Taxonomy & naming
Mastacembelus plagiostomus was described by Hubert Matthes in 1962 from Lake Tanganyika. It belongs to the family Mastacembelidae — the spiny eels — within the order Synbranchiformes, which also contains the swamp eels (Synbranchidae). This placement underscores what the fish is not: it is not a true eel. True eels belong to the entirely separate order Anguilliformes; the spiny eels share only a body shape with them, and that similarity is the product of convergent evolution on an elongate, burrowing way of life.
African members of the mastacembelid family were historically parcelled into several genera — Aethiomastacembelus, Caecomastacembelus and Afromastacembelus — and this species was long known under the name Caecomastacembelus plagiostomus. The Catalog of Fishes (Eschmeyer, CAS), the authority for valid names, now places it in the genus Mastacembelus, and that generic name is used here. The Lake Tanganyika Mastacembelus form a species flock of at least a dozen taxa that evolved in the lake's isolated rift waters; M. plagiostomus is one of the more distinctive shallow-water members of that group.
Morphology
Mastacembelus plagiostomus reaches a maximum recorded standard length of around 12 in, making it a medium-sized spiny eel. The body is greatly elongate and sinuous, tapering toward the tail, and is effectively scaleless to the touch — the tiny embedded scales are concealed under a coat of protective mucus. The snout is drawn out into a fleshy, tubular proboscis with paired tubular nostrils at its tip, a structure used to probe among rocks and substrate for prey. Along the back ahead of the soft dorsal fin runs a row of short, stiffened dorsal spines — the defining feature of the family — and there is no swim bladder, a benthic adaptation that keeps the fish pressed against the rocky lake floor.
The colour is described as cream to salmon, overlaid with a series of dark brown to near-black saddle bands that cross the back and extend partway down the flanks, producing a banded pattern that breaks up the outline against the dappled light of rocky shallows. Like all members of the family the fish lacks an obvious division between the dorsal and caudal fins, the soft-rayed dorsal merging into a continuous fringe around the tail. There is no reliable external character for sexing the fish; ripe females may appear fuller-bodied across the abdomen.
Habitat
This species is endemic to Lake Tanganyika, one of the African Great Rift Lakes, and occurs along the lake's shallow rocky shoreline from roughly 3°S to 9°S latitude. It is recorded from the rocky littoral zone and rubble slopes in shallow water, where it shelters among crevices, under boulders and in the spaces between rocks during the day and emerges to forage at night.
The water of Lake Tanganyika is chemically unlike the soft, acid lowland rivers that most spiny eels inhabit. It is permanently hard and strongly alkaline — pH typically 8.0–9.5, hardness 8–25°dH — warm (73–82 °F), well-oxygenated and exceptionally clear. These rift-lake conditions are the only environment M. plagiostomus has ever known, and any aquarium housing it must replicate them faithfully; placing this fish in the soft, neutral or acid water appropriate for South-East Asian spiny eels would be a serious error.
Feeding
Mastacembelus plagiostomus is a carnivore that forages along the rocky lake bed, using its sensitive fleshy proboscis to probe crevices and substrate for prey. In the wild the diet is understood to include small fish, crustaceans such as prawns, and bottom-dwelling invertebrates; in the aquarium it accepts prawns, cockle, mussel, earthworms and bloodworm, with live and fresh-frozen meaty foods generally preferred over dried or pelleted items.
Like all spiny eels it is largely nocturnal, spending daylight hours wedged into rocky refuges and becoming active after dark. Feeding is therefore most effective in the evening, and newly imported fish that refuse food in daylight will usually eat readily once the lights are out. It is a deliberate, bottom-oriented feeder and should not be expected to compete at the surface or mid-water for food.
Mating
Mastacembelus plagiostomus is a spiny eel, and spiny eels are egg-scattering spawners — not true eels, and therefore not catadromous migrants. It breeds in fresh water and undertakes no marine migration. In nature, spawning in Mastacembelidae is generally thought to be triggered by seasonal cues such as changes in water level, temperature or rainfall; in the stable, rift-lake environment of Tanganyika, seasonal changes in thermocline depth and littoral conditions may serve equivalent roles.
Captive observations of this particular species are scarce. In related species, a courting pair have been observed circling and entwining close to the substrate before scattering adhesive eggs among vegetation, roots or rocky surfaces. The adults form no lasting pair bond and show no territory defence around the eggs once spawning is complete.
Breeding
Mastacembelus plagiostomus, like most spiny eels, is rarely bred in the home aquarium. Where spawning has been documented in the family, adhesive eggs are scattered among fine plants or deposited against rocks and substrate, after which neither parent remains to guard or tend them. There is no parental care of any kind — no nest, no guarding, no mouthbrooding — and the tiny larvae that hatch after a few days must fend for themselves from the outset.
Replicating the hard, alkaline Tanganyika water chemistry in a dedicated breeding set-up is an additional challenge beyond those faced with soft-water spiny eels. Because captive spawning of this species has not been reliably documented in the hobbyist literature, most specimens in the trade are wild-caught from Lake Tanganyika. Buying this fish knowingly means committing to an animal that can live a decade or more in appropriate conditions.
In the aquarium
Mastacembelus plagiostomus is an unusual choice in the spiny-eel hobby precisely because of its Tanganyika origin: it demands the hard, alkaline, oxygen-rich water of the rift lake rather than the soft, warm lowland conditions most spiny eels prefer. A dedicated Lake Tanganyika biotope aquarium — or at minimum a hard-water community — is the appropriate home. The chemistry target is pH 8.0–9.0, hardness around 10–20°dH, and temperature 73–81 °F, with vigorous filtration to maintain the high oxygen levels the fish expects.
Like all mastacembelids it is scaleless, sensitive to copper-based medications and intolerant of poor water quality. A soft sand substrate is strongly preferred; coarse gravel damages the unprotected skin and can cause infections. The fish needs caves, crevices and rocky overhangs to shelter in during the day, and the tank must be fitted with a completely sealed lid — spiny eels are notorious escape artists that can squeeze through surprisingly small gaps and will leave an open tank and perish on the floor. Feed meaty items after lights-out. It can be kept with other Tanganyikan cichlids and robust invertebrates of similar size, but its nocturnal hunting means small fish or shrimp tankmates are at risk.
Conservation
Mastacembelus plagiostomus is assessed as Least Concern on the IUCN Red List (2006), reflecting a stable population across the rocky shoreline of Lake Tanganyika at the time of assessment. The lake itself is the species' only habitat, and any significant deterioration in lake conditions — sedimentation from deforestation on catchment slopes, pollution, overfishing, or climate-driven changes in thermal stratification — would affect the entire endemic fish fauna including this eel.
Lake Tanganyika is shared by four nations and faces cumulative anthropogenic pressure, particularly near centres of human population along the western and northern shores. While no specific population decline has been documented for this species, its endemic range makes it inherently vulnerable to any threat that degrades the lake's littoral zone. The aquarium trade collects it in modest numbers alongside the more commercially prominent Tanganyika cichlids, and this should be monitored as international interest in Tanganyika oddities grows.