Taxonomy & naming
Amphilius jacksonii was described by the Belgian-British ichthyologist George Albert Boulenger in 1912, with a type locality in the Hima River of the Lake George drainage, Uganda. It belongs to the family Amphiliidae, the African loach catfishes, and to the genus Amphilius, a group of small rheophilic (current-loving) catfishes distributed across tropical Africa. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Amphilius jacksonii as a valid name in this combination — it has not been shuffled through other genera the way many catfishes have.
What has changed is the species' scope. A 2015 revision by Thomson and colleagues examined the wider "Amphilius jacksonii complex" and split it, describing five new species from populations that had previously been lumped under this name. Amphilius jacksonii sensu stricto is now understood more narrowly, restricted to the Lake Edward drainage, the Kagera basin feeding Lake Victoria, and the Rufiji and Rufugu river systems. Records under this name from outside that range should be treated with caution, since some may belong to the newly split species instead.
Morphology
This is a small catfish, reaching a maximum of about 6 in total length, with a body plan clearly shaped by life in fast water. The head and trunk are noticeably depressed (flattened top to bottom) rather than compressed side to side, which lowers the fish's profile against the current. The pectoral and pelvic fins are expanded, with an enlarged first ray, and the paired pelvic fins are broadened enough to function as a feeble sucking disc that helps the fish hold station on rock in flowing water. The eyes are set high and directed upward, an adaptation for a fish that spends its life pressed against the substrate looking up rather than out.
The dorsal fin has no spine and seven soft rays; the anal fin likewise lacks a spine and carries roughly 8 to 11 soft rays, with 31 to 33 vertebrae recorded for the species. Like other Amphiliidae, Amphilius jacksonii has no scales — the skin is bare — and its gas bladder is reduced, a further current-adapted trait that limits buoyancy and keeps the fish close to the bottom. Colouration is a variable, heavily mottled pattern of browns and greys, giving rise to the common name "marbled mountain catfish"; the pattern likely helps it disappear against rock and gravel.
Habitat
Amphilius jacksonii is a hillstream specialist, found in clear, fast-flowing highland streams over rocky substrates, often at elevations above roughly 3,500 feet within its upper river drainages in the Lake Edward, Lake George, Kagera and Rufiji systems of Uganda, Kenya, Tanzania, Burundi and Zambia. It is strongly rheophilic: it needs swift current and the high dissolved oxygen that comes with it, and it is not a fish of quiet pools or slow lowland rivers.
Precise water-chemistry data for this species is sparse, but its highland, fast-water habitat points to cool-to-warm tropical stream conditions rather than the warm, still, mineral-rich water of a rift lake or the soft blackwater of a South American floodplain. What is well established is the physical habitat itself: clear, well-oxygenated, current-swept water over clean rock and gravel, with little silt.
Feeding
Amphilius jacksonii is a nocturnal benthic micro-predator, foraging over the streambed mainly after dark for aquatic invertebrates. Its diet centres on aquatic insect larvae — chironomid (midge) larvae feature prominently — along with mayfly nymphs (ephemeropterans), small worms and other invertebrates picked from among the rocks and gravel of its fast-flowing home.
In the aquarium this diet translates readily to sinking pellets, frozen bloodworm and other frozen foods, and most prepared sinking foods. Because the species is nocturnal, food is best offered after the lights go out, so this retiring bottom-dweller is not out-competed by more active tankmates during the day.
Mating
Detailed observations of courtship and spawning in Amphilius jacksonii have not been published, and nothing about mate selection or spawning behaviour can be stated with confidence from the available literature. What is known concerns the reproductive pattern of the species rather than the act of mating itself: females are iteroparous, spawning repeatedly rather than once, with asynchronous ovarian development — meaning eggs at different stages of maturity are present at the same time, consistent with more than one breeding period across the year. The precise timing of breeding seasons in the wild has not been pinned down.
Breeding
Breeding of Amphilius jacksonii has not been reported in the aquarium hobby, and captive breeding data for the species is sparse to non-existent. Given its strict requirement for strong current and high oxygenation, it is likely that few, if any, hobbyists have attempted to replicate suitable spawning conditions in a tank. As with many small rheophilic catfishes from specialised habitats, this is a species best regarded as effectively unbred in captivity — any fish in the trade should be assumed wild-collected rather than farmed, and prospective keepers should not expect to breed it without first solving the demanding flow and water-quality requirements its biology implies.
In the aquarium
Amphilius jacksonii tops out at a modest 6 in, but its care demands are anything but modest: this is a hillstream specialist that requires strong, continuous water movement and high oxygenation, essentially a powerhead- or wavemaker-driven torrent tank over rocky, minimally silted substrate. It is not a candidate for a still-water community aquarium, however small the fish itself. A single specimen likely needs on the order of 20 US gallons (about 20 US gal) or more once adequate flow and turnover are accounted for, though species-specific tank-size guidance is sparse and current requirements may push that figure higher in practice.
Like other Amphiliidae, this catfish is scaleless, so it is medium-sensitive to copper and to some medications — treat conservatively and consider a half-dose approach when medicating a tank that holds it. No locking or venomous spines are documented for this species, so handling risk is lower than for many spined catfishes, but as a benthic, nocturnal fish it should still be moved gently and given rockwork and cover to retreat into during the day. Its calm, non-aggressive temperament means it poses no threat to tankmates, but it will only thrive alongside other current-loving, well-oxygenated-water species — not general community fish.
Conservation
The IUCN Red List assesses Amphilius jacksonii as Least Concern. It occupies a reasonably broad range of highland drainages across Uganda, Kenya, Tanzania, Burundi and Zambia, and is not currently considered at elevated risk of extinction at the species level.
That said, the 2015 taxonomic revision that narrowed the species' recognised range to the Lake Edward, Kagera and Rufiji systems is a reminder that conservation assessments for this species complex may need updating as the newly split species are evaluated individually. Hillstream specialists like this one are also inherently vulnerable to habitat pressures that degrade fast, clean, well-oxygenated water — siltation from deforestation and agriculture, damming, and pollution of highland catchments — even where the broader species is not yet flagged as threatened.