Taxonomy & naming
Amphilius crassus was described by Thomson & Hilber in 2015 in a revision of the Amphilius jacksonii species complex published in Zootaxa. It belongs to the family Amphiliidae, the loach catfishes of Africa, within the genus Amphilius. The holotype (catalogued UF 184237) was collected from the Sonjo River at a bridge in Man'gula, on the road from Mikumi to Ifakara, Tanzania, at an altitude of 991 ft, within the Rufiji River basin.
The species epithet crassus is Latin for "fat" or "stout", a direct reference to the deep, thickset body that sets this species apart from its close relatives in the jacksonii complex. Before the 2015 revision, populations now recognised as A. crassus would have been lumped under the broader A. jacksonii concept; the split was based on body proportions and the notably smaller maximum size of this species relative to its relatives, only 6 of 186 examined specimens exceeding 3 in SL. The Catalog of Fishes treats Amphilius crassus as the currently valid name.
Morphology
Like all members of Amphiliidae, Amphilius crassus is a scaleless, naked-skinned catfish, an adaptation shared with the family's other rock-clinging mountain-stream specialists. Its most distinctive feature is a stout, deep body — body depth at the anus runs about 14.5–17.4% of standard length — markedly stockier than related species in the jacksonii complex, which is exactly what the name crassus describes.
The species is genuinely small: the holotype measured 3.5 in SL and is essentially the largest specimen on record, with the great majority of the 186 specimens examined in the original description falling under 3 in SL. State this plainly to any prospective keeper — this is a dwarf stream catfish, not a fish that will grow into a large tankmate, and there is no size trap to worry about here in the usual catfish sense. As in other Amphilius, a pectoral spine is present, but the family lacks the heavy, aggressively locking spines seen in many larger catfish groups.
Habitat
Amphilius crassus is known from east-central Tanzania, principally the Rufiji River basin (its type locality, the Sonjo River) with records also from the neighbouring Wami River basin. Both are Indian Ocean-draining systems running off the eastern arc of Tanzania's highlands.
It lives in the fast, rocky, highly oxygenated water typical of Amphiliidae: cool mountain and foothill streams with clear water running over stone and gravel substrate. The original description does not report measured temperature, pH or hardness values for the type locality, and no aquarium-water data has been published for this species — as a mountain-stream endemic it should be assumed to need cool, fast-flowing, well-oxygenated conditions rather than the warm, still water many community catfish tolerate; specific figures are not available and should not be guessed at.
Feeding
No feeding study exists for Amphilius crassus specifically, but the family Amphiliidae are characterised as nocturnal benthic omnivores, foraging the streambed after dark for aquatic invertebrates, detritus and small prey items. There is no reason to expect this species to differ from that pattern.
In an aquarium setting, a diet built around sinking pellets, frozen bloodworm and other small frozen or prepared foods offered after lights-out would follow the family's known ecology, though it should be noted no aquarium feeding records for this species have been published — it is not an established trade fish.
Mating
No courtship or spawning behaviour has been documented for Amphilius crassus. The species was described from a series of wild-caught museum specimens collected for taxonomic study, not from aquarium or field breeding observations, and nothing in the literature speaks to how it courts or times spawning in its native mountain streams.
Breeding
Breeding in Amphilius crassus is unrecorded. There are no published aquarium-breeding reports for this species, and it does not appear to be established in the hobby at all, so there is no basis for describing a spawning mode, parental care, or rearing method — stating otherwise would be guessing. As a small, specialised mountain-stream endemic only recently split out from the jacksonii complex, it should be treated as a species whose reproductive biology is simply unknown rather than assigned a generic catfish breeding pattern.
In the aquarium
Amphilius crassus is not a known aquarium trade fish — no records of it being kept or bred in captivity have turned up. What can be said with confidence follows from its biology: it is genuinely small (the largest confirmed specimen is 3.5 in SL), so this is a dwarf species rather than one that will outgrow a modest tank, but it is also a scaleless catfish, which means the same caution about medications applies as with loaches, puffers and eels — dose copper-based and other harsh treatments conservatively, since naked skin is more permeable and more sensitive than scaled fish.
As an Amphiliidae species built for fast, cool, highly oxygenated mountain streams, any attempt to keep it would need to replicate that: strong water movement, high dissolved oxygen, cool temperatures, and a rocky, current-swept substrate rather than the still, warm water of a typical community tank. It should be treated as a specialist fish for an experienced keeper willing to research current, undocumented husbandry from scratch, not a species with an established care routine.
Conservation
Amphilius crassus has not been formally assessed by the IUCN Red List; its status is Not Evaluated. This is unsurprising for a small stream endemic described as recently as 2015, and the absence of an assessment reflects a lack of data rather than any specific evidence of security or decline.
Its known range is narrow, restricted to the Rufiji and Wami River basins of east-central Tanzania, which makes it, by nature, a locally endemic species whose status would benefit from targeted survey work. As with many recently split stream specialists, habitat pressures typical of East African highland catchments, such as deforestation, agricultural runoff and water abstraction, are the kind of threats that would matter to a narrow-range species like this, though no species-specific conservation data has been published.