Taxonomy & naming
Mormyrus longirostris was described by the German naturalist Wilhelm Peters in 1852. It belongs to genus Mormyrus, one of the largest genera in family Mormyridae, itself part of order Osteoglossiformes, the bonytongues — a deep-time radiation named for a toothed bone on the floor of the mouth that bites against toothed plates on the roof of the mouth. Catalog of Fishes lists Mormyrus longirostris as the currently accepted name.
The genus name Mormyrus derives from Greek mormyros, an old name for an unrelated marine sea bream, later reapplied by early taxonomists to the African elephantfishes for a passing resemblance in outline. The species epithet longirostris combines Latin longus, "long", with rostrum, "beak" or "snout", describing the elongated, gently curving snout that is the fish's most obvious feature and the source of its dolphin-mormyrid nickname.
Morphology
Mormyrus longirostris is one of the larger mormyrids, reaching a maximum recorded total length of about 29.5 in, though aquarium specimens are typically much smaller. The body is elongated and laterally compressed, tapering to a slender caudal peduncle and a deeply forked tail typical of the family. The head carries the long, downward-curving snout that gives the species its common names; the small mouth sits well back on its underside, positioned for probing rather than grabbing prey in open water.
Colour is a plain silvery-grey to olive-brown over the body, without the bold banding seen in some smaller mormyrids. As in all Mormyridae, an electric organ sits in the caudal peduncle just ahead of the tail, producing a weak, pulse-type electric-organ discharge that the fish reads through electroreceptors spread across its skin — a sense used to navigate and locate prey in dark or turbid water. Mormyrids as a group also carry one of the highest brain-to-body mass ratios of any fish, with an enlarged cerebellum devoted in large part to processing this electric sense.
Habitat
Mormyrus longirostris has a wide range across southern and eastern Africa: the Zambezi, Sabi (Save) and Ludi rivers, the linked Luapula–Mweru–Bangweulu system connecting the upper Congo and Zambezi drainages, the Ruvuma and Rufiji rivers of Tanzania, and the rift lakes Malawi, Tanganyika and Rukwa. Across that range it turns up in flowing river stretches as well as lake margins, generally over sand or soft sediment where its long snout can work undisturbed.
Water across this range is warm and generally soft, running from slightly acidic to close to neutral. Hobby care guides put a workable range at roughly 75–82 °F and pH 6.0–7.5 over a soft, sandy substrate — conditions under which an electric sense is genuinely useful for finding food and other fish when visibility is poor.
Feeding
Mormyrus longirostris forages by probing sediment and vegetation with its long snout, locating buried and hidden prey through its electric sense rather than by sight alone. FishBase records its wild diet as aquatic weeds, insects, small vertebrates and fish eggs — a mixed, opportunistic bottom diet that fits a nose-down feeding style rather than active pursuit.
In the aquarium it is carnivorous and takes readily to bloodworm and other meaty frozen foods once settled. As with other mormyrids, feeding is best offered after lights-out or with the tank dimmed, since these are shy, largely nocturnal fish that are reluctant to compete for food in the open under bright light.
Mating
Little is documented about the courtship of Mormyrus longirostris specifically. As in other mormyrids, the electric-organ discharge is thought to play a role in species recognition and courtship, since each species' pulse pattern differs enough that fish can in principle distinguish potential mates and rivals by their electric signature — but the behavioural detail for this species has not been worked out. Spawning in the wild is believed to follow seasonal rains and rising floodwaters, as in many African freshwater fish, rather than occurring year-round.
Breeding
Mormyrus longirostris is essentially unbred in the aquarium hobby, and even its wild reproductive biology is poorly known. Like most mormyrids it is thought to be a seasonal, flood-triggered spawner, and some members of the family are believed to scatter eggs among vegetation with no elaborate nest, but clutch size, incubation time and whether either parent guards the eggs have not been documented for this species in the accessible literature. No reliable published account of a captive spawning exists, and this should not be treated as a fish anyone can plan to breed.
In the aquarium
Mormyrus longirostris is a large, long-lived, and genuinely demanding aquarium fish, not a starter mormyrid. Adults can reach roughly 29.5 in, and reported lifespan runs 6–10 years, so a tank able to house a fish of that scale for the long haul is a serious commitment; the small setups sold for stubby-nosed "elephantnose" type fish are not adequate for this species once grown. Give it a soft sand substrate it can probe undisturbed, dense planting or other cover for a shy, largely nocturnal fish, and strong filtration and aeration, in soft water around 75–82 °F and pH 6.0–7.5.
As a scaleless fish it is markedly more sensitive to copper, salt and many standard medications than scaled tankmates, so treatment needs conservative dosing and, ideally, prevention through good water quality rather than cure. Reported social behaviour is mixed: keepers describe the fish doing best either alone or in a genuinely large group, since pairs or trios of two or three tend to squabble and settle into a pecking order that stresses the subordinate fish. It does not fit a small community combination — keeping it well is closer to keeping a large, electrically sensitive predator than a typical community oddball.
Conservation
The IUCN Red List classifies Mormyrus longirostris as Least Concern, reflecting its wide distribution across multiple river systems and rift lakes in southern and eastern Africa and the absence of evidence for a significant population decline. It is not a species of particular conservation concern at present, though its longer-term outlook, like that of most African freshwater fish, depends on the health of the rivers and lakes it inhabits — water abstraction, sedimentation and pollution pressures affecting the Zambezi and East African rift-lake systems more broadly.