Taxonomy & naming
Distichodus rostratus was described by the German-British ichthyologist Albert Günther in 1864. It belongs to the family Distichodontidae, a lineage of African characiform fishes within the order Characiformes that also includes the deep-bodied, algae-grazing citharinids (Citharinidae) as close relatives, though the two families are now generally kept separate. The genus Distichodus is large and diverse, spanning small, peaceable species through to the much larger, more aggressive plant-shredders such as D. sexfasciatus and D. lusosso; D. rostratus sits toward the larger end of that spread, and the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference for confirming its currently accepted placement within the genus.
The common name "grass-eater" reflects its diet directly — stomach-content studies from the Bandama River in Côte d'Ivoire found grass stems made up roughly half of its wild diet by volume. The species epithet rostratus is Latin for "beaked" or "having a snout," referring to the elongated, somewhat protruding snout that distinguishes it from more blunt-headed relatives in the genus.
Morphology
Distichodus rostratus has the elongated, laterally compressed body typical of the genus, with a small, terminal mouth set on a notably drawn-out, pointed snout — the feature that gives the species its name. FishBase records a maximum published size of 24.5 in standard length, with males reaching up to 30 in total length and a recorded weight of 6.3 kg, making this one of the larger species in a genus that already runs large; aquarium specimens offered in the trade are typically juveniles far smaller than this ceiling, and keepers should plan around the adult figure, not the size at purchase.
Colouration is comparatively subdued next to the bold-barred D. sexfasciatus: the body is generally silvery to olive with a more elongated, streamlined profile than the deeper-bodied members of the genus. The dorsal and anal fins carry the scaled sheath along their base characteristic of Distichodontidae, and the overall build — long body, pointed head, terminal mouth — reads as an adaptation for grazing along submerged vegetation and riverbank grasses rather than for ambush predation.
Habitat
Distichodus rostratus has a wide West and West-Central African distribution, recorded from the Senegal River basin east through the Chad basin, the Nile, and the Cross River, though it is notably absent from the smaller coastal river systems between southern Gambia and the Sassandra River. The genus Distichodus reaches its greatest diversity in the Congo basin, and D. rostratus's range overlaps a broad swath of that West African freshwater ecoregion complex, inhabiting river channels and their vegetated margins rather than still, closed waters.
FishBase gives a temperature range of 72–82 °F and a pH range of 6.5–7.5 for the species — warm, circumneutral West African river conditions rather than the very soft, tannin-stained blackwater associated with some Congo basin fish. As a large-bodied river grazer, it is closely tied to stands of submerged and marginal grasses and vegetation, which supply the bulk of its diet in the wild.
Feeding
Distichodus rostratus is fundamentally herbivorous, with an omnivorous tolerance for other food. The clearest wild dietary data comes from the Bandama River in Côte d'Ivoire, where grass stems accounted for roughly half of stomach contents by volume, confirming the "grass-eater" common name and marking the species as a dedicated vegetation grazer rather than a generalist predator.
In the aquarium this translates into a diet built around vegetable matter: blanched vegetables, spirulina-based sinking pellets and algae or vegetable wafers should form the bulk of feeding, supplemented with occasional protein such as bloodworm or brine shrimp. Genus-mates such as D. sexfasciatus are notorious for shredding live plants in search of greenery, and while D. rostratus is a somewhat smaller, less destructive species, keepers should still expect it to graze on any soft-leaved aquarium plants rather than leave them untouched.
Mating
Little is documented about courtship behaviour in Distichodus rostratus specifically. A reproductive study on the species found a clearly seasonal spawning pattern tied to gonadosomatic index cycles, with size at first maturity estimated at 15.5 in standard length for males and 18 in standard length for females — figures that underline just how large this fish must grow before it is even reproductively mature, well beyond what most home aquaria could accommodate. The same study found a female-biased sex ratio in sampled populations, roughly 1 female to 1.61 males.
No external sexual dimorphism is documented for the species, and there is no aquarium-hobby record of courtship display or pairing behaviour to draw on; what is known comes entirely from field studies of wild, river-dwelling adults rather than from captive observation.
Breeding
Distichodus rostratus is an egg-scattering species with a defined seasonal spawning period in the wild, driven by gonadosomatic cycles rather than a fixed calendar date. Fecundity is high for a river fish of this size, with reported clutches of roughly 81,000 to just over 100,000 eggs, consistent with a broadcast-spawning strategy that relies on producing large numbers of eggs rather than on any form of parental care.
There is no meaningful record of Distichodus rostratus being bred in captivity. Its very large adult size, late maturity (well over 15.5 in standard length even for males), and apparent dependence on seasonal river cues make it an extremely unlikely candidate for home-aquarium breeding, and the honest position for a keeper is that this species should be treated as effectively unbred in the hobby — any specimens in the trade are wild-collected or, at best, produced by specialist commercial operations rather than hobbyist tanks.
In the aquarium
The single most important fact for a prospective keeper is size: Distichodus rostratus reaches 24.5 in standard length (up to 30 in total length in males) and can weigh over 6 kg, and FishBase recommends a minimum tank length of 59 in even before accounting for full adult growth — this is not a fish for a standard home tank, and juveniles sold small will outgrow most setups within a couple of years. Behaviour is reported as unpredictable, and the species is not recommended for heavily planted aquaria, since it will graze on soft vegetation even if it is less destructive than its larger relative D. sexfasciatus.
Diet should be vegetable-forward — sinking pellets, algae and vegetable wafers, with occasional protein — and water should be kept warm (72–82 °F) with a circumneutral pH of 6.5–7.5, matching its West African river origins. Given its size and unpredictable temperament, this is a fish best suited to very large, specialist aquaria or aquaculture-style systems rather than typical community or even large single-species home tanks.
Conservation
Distichodus rostratus has not been evaluated by the IUCN Red List (Not Evaluated), a status shared by many Distichodus species that lack dedicated conservation assessments despite fairly wide distributions. Its range spans a large swath of West and West-Central Africa, from the Senegal basin through the Chad basin, the Nile and the Cross River, which suggests a broad geographic tolerance, but the absence of an assessment means no formal population trend or threat analysis exists.
As with many large-river African fish, the general pressures relevant to its range — damming, water abstraction and habitat degradation along West African river systems — are plausible long-term concerns, though no species-specific data confirms an active decline. Aquarium specimens in trade are understood to be wild-collected rather than farmed, since no captive-breeding programme is documented for the species.