity peaking in February, when hundreds of pairs can be found tending fry simultaneously along the river. Numbers fall away through April and drop to very few pairs by the end of May, when the rains arrive over the Huasteca forest. As in the tribe Herichthyini, both members of the pair hold the territory jointly, the female taking the larger role in direct egg and larval care while the male patrols and defends the perimeter against conspecifics and other fishes attracted to the same rocky spawning sites; Artigas Azas noted that males of this species are unusually persistent defenders, standing their ground against a threat rather than fleeing — a departure from the more typical Central American cichlid pattern in which a hard-pressed male abandons the nest.
Breeding
Herichthys tamasopoensis is a biparental substrate spawner that uses a shallow pre-dug pit rather than an open rock face for its larvae: Artigas Azas's field observations describe pits about 1 in across and 1 in deep, where the larvae remain for roughly five days until they have fully absorbed their yolk sacs and become free-swimming. Once mobile, the fry stay in a tight, closely packed school guarded by both parents, who communicate with the group through spasmodic body movements and by opening and closing their fins. The school moves in a rotating arrangement, with fish at the rear repeatedly swimming up and over the group to take the front position, and it rarely strays far from the original pit. The fry forage on algae and detritus coating leaves, rocks and driftwood near the nest; when a predator approaches they take cover under leaves, wood or stones while the female positions herself directly over them and the male confronts the threat, standing his ground rather than fleeing — a more persistent response than is typical of Central American cichlids defending a brood, which more often flee a serious threat outright. Parental protection continues until the young reach about 1 in in length, after which they gradually disperse into the shallow water near the banks, where they are relatively safe from predators. Breeding activity peaks in February, when hundreds of pairs can be observed tending fry simultaneously along the Río Gallinas, then tapers through April to a low point by the end of May as the rainy season begins. Clutches run to a few hundred eggs in the manner of its slimmer Herichthys relatives. It is an egg-layer that has been bred by specialists, behaving much like the better-known Texas and green Texas cichlids to which it is allied.
In the aquarium
The Tamasopo cichlid is uncommon in the hobby, kept mainly by Central American cichlid specialists and by those with a particular interest in the Huasteca Potosina fauna. A pair can be kept in an aquarium of around 80 US gal, longer rather than taller, furnished to echo its rocky-stream biotope: a sand or fine-gravel bottom, plenty of smooth rockwork forming caves and broken sightlines, and a brisk current with good oxygenation to reproduce its fast, clear spring waters.
Water chemistry is important — this is a hard-water, strongly alkaline species (its wild waters run pH 7.8–8.3 with high hardness), and soft, acidic water is wrong for it. A keeping temperature in the mid-20s °C suits it. Because it is an algae grazer in the wild, its diet should include a substantial vegetable component — spirulina- and algae-based prepared foods, blanched vegetables — alongside a moderate amount of small live and frozen invertebrate foods; an all-meat diet does not suit its grazing physiology as well as it would a predatory cichlid. Tankmates should be robust, similarly sized fish able to handle a territorial pair; like all Herichthys it becomes assertive when spawning and will not tolerate crowding. Its slimmer build and grazing habit make it a somewhat less brutally aggressive option than the large snail-eaters of its valley, but it is still a species-tank or robust-community fish rather than a gentle community cichlid.
Conservation
Herichthys tamasopoensis is assessed by the IUCN Red List as Vulnerable (criterion B1ab(iii); assessed 16 August 2018), on the basis of its very small range — confined to the Río Gallinas and its tributaries — 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 those of a single-system Huasteca endemic: water extraction and diversion for agriculture and supply, tourism and recreational pressure on the Tamasopo waterfalls and pools (a heavily visited destination), pollution from domestic and agricultural runoff, and the spread of introduced and translocated fishes through the Pánuco headwaters. As a fish restricted to one small river system, it has little capacity to absorb sustained habitat degradation, and its long-term security depends on protecting the flow and water quality of the Río Gallinas itself.