Taxonomy & naming
Microglanis nigrolineatus was described in 2016 by Terán, Jarduli, Alonso, Mirande and Shibatta, one of a wave of new Microglanis species recognised in the 2010s as the genus was revised across South America. It belongs to the family Pseudopimelodidae, the bumblebee and dwarf pimelodid-relative catfishes, a group that for decades was lumped into the much larger Pimelodidae before being split out on skeletal and other characters. The holotype measures 1.5 in SL and was collected from the Quebrada Colorada, a stream in the Bermejo River basin of Salta Province, Argentina — the species' only documented locality.
The genus name Microglanis combines Greek roots for "small" and "catfish", a fitting label for a genus of diminutive, secretive bottom-dwellers found across South American lowland and piedmont drainages. The species epithet nigrolineatus is Latin for "black-lined", describing the dark line running along the middle of the body from the dorsal-fin origin to the adipose fin. As with many recently split Microglanis, older regional records from the Bermejo basin may have been folded into other species before this species was formally recognised; the Catalog of Fishes treats Microglanis nigrolineatus as the current valid name.
Morphology
This is a genuinely small catfish: published maximum size is around 1.5–2 in SL, with the type specimen measuring just 1.5 in SL. It has the typical pseudopimelodid bumblebee-catfish build — a broad, somewhat flattened head, a stout body tapering to the tail, and long, prominent barbels used to feel out food and obstacles in low light. Like other Siluriformes it carries stiff pectoral-fin spines with short, thorn-like serrations on the leading edge; these lock into the pectoral girdle and can jab a bare hand or snag a net, so this catfish (like nearly all its relatives) is best moved in a container rather than netted.
The skin is scaleless (naked), a trait shared across almost all catfishes, which makes it more sensitive than scaled fish to osmotic stress and to some medications — copper-based treatments in particular should be dosed conservatively. Coloration is the species' most distinctive feature: a dark stripe runs along the middle of the body from the origin of the dorsal fin back to the adipose fin, and a dark brown patch sits just in front of the caudal fin, ending in a bold blotch. The top of the head is uniformly dark. This pattern, combined with the small size, is what separates it from close relatives in the genus.
Habitat
Microglanis nigrolineatus is known only from the Quebrada Colorada, a stream in the Bermejo River basin in Salta Province, northwestern Argentina, at an elevation of roughly 1132–1280 ft above sea level. This is piedmont Yungas habitat — the humid montane foothill forest that fringes the eastern Andes — where the stream runs slow over sandy and muddy substrate through clear water.
The Yungas climate is strongly seasonal, with roughly 80 percent of annual rainfall falling between November and March and a pronounced dry season from June to October; stream flow and conditions likely shift substantially between the two. Recorded water conditions are warm-temperate rather than tropical-blackwater: temperature around 70–77 °F and a neutral-to-slightly-alkaline pH of 7.0–7.5, distinctly different from the soft, acidic blackwater conditions often assumed for small South American catfishes.
Feeding
Microglanis nigrolineatus is a nocturnal, benthic predator. In the wild it forages the stream bottom after dark, and the ScotCat account specifically notes it will take small fish fry — this dwarf catfish is a genuine, if small-scale, predator rather than a passive scavenger, so it should not be trusted with very small tankmates or their offspring.
In the aquarium it accepts a varied diet: sinking tablet and pellet foods, frozen bloodworm, and live or frozen microworms. As with most nocturnal catfish, feeding is best timed for after lights-out so this shy, small species is not out-competed by more active, light-loving tankmates for food.
Mating
No courtship or spawning behaviour has been documented for Microglanis nigrolineatus specifically; the species was only described in 2016 and appears to remain essentially unstudied in captivity. General notes for the genus describe Microglanis as rarely bred in the aquarium, with no established guarding or pairing behaviour reported, and any successful spawning attempts reportedly favouring soft, slightly acidic water rather than the neutral-to-alkaline conditions recorded at the type locality. None of this is confirmed for this species itself, and it should be treated as generic genus-level background rather than a documented account of this fish.
Breeding
Breeding of Microglanis nigrolineatus has not been documented, in the wild or in captivity — data here is sparse to the point of absent. It is not a known bubble-nest builder, cave-spawner, or livebearer; among Pseudopimelodidae and Microglanis generally, egg-laying with no reported parental guarding appears to be the norm, but this has not been specifically confirmed for nigrolineatus. Given the tiny known range and lack of an established aquarium trade, this species should be assumed essentially unbred in the hobby, and any claim to the contrary should be treated with suspicion until it is properly documented.
In the aquarium
This is one of the true dwarf catfishes — adults top out around 1.5–2 in SL — so tank size can be modest, but the fish itself needs the same careful handling as any of its larger relatives. It is scaleless, so treat medications, and copper-based ones especially, conservatively; and it carries locking pectoral spines, so move it in a cup or net-free container rather than a net, both to protect the fish's spines and the keeper's fingers.
Provide fine sandy or muddy substrate, subdued lighting, and plenty of cover — rockwork, wood or dense planting — to suit a small, nocturnal, secretive predator, and expect to see it mostly at dusk and after dark. Water should be kept close to the wild readings of 70–77 °F and pH 7.0–7.5 rather than warmer, softer tropical-community norms. Because it will take fry and very small fish given the chance, keep it only with tankmates too large to be eaten. This species is essentially unavailable in the hobby — it is not a farmed or commonly traded catfish, and anyone offering it should be able to document its origin.
Conservation
The IUCN Red List assesses Microglanis nigrolineatus as Vulnerable (VU). The species is known from a single stream, the Quebrada Colorada in the Bermejo basin, which puts its entire known population at risk from any localised disturbance.
Documented threats are habitat loss driven by agricultural expansion, deforestation, and industrial contamination within the Yungas piedmont — pressures affecting a great many small-range streams along the Andean foothills. No specific conservation actions are documented in the sources available. Because the species is confined to a single known locality, its status depends entirely on the health of that one stream system, making it a strong example of why granular, water-body-level habitat protection matters for South America's least-known catfishes.