Taxonomy & naming
Thorichthys affinis was described by Albert Günther in 1862 as Heros affinis, on syntypes (BMNH 1864.1.26.83-86) collected at Lake Petén (Petén Itzá) in Guatemala. Günther chose the name affinis, Latin for "related" or "allied," precisely because he regarded the fish as very closely allied to Thorichthys aureus, which he described in the same work. For most of the twentieth century it was placed in Cichlasoma as Cichlasoma affine; it was returned to the resurrected firemouth genus as Thorichthys affinis (Burgess 2000; Kullander in Reis et al. 2003) and has been retained as valid in subsequent treatments of the Middle American cichlids (Říčan et al. 2016; McMahan et al. 2015; Fricke et al. 2024). It belongs to the cluster of yellow-bellied, blue-spangled Thorichthys of the Usumacinta system that also includes Thorichthys meeki and Thorichthys pasionis, and the close resemblance to the firemouth is the source of its trade name "yellow meeki." Note a genuine conflict in the sourcing here: Ad Konings, writing in the Cichlids Yearbook, attributes the "Yellow Meeki" trade name instead to the closely related T. pasionis, describing that species as differing from T. meeki mainly in its yellow coloration. Both affinis and pasionis are yellow-toned meeki relatives from the same drainage system, and current hobby and reference usage (FishBase, Seriously Fish) applies "yellow meeki" to T. affinis — but given Konings' explicit attribution to T. pasionis, readers encountering the name applied to either species in older or mixed sources should not assume it pins down the species with certainty.
Morphology
Thorichthys affinis is a small-to-medium firemouth, reaching about 5.5 in standard length according to FishBase. In form it is the typical deep-bodied, laterally compressed, large-headed Thorichthys. Its ground colour is a warm yellow-gold to fawn — the basis of the "yellow" in its hobby name — patterned with the rows of iridescent blue-green spots and streaks characteristic of the genus across the flanks, cheeks and unpaired fins. A dark, light-ringed ocellus marks each gill cover. The genus's signature throat flash is where affinis actually parts company with its closest relatives rather than merely toning them down: Konings records the breeding throat as red in T. meeki, black in T. pasionis, and yellow — not red — in T. affinis, a clean three-way diagnostic within the meeki group. Normally hidden, the throat colour is flashed by lowering the buccal membrane in a threat display, an abrupt burst of colour that (as in its relatives) likely serves to startle or deter rivals. A series of dark midline marks and a caudal-base spot complete the pattern. Males grow larger and develop more extended fin tips and stronger colour, while females are smaller and rounder; the sexes are otherwise broadly similar, as is usual in the genus.
Habitat
The species is native to the Atlantic slope of Central America in the upper Usumacinta River basin, principally the lake district of the Petén in Guatemala — including the type locality, Lake Petén Itzá — and extending into Belize. FishBase describes it as a fish of both lakes and lower-valley rivers, where it prefers sandy or soft bottoms scattered with small rocks. It is a warm-water species, recorded at temperatures of roughly 79–86 °F, occupying the quiet, vegetated margins and shallows typical of the limestone lakes and slow rivers of the region. As with the rest of the genus, detailed in-situ chemistry is sparsely published, but the karst catchments of the Petén imply generally hard, alkaline water.
Feeding
Thorichthys affinis is a benthic omnivore that forages over soft bottoms in the manner of its relatives, taking in mouthfuls of sand and fine sediment to sift out small invertebrates, insect larvae, crustaceans and organic detritus, supplemented by algae and plant material. Its preference for sandy and soft substrates with small rocks reflects this sifting habit. FishBase places the genus at a trophic level around 3.3. In the lakes and slow rivers it inhabits, it functions as a small generalist bottom-feeder, part of the same invertebrate-and-detritus guild occupied by the other firemouths.
Mating
As a firemouth, Thorichthys affinis is expected to be a monogamous pair-former that relies on the genus's frontal threat display — flared gill covers, presented eyespots and a reddened throat — to settle disputes with rivals and to court mates. Pair bonds form after mutual display, after which the two fish select and clean a spawning site and jointly defend a surrounding territory. Specific field observations of courtship and territoriality in affinis are not well documented in the literature; its behaviour is inferred from its close relatives, with which it shares the same displays and body plan.
Breeding
Breeding in Thorichthys affinis has not been described in detail in the wild, but the species is a biparental open-substrate spawner like the rest of the genus. By analogy with its congeners — particularly Thorichthys meeki, whose reproduction is well documented — a pair cleans a flat hard surface or shallow pit, lays a clutch on the order of a few hundred eggs, and both parents guard the eggs, move the hatched larvae to pits in the substrate and escort the free-swimming fry. Parental care is biparental and extends through the early fry stage. Where data specific to affinis are lacking, this genus-typical pattern is the honest best description.
In the aquarium
Thorichthys affinis is an occasional aquarium fish, kept and bred by Central-American-cichlid specialists rather than seen routinely in the general trade, where it is sometimes sold as "yellow meeki." Husbandry mirrors that of the firemouth. A pair is comfortable in a tank of about 40–55 US gal with a footprint of roughly 39.5–47 in, 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 on the hard, alkaline side, reflecting its native limestone lakes and rivers: temperature about 77–84 °F and pH roughly 7.2–8.2, with medium-hard to hard water. Clean, well-filtered water and regular partial changes are important, as the genus is susceptible to head-and-lateral-line erosion in poor conditions. Feeding is undemanding — quality sinking pellets and flake, frozen and live invertebrate foods, and some vegetable matter to suit its omnivorous habit.
Temperament is firemouth-like: relatively peaceful outside breeding and reliant on bluff and display, but a spawning pair becomes territorial. Suitable tankmates are sturdy, even-tempered fish of similar size — larger characins, robust barbs, peaceful catfish and other moderate Central Americans. It is a good choice for a hobbyist wanting a less-common Thorichthys that behaves much like the familiar firemouth.
Conservation
The IUCN Red List assesses Thorichthys affinis as Least Concern (assessed 9 August 2018, published 2019). The species is reasonably widespread across the upper Usumacinta lake country of the Petén and into Belize, and is not considered to face species-level threats. Eschmeyer's Catalog of Fishes records its distribution as the Atlantic slope of southern Mexico, Belize, Guatemala and Honduras, a broad range consistent with the Least Concern listing. As with the rest of the genus, the general pressures on the rivers and lakes of the Usumacinta basin — habitat alteration, pollution, water extraction and introduced species — form the backdrop to its status, but none is identified as a present danger to this fish, which remains common within its range.