Taxonomy & naming
The Panamanian eartheater was described by the Austrian ichthyologist Franz Steindachner in 1876 (in the Ichthyologische Beiträge der Akademie Wien), on a unique holotype from a stream on the 'Isthmus of Panama near Candelaria.' It was the first of what later authors would call the red-humped Geophagus to be formally named — Geophagus pellegrini and the redescription that stabilised Geophagus steindachneri both followed decades later, in the twentieth century. Steindachner himself, however, did not recognise crassilabris as founding a distinct lineage: he placed it in the subgenus Satanoperca, the same catch-all grouping nineteenth-century authors used for several eartheater-like fishes later split into separate genera. That placement stuck for a long time — as late as 1891, Eigenmann & Eigenmann's list of South American freshwater fishes still filed crassilabris under subgenus Satanoperca, alongside Geophagus acuticeps, pappaterra, daemon and jurupari (all now the valid genus Satanoperca after Kullander's 1986 revalidation) and even alongside Chromys lapidifera, the species that became the basal, non-geophagine genus Retroculus. It is a small illustration of how thoroughly tangled these lineages were before modern taxonomy separated Geophagus, Satanoperca and Retroculus into their current, phylogenetically distinct places. The specific name crassilabris is Latin for 'thick-lipped' (crassus, 'thick' + labrum, 'lip'), in reference to the fleshy, enlarged lips that characterise the species. Eschmeyer's Catalog of Fishes lists it as valid as Geophagus crassilabris Steindachner 1876, and FishBase, GBIF and the Catalogue of Life concur. Its generic placement, however, still sits at the edge of the genus: crassilabris belongs to the Geophagus steindachneri species group, the small north-western assemblage (with Geophagus steindachneri and Geophagus pellegrini) whose members are immediate maternal mouthbrooders rather than the delayed biparental brooders of Geophagus in the strict sense — and whose closest relative, steindachneri, went through its own tangled history, having been misidentified by Steindachner as Geophagus brasiliensis in an 1880 paper on the cichlids of Colombia's Cauca basin before Eigenmann & Hildebrand puzzled out its true identity in 1909, formally describing it as Geophagus steindachneri the following year. Because of this history, some authors flag the group as 'Geophagus' in quotation marks, anticipating that it may eventually be split from Geophagus sensu stricto once the genus is fully revised — a caveat worth keeping in mind even though the combination Geophagus crassilabris remains the valid, catalogued name. The species is endemic to Panama and is the only Geophagus, alongside its range-mate Geophagus pellegrini complex on the Colombian side, to occur in Central America at all.
Morphology
Geophagus crassilabris is a moderately large eartheater, reaching about 9.5 in in total length (FishBase), making it one of the bigger members of the steindachneri group. It has the standard Geophagus build — a deep, laterally compressed body, a steep frontal profile and the small, low, strongly protrusible mouth of a dedicated substrate-sifter — but its signature feature is the one the name records: noticeably thick, fleshy lips. Ground colour is a silvery to brassy or greenish-grey overlain with iridescent blue-green spangling on the flanks, cheeks and operculum, a dark mid-lateral spot, and rows of light spots and streaks on the dorsal, anal and caudal fins; breeding and dominant fish intensify the metallic sheen. The most striking dimorphism in the steindachneri group is the male's nuchal hump: dominant males of crassilabris develop a pronounced fatty bump on the forehead, sometimes flushed with red or coppery colour, and grow distinctly larger than females. This sexual size and head-shape difference is functional — these are polygamous mouthbrooders in which large, ornamented males compete for females — and makes the sexes relatively easy to tell apart in mature fish, unlike most eartheaters.
Habitat
The species is endemic to Panama, occurring on both the Pacific and the Atlantic (Caribbean) slopes of the central and eastern part of the country, in drainages such as the Bayano and the rivers of the Darién and the canal-divide region around its type locality. FishBase characterises it as a benthopelagic freshwater fish of warm tropical streams, with a recorded preferred temperature range of roughly 77–86 °F. Unlike the still-water blackwater eartheaters of the Guianas, the Panamanian eartheater is a fish of moving water: it prefers moderate to fast-flowing streams over a substrate of small stones, sand, gravel and rock, among the leaf debris that falls from the forest canopy. This combination of clear-to-tannic running water and a mixed sand-and-stone bottom is the habitat its sifting habit exploits — it works the patches of sand and accumulated leaf litter between the stones. Its range straddles the continental divide of lower Central America, an unusual distribution for a genus otherwise confined to South America, and reflects the relatively recent biotic interchange across the Isthmus of Panama.
Feeding
Geophagus crassilabris feeds in the manner the genus name advertises: it is a benthic substrate-sifter. The fish takes mouthfuls of sand and fine sediment from the stream bed and from accumulations of leaf litter, sorts edible matter — aquatic insect larvae, small crustaceans, worms, seeds and organic detritus — from the inorganic grit over its gill rakers, and expels the cleaned material through mouth and gills. The thickened lips that give the species its name may assist in working the substrate and the leaf-litter packs of its fast-stream habitat, though their precise function is not documented. As with the rest of the genus, the protrusible, down-turned mouth and well-developed gill rakers are the apparatus of a winnowing invertivore-detritivore rather than a predator. Within its stream community it functions as a bottom-feeding generalist, processing the productive interface of sand, gravel and decaying leaf matter.
Mating
Geophagus crassilabris is an immediate, maternal, ovophilous mouthbrooder — the reproductive mode that sets the steindachneri group apart from the rest of Geophagus and from most New-World cichlids. Rather than forming a lasting pair bond, the species is polygamous: large, hump-headed, brightly coloured dominant males hold and display over a site and court multiple females in succession. After a brief courtship the female lays a small batch of eggs and, almost immediately, takes them up into her mouth, where they are fertilised and incubated. From that point the male's role is over; there is no biparental care and no shared defence of a nest. This is the same strategy used by the African mouthbrooding cichlids of the Rift lakes but evolved independently in this small South-and-Central-American lineage — a striking case of convergence. The male's exaggerated nuchal hump and size are the products of this mating system: ornaments and weapons in the competition for the chance to fertilise a female's clutch.
Breeding
Breeding follows the maternal-mouthbrooder pattern of the steindachneri group, which is reasonably well documented in aquaria for the group as a whole even where crassilabris-specific figures are thin. The female lays a comparatively small clutch — on the order of a few dozen eggs, the modest fecundity that goes with intensive oral incubation — and immediately gathers them into her buccal cavity, where they are fertilised (the female may mouth the male's vent or a spawning surface to take in milt) and then carried. She broods the eggs and the developing larvae in her mouth for roughly two to three weeks, not feeding or feeding little during this time, periodically rolling and rotating the clutch to oxygenate it. When the fry are released they are already well developed and free-swimming, and the female may continue to take them back into her mouth for shelter for a few days before they become fully independent. There is no paternal involvement after spawning. This single-parent oral brooding, with its small broods of large, well-provisioned young, is the reproductive hallmark that distinguishes crassilabris and its relatives from the delayed biparental mouthbrooders of Geophagus sensu stricto.
In the aquarium
In its home country the Panamanian eartheater is a familiar fish — taken locally for food and kept by Panamanian and specialist hobbyists — but it is uncommon in the international aquarium trade, where the closely related Colombian 'redhump eartheater' Geophagus steindachneri fills the same niche and is far more widely available. A keeper who obtains crassilabris should treat it much like steindachneri, with allowance for its larger adult size and its preference for moving water.
Give it a spacious tank with a deep bed of fine, soft sand — this is an obligate sand-sifter, and gravel frustrates the feeding behaviour and risks gill abrasion. Reflect the natural fast-stream habitat with good water movement, scattered smooth stones and rounded gravel, driftwood and some leaf litter, and plenty of open sand to forage over. Water should be warm (around 77–86 °F) and clean, near neutral; the species is not a soft-acid blackwater specialist like the Guianan eartheaters but a running-water fish that tolerates a range of moderate conditions, so stable, well-filtered, well-oxygenated water matters more than a precise pH target. Strong filtration and regular water changes are essential for a fish that constantly stirs the substrate, and clean conditions guard against the 'hole-in-the-head' and bacterial complaints that afflict stressed eartheaters. The mating system shapes the social setup: because dominant males are large, humped and can be pushy, keep one male with several females, or a group large enough to spread aggression, rather than a single rival pair of males in a small tank. Feed a varied diet of sinking pellets and frozen or live invertebrates taken off the bottom. Tankmates should be robust but peaceable mid-sized fish that can share running water without competing for the same patch of sand.
Conservation
The IUCN Red List assesses Geophagus crassilabris as Least Concern (assessed 2 August 2019). The species is reasonably widespread across central and eastern Panama on both slopes, occupies a range of stream habitats, and is not dependent on any single narrow locality, which buffers it against localised disturbance. The familiar pressures on lower-Central-American freshwaters nonetheless form the backdrop to its status — deforestation and the resulting sedimentation of streams, agricultural and urban pollution, mining, and the large-scale hydrological alteration of the central isthmus around the Panama Canal watershed near its type locality. None of these is currently documented as causing a population decline in this particular cichlid. It is harvested locally for food and for the aquarium trade but carries no CITES listing and is not considered threatened.