Taxonomy & naming
Microglanis cottoides was described by the Belgian-British ichthyologist George Albert Boulenger in 1891. It belongs to Pseudopimelodidae, the bumblebee and dwarf marbled catfish family, a group of small, scaleless, big-headed pimelodid relatives that was only formally separated from Pimelodidae in the early 2000s; older literature and some aquarium references may still place species like this one loosely among the "pimelodids" in a broader sense. The Catalog of Fishes treats Microglanis cottoides as the currently valid combination, and it is not to be confused with the similarly patterned M. malabarbai, a distinct species described more recently from the Uruguay River basin that was previously misidentified as M. cottoides in that drainage.
The genus name Microglanis combines the Greek mikros, "small", with glanis, an old Greek word for a catfish — literally "small catfish", a fitting name for this genus of diminutive pseudopimelodids. The species epithet cottoides means "cottus-like" or "sculpin-like" (from the genus Cottus plus the Greek suffix -oides, "resembling"), a reference to the broad, flattened, sculpin-like head shape typical of the species.
Morphology
Microglanis cottoides has the classic pseudopimelodid build: a wide, somewhat flattened head, a stout body that tapers toward the tail, and three pairs of long, sensitive barbels used to feel out food and obstacles in low light or murky water. Like all catfish in this family it is completely scaleless, its skin bare rather than plated or covered in bony scutes.
Adults reach roughly 2.5–3.5 in standard length in most accounts, with the species topping out under 4.5 in SL even at maximum size — a genuinely small catfish rather than one that quietly outgrows its tank. Coloration is a mottled, marbled brown-and-cream pattern that provides good camouflage against gravel and leaf litter, in keeping with the "dwarf marbled catfish" name sometimes used for the family. Distinguishing features used in taxonomic work include head depth and the number of lateral-line pores, which runs roughly 7–13 (typically around 11) in this species.
Habitat
This species is native to coastal river drainages of southern Brazil, from around São Paulo state south to Rio Grande do Sul, with its type locality in the Camaquã River, a tributary of the Laguna dos Patos basin. It is a freshwater, demersal (bottom-dwelling) fish of tropical to subtropical streams and cannot tolerate saltwater. Populations once attributed to it in the Uruguay River basin are now recognised as the separate species M. malabarbai, narrowing the confirmed range of true M. cottoides to the Laguna dos Patos coastal system.
In these streams it lives among submerged debris, leaf litter, rocks and root tangles on the bottom, sheltering there through the day. The cache notes the species is not especially demanding about water chemistry in captivity, but its wild home is warm-temperate to subtropical running freshwater rather than a stable, mineral-rich lake system.
Feeding
Microglanis cottoides is a nocturnal, benthic micro-predator and omnivore. In the wild it forages along the stream bed after dark for small aquatic invertebrates — worms, small crustaceans and insect larvae — using its barbels to locate prey it cannot see.
In the aquarium it takes readily to live and frozen foods such as daphnia, brine shrimp and mosquito larvae, alongside standard sinking pellets and other prepared foods once settled in. Because it is most active at night, offering food after the lights go out helps ensure this shy little catfish gets its share rather than being out-competed by more visible daytime tankmates.
Mating
Detailed accounts of courtship and spawning behaviour for Microglanis cottoides are sparse in the available literature. What is documented from aquarium keepers is that spawning is rare and poorly observed in captivity, with softer, slightly acidic water conditions cited as more conducive to reproductive activity than the harder water the species otherwise tolerates day to day. As with many small pseudopimelodids, courtship details such as pairing behaviour or triggers have not been well characterised; this account does not speculate beyond what is reported.
Breeding
Breeding of Microglanis cottoides in the home aquarium is uncommon and only loosely documented — this is not a species with a reliable, repeatable home-breeding protocol the way some catfish groups are. Reports describe eggs being deposited among aquatic plants rather than in a bubble nest or a guarded cave, with softer, slightly acidic water suggested as helpful for encouraging spawning. There is no confirmed account here of dedicated parental care such as egg-fanning or fry-guarding.
Given the sparse data, aquarists should treat any successful spawn as opportunistic rather than something to plan around; providing fine-leaved plants and stable, slightly soft-to-neutral acidic water is the best-documented approach, but this remains a fish that is far more often kept than bred.
In the aquarium
Microglanis cottoides is a genuinely small catfish — adults stay under about 3.5–4.5 in SL — so it does not need the outsized tank that many long-whiskered cats eventually demand, but it still wants a mature aquarium with soft substrate and plenty of cover: driftwood, rockwork, leaf litter and dense planting to shelter in through the day. The cache reports it as unfussy about water chemistry, tolerating typical tap-water parameters, with a comfortable temperature range around 72–82 °F.
Being scaleless, like other members of its family, it is more sensitive than scaled fish to copper-based medications and should be dosed conservatively if treatment is ever needed. It does not carry the heavy locking spines of some larger catfish, but standard care still applies: net gently or move it in a container to avoid stress and injury. It is strictly nocturnal, hiding by day and emerging to forage after the lights go down once acclimated, so patient keepers will see far more activity from a red-light check at night than during the day.
Conservation
The IUCN Red List assesses Microglanis cottoides as Least Concern, most recently reflecting an assessment dated 16 November 2020. It is a small-bodied species restricted to coastal river drainages of southern Brazil, but within that range it is not currently considered under significant threat.
As with many small, narrow-range South American stream fish, ongoing pressures such as habitat degradation, water pollution and land-use change in its coastal Brazilian catchments are the long-term factors worth watching even though the current assessment is not concerning.