Taxonomy & naming
The yellowhump eartheater was described by the British ichthyologist Charles Tate Regan in 1912, in a short paper on new South American cichlids in the British Museum (Annals and Magazine of Natural History, series 8, volume 9, pages 505–507). The type locality is the Río San Juan at Tadó, in the Chocó Department of northwestern Colombia. The name has been treated as valid as Geophagus pellegrini ever since; Eschmeyer's Catalog of Fishes records a lectotype (BMNH 1910.4.11.89) and two paralectotypes in the British Museum and the Paris museum, the lectotype having been designated by Gosse in his 1976 revision of the genus. In that same 1912 paper Regan described a second Colombian eartheater, Geophagus hondae, from Honda on the Magdalena — a name later judged conspecific with Geophagus steindachneri, the redhump eartheater, by Gosse (1976).
The placement of pellegrini within Geophagus is unsettled, which is why careful authors and dealers write the genus in quotes as 'Geophagus' pellegrini. When Sven Kullander rediagnosed Geophagus in 1986 he restricted the genus to species sharing a particular swim-bladder anatomy (paired caudal extensions lined by epihaemal ribs) and a specific vertebral arrangement; several lineages that did not fit, including the brasiliensis group and the steindachneri group, were left as informal assemblages awaiting a formal home. pellegrini is one of just three species placed in the steindachneri group, alongside Geophagus steindachneri itself and Geophagus crassilabris, all tied to the trans-Andean drainages of northwestern South America and lower Central America. Within the eartheater subfamily Geophaginae, the morphological work of Kullander (1998) and the molecular analysis of López-Fernández et al. (2005) both place this assemblage in the species-rich 'Geophagini' radiation rather than near the true Amazonian Geophagus, but the group's exact rank and relationships remain genuinely open.
Morphology
This is a medium-sized eartheater, reaching a maximum of roughly 5–6 in standard length (FishBase lists 6 in SL); total length, including the tail, runs somewhat longer. The body is the moderately deep, laterally compressed form typical of the genus, with a gently sloping forehead, a smallish protrusible mouth set low on the head, and the long-based dorsal and anal fins of a benthic browser. The defining feature in the steindachneri group is sexual: mature males grow conspicuously larger than females, carry longer trailing lobes on the soft dorsal and anal fins, and — most strikingly — develop a fatty nuchal hump on the forehead that in well-conditioned dominant males can swell dramatically. Females stay smaller, plainer and hump-less.
Colour is what separates pellegrini from its near-identical relatives in the trade. Where Geophagus steindachneri shows the bright red blotch on the snout and gill cover that earns it the name redhump, pellegrini wears an overall yellowish to brassy-gold cast across the head, hump and upper flanks, broken by iridescent blue-green speckling on the cheeks and fins and by faint vertical bars or a mid-flank smudge that the fish can intensify or wash out with mood. A Río Pepe collection is reported to run a deeper orange-red with yellow-green blotches on the back and dorsal fin, a geographic variant distinct from the more typically yellow Río Tirado fish. Because the differences are subtle and the fish are often sold young and uncoloured, pellegrini has historically reached aquaria mislabelled as Geophagus steindachneri or even Geophagus crassilabris; the yellow tone of an adult male is the most reliable field mark.
One field report is worth flagging rather than folding in outright: Ad Konings, collecting live fish in the Río San Juan drainage, described large wild males of 10–12 in and females of 6–8 in with humps 'much bigger than we had ever seen in Geophagus steindachneri' — figures that would make this the largest fish in the steindachneri group by a wide margin and sit far above FishBase's 6 in SL maximum, which is itself built mostly from aquarium and smaller field-collected fish. The discrepancy is large enough that it should be read as an open question about the upper size this species reaches in remote, lightly fished stretches of its range, not as a revision of the documented maximum.
Habitat
Geophagus pellegrini is endemic to the wet trans-Andean rivers of northwestern Colombia, recorded from the Atrato, San Juan and Baudó basins of the Chocó and adjacent Antioquia. These drainages fall within the freshwater ecoregion FEOW calls the North Andean Pacific Slopes — Río Atrato (ecoregion 301): a band of short, steep, independent rivers running off the Cordillera Occidental and the coastal Serranía del Baudó, most discharging into the Pacific while the Atrato itself drains north into the Caribbean through the Gulf of Urabá. It is one of the rainiest regions on Earth — parts of the Chocó receive 9,000–12,0.5 in of rain a year — and the fauna is highly distinct, with species-level endemism exceeding seventy percent.
Weidner frames pellegrini's range against the neighbouring Magdalena basin, home to its close relative Geophagus steindachneri: the Rio Magdalena runs some 1,0 mi through a single cordillera-bounded valley, geologically young — formed, like the African rift valleys, by a sinking of the Earth's crust — and consequently more mineral-rich than most South American blackwater systems, with hardness reaching about 10° dGH and pH sitting in the neutral zone. That Magdalena valley is 'famous' for its red-humped eartheaters, above all steindachneri itself, and both of its close relatives occur there too — but Weidner notes pellegrini is found more on the trans-Andean side, the narrow Pacific-slope coastal strip separated from the Magdalena by the Cordillera Occidental. That trans-Andean, rain-soaked Chocó habitat, softer and more acidic than the Magdalena's, is the one this species primarily occupies.
Field collections describe pellegrini as something of an ecological generalist rather than a specialist of one biotope. A 1989 sampling trip took it from a narrow stream barely 10 ft wide with shallow (to about 3 ft), clear, moderate-to-fast water over gravel, mud, clay and submerged logs fringed by water lilies and dense marginal forest; from a wider forest stream of 7–16 ft over mixed sand and gravel; and from a very shallow stream, around 19.5 in deep, that widened into a 20-m pool over sand and rock under overhanging grasses. Across these sites it shares the water with poeciliids (Priapichthys), other cichlids ('Cichlasoma' atromaculatum, Andinoacara latifrons, Andinoacara biseriatus), and a host of characids including the well-known rainbow tetra, Nematobrycon palmeri. Water there is soft and acidic: FishBase gives a pH range of about 5.5–6.0 and temperatures of 77–86 °F, and the documented stream depths place the fish in a shallow band of roughly 2–3 ft. A direct field reading from one such tributary, informally called Río Tirado for the nearby village, recorded pH 6.8, water temperature 82 °F, and a very low conductivity of 24 µS/cm — hardness too low to register on the meter used — with shaded air temperatures of 84 °F rising to 99 °F in direct sun, a useful snapshot of just how dilute and thermally buffered this forest water is.
Adults were seen swimming in small, loose groups of around ten individuals over open stretches of stream bottom, while smaller, younger fish foraged singly among the stones and roots rather than schooling — a size-structured social pattern consistent with the polygynous mating system described below.
Feeding
Like the rest of its group, pellegrini is a benthophagous sand-sifter — the behaviour that gives all these fish the name 'eartheater'. It takes mouthfuls of the bottom substrate and winnows them in the buccal cavity, swallowing edible particles and ejecting the cleaned sand and grit through the gill openings and mouth. Stomach contents of wild fish are a generalist's mix: small aquatic and terrestrial invertebrates, plant material such as seeds, organic detritus, and incidental sediment. FishBase estimates a low trophic level near 2.3, consistent with an omnivore working the bottom rather than a predator. The mouth is small even in adults, so the fish cannot manage large prey; in the wild it grazes the sediment more or less continuously, rising into the water column only opportunistically when food drifts past. Functionally it is one of the bottom-turning, sediment-processing members of these Chocó stream communities.
Mating
The steindachneri-group eartheaters do not form the lasting biparental partnerships of many Amazonian cichlids. Geophagus pellegrini is a maternal mouthbrooder with, at most, a weak and temporary pair bond struck only for the act of spawning. The system is effectively polygynous: a large, humped, brightly coloured male holds a loose territory and will court and spawn with several ripe females in succession where enough are present, as FishBase notes for the species ('males often spawn with many females at the same time if there is a surplus of them'). The male's exaggerated secondary sexual characters — his size, his trailing fin lobes and above all his swollen yellow nuchal hump — are display gear in this competitive, female-choice arena. Outside of spawning the fish is, by cichlid standards, notably peaceful; serious aggression is largely confined to rival males and to the brief, intense business of courtship rather than the prolonged nest-guarding territoriality seen in substrate-spawning relatives.
Breeding
Breeding follows the ovophilous maternal-mouthbrooding pattern of the steindachneri group, and is among the more straightforward in the eartheater clan. Courtship opens with the male stationing himself in front of the female, fins erect, shaking his whole body — behaviour that can shade into what looks like the male 'vacuum-cleaning' the bottom with his mouth. The female then selects a spawning site, typically a stone or a root, and cleans it of algae and sediment before the pair circles it together in the usual cichlid manner. Spawning itself proceeds a few eggs at a time — the female lays just one to three eggs per pass over the cleaned site rather than a continuous strip — and fertilisation is essentially instantaneous: the male releases milt over the eggs at the same moment the female takes them up into her mouth, a technique reported as similar to that of the closely related Geophagus steindachneri. From that point the male's role is essentially over. The female alone broods the developing eggs and then the larvae in her buccal cavity for about 14 days before releasing free-swimming fry of roughly 0.5 in, which are immediately able to take newly hatched Artemia nauplii and Cyclops; she continues to offer them shelter in her mouth for some days afterward. Clutches in this group are modest — on the order of a few dozen to perhaps a couple of hundred eggs, scaling with the size and condition of the female — the trade-off these fish accept for the security of oral brooding over the larger, exposed clutches of open or cave spawners. In aquaria the species will spawn readily once a dominant male has coloured up and ripe females are present; a sandy substrate, a few flat stones and stable, soft, warm water are the main requirements, and the carrying female is best left undisturbed or moved gently to a quiet tank if the fry are to be reared in number.
In the aquarium
Geophagus pellegrini was for a long time rare in the hobby, with most fish sold under the name reaching tanks mislabelled as Geophagus steindachneri or 'crassilabris'; it has become more reliably available in recent years, often offered by collection locality (for example 'Rio San Juan'). It is a rewarding, comparatively peaceful eartheater, but it is not a fish for an unprepared tank. A single specimen or a group needs a base footprint of at least about 120 by 17.5 in (roughly 65–80 US gal); more room is better, both because adult males grow substantial and because keeping a group with a surplus of females spreads the breeding male's attention and reduces harassment.
The single non-negotiable item of decor is a soft, fine sand bed deep enough for the fish to sift naturally. Gravel or sharp substrate defeats the point — it inhibits the browsing behaviour, can abrade or be packed into the gill filaments, and may be swallowed. Beyond that, a dim, tannin-stained set-up with driftwood, scattered roots and a couple of flat water-worn stones (which double as spawning sites) suits the species and its Chocó-stream origins. Leaf litter, authentic as it is, tends to be shredded into a haze of suspended debris by the constant sifting and is better left out of the display.
Water quality matters more than exact chemistry. These cichlids are sensitive to deteriorating conditions and to swings in parameters, and are prone to head-and-lateral-line erosion and stunting if maintenance lapses, so they should only go into a biologically mature tank kept on heavy filtration (canister and/or sump) and large weekly water changes of 50–70 percent. Filtration should be set to trap the fine particles the fish stir up while avoiding strong current — these are not white-water fish. Target soft, acidic, warm water: about 77–86 °F and pH in the 5.0–6.5 range mirrors the wild, though the species tolerates somewhat harder, more neutral water than blackwater Amazonian eartheaters and can be kept and bred without extreme softness as long as stability is maintained.
Feed a varied diet of high-quality fine-grade prepared foods with regular small live or frozen items — bloodworm, brine shrimp, Tubifex, mosquito larvae — and a vegetable component such as spirulina; several small meals a day suit the browser's metabolism better than one large feed. Tankmates are easy to choose because, outside of spawning, pellegrini ignores anything it cannot swallow: peaceful mid- and upper-water dithers such as larger tetras, and other calm, similarly-sized cichlids, work well, while boisterous or fin-nipping species are poor company. The main behavioural caution is intraspecific: a dominant breeding male can pester females and bully rival males, so give the group space and cover.
Conservation
The IUCN Red List assesses Geophagus pellegrini as Data Deficient, assessed on 8 October 2014 (published 2016) by Villa-Navarro, Mesa-Salazar and Sanchez-Duarte, with an unknown population trend. The Data Deficient category reflects a genuine shortage of information rather than evidence of safety: the species is endemic to a restricted set of Chocó drainages, but its population size, trend and the specific pressures on it have not been quantified. It carries no CITES or CMS listing and is harmless to people. The fish is taken for both the local commercial and the ornamental-aquarium trades, though at a level the assessment does not flag as a population threat.
The broader context is the Chocó itself. This is one of the wettest and most biologically distinctive corners of the Neotropics, with a freshwater fauna more than seventy percent endemic, and it faces mounting pressure from deforestation, sediment loads from land-use change in the Atrato and San Juan watersheds, and — acutely in this region — alluvial gold mining and its associated mercury contamination and channel destruction. None of these is documented as a present, species-specific danger to the yellowhump eartheater, but as a narrow-range endemic of exactly these rivers it is more exposed to habitat degradation than the wide-ranging eartheaters of the Amazon, and its Data Deficient status is best read as a call for the basic survey work that would let its status be judged at all.