Taxonomy & naming
The blackgullet cichlid was described by Luis René Rivas in 1962 as Cichlasoma pasionis, from the Río de la Pasión at Sayaxché in the Usumacinta drainage of the Department of El Petén, Guatemala (holotype USNM 203156). The epithet pasionis is the Latin genitive of the type-locality river, the Río de la Pasión. As with the rest of the genus it was long known as Cichlasoma pasionis before being transferred to the resurrected firemouth genus as Thorichthys pasionis (Burgess 2000; Kullander in Reis et al. 2003), a placement retained in subsequent revisions of the Middle American cichlids (McMahan et al. 2015; Říčan et al. 2016; Fricke et al. 2024). Eschmeyer's Catalog of Fishes records its distribution across the Río Grijalva and Río Usumacinta systems of Mexico and Guatemala. It belongs to the Usumacinta cluster of firemouths alongside Thorichthys meeki and Thorichthys affinis, but is readily distinguished from them by its dark rather than red throat. So close is that resemblance to Thorichthys meeki, in fact, that pasionis has long circulated in the hobby under the trade name "Yellow Meeki" — Ad Konings, writing in the Cichlids Yearbook, notes that the only real differences between the two are pasionis's distribution and its yellow-toned coloration, the firemouth's familiar red throat traded for pasionis's dark one.
Morphology
Thorichthys pasionis is a deep-bodied, laterally compressed firemouth and one of the larger members of the genus, reaching about 6.5 in standard length (FishBase). Ad Konings describes the ground colour as violet-blue over the body, with the head and lower flanks washed in yellow, over which run the iridescent blue-green spots and streaks characteristic of the genus. Its defining feature is the throat: where the firemouth and most congeners flush vivid red, pasionis carries a dark bordeaux-red to almost-black tone across the lower head, gular region and breast — the "black gullet" of its common name. That colour is not a constant, visible wash, though: Konings notes it is normally hidden and becomes strikingly obvious only when the fish lowers its buccal cavity in a threatening display, the sudden dark flash against the yellow head reading as a dramatic warning to rivals. Each gill cover bears a dark, light-ringed ocellus used in frontal display, and a dark midline marking and caudal-base spot are present. Males grow larger, with more produced fin tips and stronger colour; females are smaller and rounder. The dark-throated display, combined with the genus's flaring threat posture, gives a confronting pasionis a distinctly grim, shadow-faced appearance.
Habitat
The species is distributed across the Atlantic slope of southern Mexico and northern Guatemala in the Río Grijalva and Río Usumacinta systems, including the Río de la Pasión of the Petén and, by name, Lago Petén Itzá itself. FishBase reports that it inhabits both rivers and lakes with softer substrate, occupying the warm, slow-flowing lowland reaches and quiet vegetated margins typical of the genus, and Ad Konings records it foraging over sometimes muddy bottoms. In the seasonal cienega ponds around Villahermosa, Tabasco, it turns up alongside Thorichthys meeki in shallow water — often under a metre deep — over a muddy bottom, shaded by floating water lilies and littered with fallen leaves and branches, where temperatures can run from the high 20s past 90 °F; these crowded ponds dry down in the dry season and are restocked from overflowing neighbours once the rains return. In the slower, sandy-bottomed reaches of Campeche's Chompán, Candelaria and Champotón rivers — where dry-season visibility can reach 3–16 ft — it occupies the same leaf- and driftwood-littered stretches as Thorichthys meeki and Thorichthys helleri. Across its range it also shares habitat with a broader community of Central American cichlids, among them Chuco intermedius, Herichthys pearsei, Nandopsis salvini and Nandopsis friedrichsthalii, and various Theraps and Vieja species, alongside characins, livebearers such as Poecilia and Xiphophorus helleri, and the catfish Rhamdia guatemalensis. Recorded conditions elsewhere in its range are temperatures of about 79–86 °F and notably hard, alkaline water — FishBase gives a pH range of roughly 8.0–9.0 and hardness from about 5 °dH upward — reflecting the limestone catchments of the Usumacinta and Grijalva lowlands. Its occupancy of both rivers and lakes, and its tolerance of a wide range of hard-water conditions, give it a fairly broad lowland range.
Feeding
Thorichthys pasionis is a benthic omnivore. Ad Konings' field notes record it foraging over sometimes muddy bottoms for small prey such as insect larvae and crustaceans, and, in the manner typical of the genus, taking in mouthfuls of sand and sediment to sift out whatever else it can extract, supplemented by algae and organic detritus. Its jaw shape may matter here: pasionis and its close relatives have a lower jaw that protrudes slightly beyond the upper (prognathous), unlike the equal-jawed (isognathous) Thorichthys aureus subgroup, and it is plausible that the prognathous species scoop food from the substrate while the isognathous ones pick it from settled sediment — but by Konings' own account nothing factual is actually confirmed about how the two groups' natural feeding techniques differ. FishBase places the genus at a trophic level around 3.3. In its rivers and lakes it functions as a small-to-medium generalist bottom-feeder, occupying the same invertebrate-and-detritus guild as the other firemouths.
Mating
Like its relatives, the blackgullet cichlid is a monogamous pair-spawner that uses the genus's frontal threat display to repel rivals and to court — but with a darkened rather than reddened throat presented during the flare. Pairs form after mutual display, then select and clean a spawning site and defend a surrounding territory. Outside breeding it is comparatively peaceful and inclined to bluff. Detailed in-situ behavioural accounts are limited, and its social biology is understood largely by analogy with the better-studied members of the genus.
Breeding
Thorichthys pasionis is a biparental open-substrate spawner. A pair cleans a flat hard surface or shallow pit and deposits a clutch on the order of a few hundred eggs; both parents guard the eggs and then move the hatched larvae to pits in the substrate before escorting the free-swimming fry. Parental care is shared through the early fry stage, with the female typically staying closest to the brood while the male patrols the territory. Wild clutch counts specific to pasionis are not richly documented, so this follows the genus-typical pattern; in the aquarium it spawns biparentally in the same manner as its congeners when given suitable cover and water quality.
In the aquarium
The blackgullet cichlid is an occasional aquarium fish kept by Central-American-cichlid enthusiasts, distinctive for its dark-throated display and its larger size among the firemouths. A pair needs a tank of about 55–65 US gal with a footprint near 47 in — a little more than the smaller firemouths — furnished with a soft sand bed for natural sifting, rock and wood to provide caves and break up sightlines, and open foreground space.
Water should be warm and distinctly hard and alkaline, reflecting its native limestone rivers and lakes: temperature about 77–84 °F and pH roughly 7.5–8.5, with medium-hard to hard water. Clean, well-filtered water and regular partial changes are essential to prevent head-and-lateral-line erosion, the common pitfall of the genus. Feeding is easy — quality sinking pellets and flake, frozen and live invertebrate foods such as bloodworm and brine shrimp, and some vegetable content.
Temperament is firemouth-like: relatively peaceful outside breeding and reliant on display, though a spawning pair becomes territorial and, being a larger fish, can throw its weight around more than the smaller firemouths. Choose sturdy, even-tempered tankmates of similar size — larger characins, robust barbs, peaceful catfish and other moderate Central Americans. It is a good choice for a hobbyist wanting a bigger, less-common Thorichthys with a strikingly different, dark-faced threat display.
Conservation
The IUCN Red List assesses Thorichthys pasionis as Least Concern (assessed 8 August 2018, published 2019). The species is reasonably widespread across the Grijalva and Usumacinta lowlands of Mexico and Guatemala, occupies both rivers and lakes, and is not considered to face species-level threats. The general pressures on the Usumacinta and Grijalva basins — pollution, water extraction, dam development, habitat alteration and introduced species — form the backdrop to its status, but none is identified as a present danger to this adaptable, hard-water firemouth, which remains common within its range.