Taxonomy & naming
Tomocichla tuba was described by the American ichthyologist Seth Eugene Meek in 1912 as Cichlasoma tuba, on material from the Atlantic-slope rivers of Costa Rica; Meek (1914) gave the type locality as the vicinity of Victoria, Costa Rica. As with most of the broad, catch-all 'Cichlasoma' of that era, the name was later moved as the heroine cichlids of Middle America were broken into smaller, more natural genera. The genus Tomocichla itself had been erected by Charles Tate Regan in 1908, and the placement of tuba within it has been confirmed by successive checklists — Burgess (2000), Kullander in the Reis et al. (2003) Check List of the Freshwater Fishes of South and Central America, and the comprehensive molecular revision of Říčan, Piálek, Dragová & Novák (2016).
The species sits at the centre of a long-standing tangle. Regan (1908) had also described Tomocichla underwoodi from the Río Iroquois, Costa Rica (syntypes BMNH 1909.3.13.99-108). Because Regan had already used the epithet 'underwoodi' for another heroine (Herichthys underwoodi, 1906), the name underwoodi became a secondary homonym whenever both fish were placed together in Tomocichla. For much of the late twentieth century underwoodi was treated as a valid species — Bussing (1998) listed it as Theraps underwoodi — and many aquarium and field references, including breeding accounts from Lake Nicaragua, used that name. The Catalog of Fishes and the modern revisions, however, treat Tomocichla underwoodi (and Theraps underwoodi) as a junior synonym of Tomocichla tuba: they are the same fish. Anyone reading older literature should treat 'underwoodi' records on the Atlantic slope as referring to Tomocichla tuba.
A further wrinkle concerns the Panamanian populations. Říčan et al. (2016) showed that fish from Bocas del Toro in northwestern Panama, long lumped with tuba, in fact represent the closely related Tomocichla asfraci (described by Allgayer in 2002), so the southern edge of the true tuba range needs reading with that split in mind. Within the heroine radiation of Middle America, Tomocichla is a small genus of riverine, current-loving, largely herbivorous cichlids; its precise sister relationships in the Astatheros/Amphilophus part of the tree have shifted between studies and are best regarded as still being resolved.
Morphology
Tomocichla tuba is a large, elongate, muscular cichlid built for life in moving water — deeper-bodied than a true torpedo-shaped rheophile but more streamlined than the disc-shaped 'Cichlasoma' of still backwaters. Maximum reported size is given as 12 in total length by FishBase (after Conkel 1993), while Bussing (1998) cites about 11 in TL; either way this is a substantial fish, and wild adults of 10 in or more are routine. The base colour is an olive to brassy green-grey, often with a paler belly and a scattering of dark blotches or an interrupted mid-lateral band along the flank; breeding and dominant fish intensify markedly, and the Spanish common names recorded by the IUCN — 'moga verde' (green moga) and simply 'verde' alongside 'tuba' — reflect that green cast.
The most conspicuous secondary-sexual feature is the nuchal hump. Mature males develop a swollen forehead bump that, as Barlow and Siri's work on Central American cichlids showed, swells during courtship and shrinks during parental care, and is filled largely with water rather than fat; in the wild only males carry it and it is thought to function mainly in sex recognition and mate choice (Coleman, Cichlid Room Companion). Males also grow larger and carry somewhat longer dorsal and anal fin extensions; ripe females are rounder-bellied. Outside of breeding condition the sexes are otherwise fairly similar, so the species is best described as weakly to moderately dimorphic, with the male's hump the clearest tell.
Habitat
Tomocichla tuba is an Atlantic-slope (Caribbean-drainage) species of southern Central America. The IUCN range runs from the Escondido River in Nicaragua south through the Caribbean rivers of Costa Rica to the Sixaola and adjacent drainages on the Costa Rica–Panama border and into western Panama. In Costa Rica it is recorded from the Lake Nicaragua, Río Frío, San Juan, San Carlos, Sarapiquí, Tortuguero, Parismina, Matina and Sixaola drainages, at elevations between roughly 0 and 1772 ft above sea level (Angulo et al. 2013). Its estimated extent of occurrence is about 54,0 mi².
This is, above all, a fish of fast water. It typically inhabits lotic (flowing) environments of moderate to high current velocity — the riffles, runs and rocky channels of lowland and foothill rivers — over gravel, cobble and bedrock, in clear, well-oxygenated water (Bussing 1998; Conkel 1993). Recorded water temperatures span roughly 72–91 °F, and FishBase gives a maintenance range of about pH 6.8–8.0 with soft to moderately hard water (dH from about 4 upward), consistent with rivers running off volcanic and limestone terrain. Juveniles favour shallow riffles and the margins of creeks. A telling observation from McCrary et al. (2005) documented the species along the shore of Ometepe Island in Lake Nicaragua — a lake, not a river — which the authors attributed to young fish dispersing beneath drifting rafts of floating vegetation (water hyacinth and the like), a reminder that even a committed river fish can colonise standing water under the right conditions. The IUCN classifies its habitat as permanent rivers, streams and creeks plus, secondarily, large permanent freshwater lakes.
Feeding
Tomocichla tuba is, for a large predatory-looking cichlid, surprisingly herbivorous — FishBase places its trophic level near 2.2, low for the family. Its diet is dominated by plant matter: terrestrial vegetation, aquatic macrophytes and algae, supplemented by nuts, fruit, seeds, aquatic insects and other freshwater invertebrates (Bussing 1975, 1998; Conkel 1993). Much of this is forest subsidy — leaves, fruit and seeds shed from the gallery forest into the river and swept downstream — so the species functions partly as a processor of allochthonous (land-derived) plant material in these fast streams. A specialised behaviour was documented by Romero (1985), who described the fish feeding on petioles (leaf stalks), an unusual dietary niche that shows how thoroughly tuba exploits plant tissue. Juveniles lean more carnivorous, taking aquatic insects in the riffles before shifting toward the adult plant-heavy diet. In its rivers the species is essentially a large grazer-browser, a role more often filled by characins than by cichlids.
Mating
As in most heroine cichlids, Tomocichla tuba forms monogamous pairs that jointly hold and defend a territory through the spawning and rearing cycle. Pair formation is preceded by the usual cichlid repertoire of lateral displays, fin-spreading and colour intensification, and in this species the male's nuchal hump becomes a central signal: it enlarges during courtship and is thought to advertise sex and condition to prospective mates (Barlow & Siri 1997; Coleman, Cichlid Room Companion). Larger size is favoured in both sexes — bigger males defend territory better and bigger females carry more eggs — so pairs tend to be size-assorted. These are robust, assertive fish, and the same strength that suits them to a turbulent river makes them combative at breeding time; pairs become strongly territorial and intolerant of intruders, and ill-matched pairs in confinement can turn on one another. Spawning in the wild is tied to current-swept, structured habitat where a defensible hard surface and adjacent shelter are available.
Breeding
Tomocichla tuba is a biparental, open/substrate-guarding spawner in the heroine mould: the pair cleans a firm surface — rock, cobble or a similar hard substrate in or beside the current — and the female deposits an adhesive sheet of eggs that both parents then guard and tend while fanning fresh, oxygen-rich water over them, with the wrigglers later moved to pits and the free-swimming fry shepherded and defended by both adults. Detailed wild egg counts for the species are not well documented in the accessible literature; congeneric and comparably sized Central American substrate-spawners typically produce clutches on the order of several hundred to over a thousand eggs, and that is the right order of magnitude to expect here. Brood care is prolonged and vigorously biparental — a behaviour that has been filmed and photographed in the wild — and is reflected in the male's hump shrinking once parental duties take over from courtship. McCrary et al. (2005) reported a breeding population in the atypical lake-shore habitat of Ometepe Island, showing the species can complete its cycle outside fast rivers when it has to. The strong current of its native streams keeps eggs clean and oxygenated, and replicating that flow is the key to coaxing the same behaviour in captivity.
In the aquarium
Tomocichla tuba is a specialist's fish rather than a community-tank staple, and it is only occasionally offered in the hobby. Two things drive its requirements: its adult size (approaching 12 in) and its origin in fast, oxygen-rich rivers. A single adult or a pair needs a large tank — realistically 78.5 in of length and several hundred litres (on the order of 180 US gallons / 185 US gal or more for a pair), with serious filtration and strong, well-oxygenated flow. This is one of the relatively few cichlids that genuinely benefits from a 'river-tank' arrangement: powerheads or a closed-loop manifold to generate brisk linear current, a substrate of sand, gravel and water-worn cobble, and robust rockwork or driftwood arranged to break sightlines and provide defensible territories. Keepers who house tuba in calm, under-filtered tanks tend to see poor colour, stress and disease; the current is not optional.
Water should be warm (about 75–86 °F) and run from slightly acidic to alkaline (roughly pH 6.8–8.0), with soft to moderately hard water acceptable — these fish come from a range of volcanic and limestone drainages and are not fussy about exact chemistry so long as it is clean and stable. Given the plant-heavy natural diet, the captive menu should be built around vegetable matter: spirulina- and vegetable-based pellets and flake, blanched greens, peas and similar, rounded out with occasional invertebrate foods rather than a meat-dominated regime. Overfeeding protein to a low-trophic-level grazer invites digestive and water-quality problems.
Temperament is the other watch-point. Tuba is large, strong and seasonally aggressive, especially when breeding; hobbyist accounts describe pairs that are 'very rough on each other' and note specimens kept essentially solo in large systems. Tankmates, if any, should be comparably large, current-tolerant, robust fish that can hold their own — large Central American cichlids, big characins or other sturdy riverine species — never small or delicate tankmates that will be bullied or eaten. Plants will be uprooted or eaten. In short: big footprint, strong flow, vegetable-forward diet, heavy filtration, and respect for a powerful, territorial temperament.
Conservation
Tomocichla tuba is assessed by the IUCN Red List as Least Concern (assessed 17 June 2019 by T.J. Lyons of the ABQ BioPark, published 2020). The assessment cites a reasonably wide distribution across the Atlantic slope of Nicaragua, Costa Rica and Panama, an estimated extent of occurrence of about 54,117 mi², and the species' status as common in fast-flowing Caribbean rivers of Costa Rica, including within several protected areas (for example the Tirimbina Biological Reserve). Total population size and trend are listed as unknown, and the species carries no CITES listing. The fish is taken in subsistence and artisanal fisheries through much of its range and is occasionally collected for the aquarium trade, but neither is flagged as a population-level threat.
The pressures the assessment does note are the diffuse, habitat-level ones typical of lowland Central American rivers: pollution and runoff from banana, pineapple and plantain plantations (pesticides, herbicides and nutrient loading), municipal and sewage wastewater, sedimentation and land-use change from agricultural expansion, and the spread of introduced fishes such as tilapia (Oreochromis spp.) and suckermouth armoured catfishes (Hypostomus spp.). These are inferred to degrade habitat quality continuously, but with no documented range-wide population decline, the species is not presently considered at risk. As a current-dependent river specialist, however, it is precisely the kind of fish whose fortunes track the health of its watersheds, so ongoing monitoring of those drainages is the sensible safeguard.