Taxonomy & naming
Osphronemus exodon was described by Tyson R. Roberts in 1994, in a paper published in the Natural History Bulletin of the Siam Society (vol. 42: 67–77). The type specimen — a holotype measuring 18 in standard length — was collected from the Mekong basin, and the species is known from Thailand, Laos, and Cambodia. Eschmeyer's Catalog of Fishes records the valid name as Osphronemus exodon Roberts, 1994.
The genus Osphronemus belongs to the family Osphronemidae and contains just four recognized species of giant gouramis, all large, plant-feeding fishes of mainland and insular Southeast Asia. Within the genus, O. exodon is distinguished by a diagnostic and immediately recognizable character: teeth that are externalized — projecting outward beyond the lip margins — an extraordinary dental arrangement unique within the group. The three other species (O. goramy, O. laticlavius, and O. septemfasciatus) lack this trait entirely.
The specific epithet exodon is Greek, from exo- (outside) and odon/odous (tooth), a direct reference to this protruding dentition. The species has no single settled vernacular name in the aquarium hobby; 'elephant ear gourami' and 'exodon giant gourami' are both in use, the former alluding to the broad, rounded pectoral and pelvic fins that overlap when extended.
Morphology
Osphronemus exodon is a large, robust fish. Roberts (1994) gives a holotype of 18 in SL, and FishBase lists the maximum recorded size as 23.5 in SL — placing it among the largest of the giant gouramis and among the largest labyrinth fishes. The body plan is typical for the genus: deep and laterally compressed, with a rounded head, a large mouth, and long, filamentous pelvic fins that are a hallmark of the family.
The defining diagnostic feature is the externalized teeth. Where other Osphronemus carry their teeth inside a normal oral cavity, O. exodon has teeth that protrude visibly beyond the lips, creating an almost tusk-like appearance unique among osphronemids. Roberts (1994) recorded 14–16 dorsal spines and 10–11 dorsal soft rays, with 11–13 anal spines and 17–19 anal soft rays.
Juveniles are strikingly patterned, with 6–7 complete vertical bars crossing the body against a pale ground colour and an orangish-red lateral stripe. Adults lose much of this banded patterning and become more uniform in colouration — a developmental shift also seen in O. goramy. The labyrinth organ, present in all osphronemids, sits in a chamber above the gills and allows the fish to breathe atmospheric air directly, enabling survival in warm, oxygen-poor water.
Habitat
Osphronemus exodon is a Mekong basin species, recorded from Thailand, Laos, and Cambodia. FishBase characterizes it as potamodromous — undertaking seasonal migrations within fresh water — and notes that it inhabits the pools of large rivers as well as flooded forests during the rainy season, when rising waters inundate the surrounding lowland forest. Roberts (1994) specifically records the fish feeding on fruits of Crataeva nurvala, a flowering tree whose branches overhang backwater habitats along the Mekong system.
Such flood-pulse ecology is central to the species' biology: floodplain inundation unlocks a vast, seasonal food resource in submerged and overhanging vegetation, and the fish's deep body, powerful jaws, and externalized teeth appear to be adaptations for processing tough plant material and fruits. As a labyrinth fish, O. exodon can exploit the warm, seasonally stagnant or low-oxygen conditions of floodplain pools that would exclude obligate gill-breathers.
Water conditions in the Mekong lowlands are typically soft to moderately mineralised with a mildly acidic to neutral pH. Seriously Fish reports water chemistry of approximately pH 6.5 and 75 °F (75 °F) at 5 dGH for this species, consistent with lowland Mekong conditions.
Feeding
Osphronemus exodon is primarily herbivorous. FishBase records the diet as mainly plant matter — fruits, leaves, and flowers — supplemented with insects and crustaceans. Roberts (1994) provides a precise note on wild foraging: fish were recorded consuming fruits of Crataeva nurvala dropping from branches overhanging Mekong backwaters, demonstrating the species' exploitation of allochthonous (terrestrially derived) food inputs during flood season.
The externalized teeth may be particularly adapted to grasping and processing tough fruits and fibrous vegetation — a dietary specialism that sets this species apart from its congeners, although all Osphronemus are broadly plant-feeders. Like O. goramy, the fish almost certainly also takes invertebrates and organic detritus opportunistically.
In captivity, Seriously Fish recommends a primarily vegetable diet: dried vegetable-based pellets, blanched spinach, lettuce, cucumber, and tomato. Some frozen or live foods (such as prawns or earthworms) round out the diet. Adequate vegetable bulk is important for both health and behaviour, as a fish of this size denied suitable plant material may turn destructive in a planted aquarium.
Mating
Osphronemus exodon has not been documented breeding in captivity, but spawning behaviour is presumed to follow the pattern established for its congener O. goramy, from which it is behaviourally similar. In O. goramy, the male constructs a large, floating nest built from plant stems, roots, and other fibrous vegetation bound together near the water surface or among emergent plants — the giant-gourami expression of the bubble-nest that characterizes the broader osphronemid family.
Male giant gouramis are territorial around the nest and engage in courtship displays toward females prior to spawning. The female is typically chased away once eggs have been deposited, and nest-guarding falls to the male. The externalized teeth of O. exodon may play a role in male–male competition or territory defence, though this remains speculative.
The species' potamodromous migrations into flooded forest at high water suggest that natural spawning is likely tied to the seasonal flood pulse, when shallow, vegetated inundation zones provide both nest-building material and the sheltered, warm conditions required for egg incubation.
Breeding
No confirmed captive breeding of Osphronemus exodon has been reported in the literature. FishBase notes that the species builds a small nest of leaves and plant roots and that one parent guards the eggs and early young — a pattern consistent with the genus's general approach of parental investment by the male after the female departs.
For O. goramy, the closely related congener for which captive breeding is documented, the male assembles a large platform of plant material at the surface, the female deposits buoyant eggs that rise into the nest, and the male maintains and defends the nest through egg incubation and into early larval stages. Incubation in giant gouramis is typically completed within 24–36 hours at warm temperatures, and fry are free-swimming within a few days. Should O. exodon become available in significant numbers for captive trials, a large aquarium with abundant floating plant material and surface calm would be essential — the nest-building impulse requires space and material.
Given the IUCN Vulnerable assessment and the species' apparent rarity in the hobby, captive breeding represents both a conservation and a husbandry opportunity that remains largely unexplored.
In the aquarium
Osphronemus exodon is an extreme rarity in the aquarium trade. At a potential 23.5 in SL, it requires enormous facilities — Seriously Fish recommends a minimum of 180 US gal (approximately 72 × 24 × 24 inches) for an adult, and even that figure assumes a single specimen or compatible pair in a species-only or large-fish arrangement. No planted aquarium can contain an adult giant gourami without the plants being consumed; bare or lightly decorated setups with robust filtration are the practical norm.
Water chemistry should reflect the species' Mekong lowland origins: soft to moderately hard, mildly acidic to neutral water at 75–82 °F (75–82 °F). As a labyrinth fish O. exodon must have unobstructed access to the surface at all times; the area above the waterline should be kept warm and humid to protect the labyrinth organ from cold air drafts, which can be fatal. Filtration must cope with the waste output of a large, heavily feeding fish, but surface turbulence should be moderate — the fish is not a fast-water species.
Compatible tankmates, if any are attempted, must be large enough not to be threatened by the fish's size and powerful dentition and must be robust enough to withstand periodic territorial behaviour. Most keepers housing giant gouramis of any species maintain them alone or with other very large fish in a species-appropriate biotope display.
Conservation
Osphronemus exodon is assessed as Vulnerable (VU) under criterion A2ce on the IUCN Red List, most recently evaluated in 2024. The iNaturalist / IUCN sources also note a stricter national assessment: the species is listed as Endangered (EN) in Thailand, reflecting the more acute pressures at the northern margin of its range.
The primary drivers of decline are consistent with those affecting large Mekong fish generally: overfishing for food (O. exodon, like O. goramy, is a food fish of significant size), habitat loss through floodplain modification, and the disruption of seasonal flood-pulse dynamics by dam construction along the Mekong mainstream and its tributaries. These dams alter or eliminate the seasonal inundation of floodplain forests that the species depends on for foraging and likely for spawning. A potamodromous species reliant on seasonal flood cycles is particularly exposed to the hydrological changes that large dams impose.
The species has a relatively restricted distribution compared to the more widespread O. goramy, and this limited range increases its vulnerability to basin-wide threats. Conservation of functional floodplain connectivity and habitat in the lower and middle Mekong remains the most significant lever for the species' long-term prospects.