Taxonomy & naming
The San Domingo cichlid was described by Charles Tate Regan in 1904 as Heros (Heros) callolepis, from Santo Domingo de Guzmán on the Isthmus of Tehuantepec, Mexico — in the upper Coatzacoalcos drainage (syntypes BMNH 1890.10.10.112-113). The epithet combines Greek kallos, "beautiful," with lepis, "scale," a reference to the small, light-blue spot at the base of each scale on the body. Long placed in the omnibus genus Cichlasoma (as Cichlasoma callolepis), it was returned to the resurrected firemouth genus as Thorichthys callolepis (Burgess 2000; Kullander in Reis et al. 2003) and retained as valid in subsequent revisions of the Middle American cichlids (McMahan et al. 2015; Říčan et al. 2016; Fricke et al. 2024). It is the headwater specialist of the upper Coatzacoalcos, sharing that basin with the more recently described Thorichthys panchovillai of the same river system.
Morphology
Thorichthys callolepis is a deep-bodied, laterally compressed firemouth reaching about 5.5 in standard length (FishBase); Ad Konings records wild males at the type locality maturing around 4.5 in total length and females at only about 3 in, a wider size gap between the sexes than is typical for the genus. Its distinguishing mark is the one Regan named it for: a small, pale blue spot at the base of each flank scale, which lends the body a finely spangled appearance over a warm golden to grey-olive ground. The fins and cheeks carry the iridescent blue-green flecking typical of the genus, and the lower head and throat show the characteristic reddish wash. Uniquely among Thorichthys, however, callolepis lacks the dark, light-ringed gill-cover ocellus that every other species in the genus uses in its frontal threat display — Konings singles this out as the one feature that separates it from all its congeners, including its close neighbour T. helleri. Males grow larger, with more produced fin tips and stronger colour; females are smaller and somewhat plainer. Adapted to faster water, it is a relatively streamlined and robust Thorichthys.
Habitat
The species is endemic to the headwaters of the Río Coatzacoalcos on the Atlantic slope of Mexico, in Oaxaca and Veracruz — one of only two cichlids endemic to this upper drainage, the other being Vieja regani. Unlike most of its lowland-dwelling congeners, FishBase reports that adults inhabit rocky substrate in the faster-flowing waters of the upper river sections and headwaters — a clear, well-oxygenated, current-swept biotope. Field data recorded by Ad Konings at Puente Ajal on the Río Grande (also called Río Almoloya in the literature), about 6 mi south of Matías Romero, put the water at 82 °F, pH 8.0 and total hardness 14 °DH in March; the river there runs about 16 ft wide and 19.5 in deep on average, cascading over rocky dams into pools up to 10 ft deep and 33 ft across, with visibility good enough to watch fish from several metres away. The bottom is sandy but silted with mud despite the current, and the deeper pool sections are carpeted with a thick layer of fallen tree leaves, with rocks, tree stumps and roots providing cover in the shallows. In this stretch T. callolepis lives sympatrically with Thorichthys helleri, though the two are far from equally common — Konings found roughly a hundred T. helleri for every T. callolepis. Recorded temperatures elsewhere in its range run 75–79 °F, and the water is reported as alkaline (pH from about 7.9 upward). Its restriction to the upper Coatzacoalcos and its preference for fast, clean headwater habitat give it a narrow and ecologically specific range.
Feeding
Thorichthys callolepis is a benthic omnivore, but — contrary to the sand-sifting reputation the body shape of Thorichthys invites — Ad Konings is explicit that the genus does not sift substrate the way the superficially similar Amphilophus do; instead it picks prey directly off the bottom and out of the sediment covering it. At its type locality it forages mainly in the deeper parts of the river, picking bits and pieces of insect larvae, crustaceans and other small invertebrates from the leaf litter that carpets the substrate. Konings found no difference in feeding technique between T. callolepis and the far more abundant T. helleri, which feed side by side from the same leaf-litter substrate; the disparity in their numbers appears to owe more to some form of dominance or interference by T. helleri than to any difference in diet or foraging style. FishBase places the genus at a trophic level around 3.3.
Mating
As a firemouth, the San Domingo cichlid is a monogamous pair-spawner that relies on the genus's frontal threat display — gill covers flared open, gular pouch distended to look larger, small ritualized runs forward and backward toward a rival, occasionally escalating to a brief mouth-lock without real damage — to settle disputes and to court. Because callolepis is the one Thorichthys without a gill-cover eyespot, its display lacks the bluffing 'giant eye' effect the rest of the genus relies on, and presumably leans more on sheer gill-cover size and the reddened throat. Thorichthys typically breed in loose colonies of several neighbouring pairs, with frequent ritualized confrontations between adjacent territories; whether callolepis forms colonies at the low densities Konings recorded for it is not documented, but the genus-typical pattern gives the best working picture. Pairs form after mutual display, then select and clean a spawning site among the rocks and defend a territory around it. Detailed field observations of courtship specific to callolepis are otherwise sparse; its social behaviour is understood largely by analogy with the better-studied members of the genus, adjusted for its rocky, fast-water setting.
Breeding
Thorichthys callolepis is a biparental open-substrate spawner. At the type locality, Konings found only three breeding pairs over the course of his observations: one was defending eggs laid on a small rock rather exposed to view, and the other two were already guarding free-swimming fry. Brood sizes for both T. callolepis and its far more numerous neighbour T. helleri ran 50–100 fry — a modest clutch, smaller than the couple of hundred eggs reported for some other Thorichthys. Guarding pairs of both species keep their fry close to the nest rather than leading them on excursions through the habitat, and the fry forage for themselves from the sediment on leaves and sand; a school of the locally abundant tetra Astyanax fasciatus, which feeds heavily on cichlid fry, is a persistent hazard to a guarding pair. Breeding T. callolepis lose most of the pinkish-orange colour on the lower body while tending a brood. Egg incubation and wriggler-pit behaviour have not been documented specifically for callolepis, but match the genus-typical pattern of eggs hatching within a couple of days in warm water and larvae being moved into a shallow pit before becoming free-swimming.
In the aquarium
The San Domingo cichlid is regarded as one of the more challenging firemouths to keep, precisely because of its headwater origins: it has a high oxygen requirement and is intolerant of the warm, sluggish, organically loaded water that the lowland firemouths shrug off. A pair needs a tank of about 55 US gal or more with a footprint near 47 in, aquascaped with rounded rocks and cobbles to recreate its rocky-stream biotope, some open sand or gravel, and strong, well-oxygenated water movement from the filtration.
Water should be cooler than for the typical firemouth, clean and well-aerated: temperature about 73–79 °F, pH roughly 7.5–8.2, with medium-hard to hard, alkaline water. Powerful filtration and brisk flow are important both for oxygenation and to mimic the current of its native streams, and large, frequent partial water changes help maintain the pristine conditions it expects. Like all Thorichthys it is prone to head-and-lateral-line erosion in stale water, and this species is less forgiving of lapses than its lowland kin. Feeding is straightforward — quality sinking foods, frozen and live invertebrates, and some algae- or vegetable-based food.
Temperament follows the genus: relatively peaceful outside breeding, reliant on display, but territorial when spawning. Choose sturdy, even-tempered tankmates that share its tolerance for cooler, fast, oxygen-rich water. This is a fish best suited to a keeper prepared to provide a properly oxygenated, current-driven setup rather than a standard warm community tank.
Conservation
The IUCN Red List assesses Thorichthys callolepis as Data Deficient (assessed 10 August 2018, published 2019). As an endemic confined to the headwaters of a single river system — the upper Coatzacoalcos — and dependent on clean, fast, well-oxygenated water, it is inherently more vulnerable to localised habitat degradation than the wide-ranging firemouths; but the assessors judged that available data on its population trend, size and distribution were insufficient to assign a definitive threat category. The general pressures on the Coatzacoalcos basin — pollution (the lower basin has a long history of petrochemical industry), sedimentation, water extraction and habitat alteration in the uplands — are the backdrop to its status, and clearer survey data are needed to determine whether this narrow-range headwater specialist is at risk.