Taxonomy & naming
Microglanis eurystoma was described by Luiz Malabarba and Cristiano Mahler in 1998 from the upper rio Uruguay drainage in southern Brazil. It belongs to the family Pseudopimelodidae — the bumblebee and dwarf pimelodid catfishes — a group that was long lumped into Pimelodidae before being recognised as its own family; older literature and some aquarium sources may still place related genera under Pimelodidae, but the genus placement Microglanis itself is unaffected by that reshuffling.
The genus Microglanis ("small catfish", from Greek mikros, small, and glanis, an old name for a catfish) contains a couple of dozen small Neotropical species, most of which stay under 2 in. The species epithet eurystoma is Greek for "wide mouth" (eurys, wide + stoma, mouth), a fitting name given that M. eurystoma is described as one of the largest and most robust species in the genus.
Morphology
Microglanis eurystoma has the classic pseudopimelodid build: a broad, somewhat depressed head, a wide mouth, short maxillary and mental barbels, and a stocky, tapering body. The skin is naked — like all catfishes in this family it has no scales, which matters directly for how the fish should be medicated (see "In the aquarium").
At up to about 3 in standard length (roughly 3.5–4 in total length), M. eurystoma reaches a genuinely respectable size for its genus — most Microglanis top out well under that. This is the size trap for this species: it is easy to assume any "micro catfish" genus stays tiny, but this particular species is one of the largest in the group and should be planned for accordingly. Like other pseudopimelodids it carries a strong, serrated, locking dorsal spine along with locking pectoral spines, an adaptation for wedging into crevices and deterring predators.
Habitat
The species is native to the rio Uruguay basin, ranging through its upper, middle and lower stretches in the Brazilian states of Santa Catarina and Rio Grande do Sul, with the drainage's southern reach extending the range into Uruguay itself (first recorded there around 2012). This sits at the southern edge of the genus Microglanis's overall distribution.
Rio Uruguay tributaries are subtropical South American streams and rivers rather than Amazonian blackwater, and Microglanis as a genus is generally kept in soft to slightly acidic water, particularly for conditioning and breeding. Specific published water-chemistry data for M. eurystoma itself is sparse; keepers should default to moderate, stable river conditions rather than extreme soft acidic blackwater parameters.
Feeding
Microglanis eurystoma is a nocturnal benthic micro-predator and omnivore. By day it hides among cover on the substrate; after dark it forages over the bottom for small aquatic invertebrates — worms, small crustaceans and insect larvae — and likely some detritus, consistent with the feeding habits documented across the genus.
In the aquarium it takes small sinking foods readily: live or frozen daphnia, brine shrimp and mosquito larvae, plus standard sinking pellets and prepared foods of a suitable size. Because it is most active after lights-out, offering food in the evening or after the tank has darkened helps ensure it gets fed rather than being out-competed by diurnal tankmates.
Mating
No published account of courtship or spawning behaviour for Microglanis eurystoma was found in the sources consulted. Sexual dimorphism has not been documented for this species specifically. Data is sparse here; this should not be assumed to resemble any particular breeding style until confirmed.
Breeding
Microglanis eurystoma is not reported to be bred in the home aquarium, and no spawning mode has been documented for the species in the sources available. Small pseudopimelodids of this kind are occasionally produced by specialist or commercial breeders, but this cannot be confirmed for M. eurystoma specifically — it is honest to say breeding in captivity is essentially unrecorded for this species rather than to guess at a mode such as egg-scattering or cave-spawning.
In the aquarium
Because M. eurystoma is one of the largest species in its genus, reaching roughly 3.5–4 in total length, it needs more room than the "dwarf catfish" reputation of Microglanis in general would suggest — the size trap to avoid here is assuming it will stay pygmy-sized. A tank in the 23.5 in-plus range with soft substrate, driftwood and plenty of hiding places suits its nocturnal, secretive habits.
Like all pseudopimelodids it is scaleless, so it is more sensitive than scaled fish to copper-based medications and should be dosed conservatively, ideally at reduced strength, if treatment is needed. It also carries stiff, serrated, locking pectoral and dorsal spines that can snag in a net and jab the handler; move it in a cup or container rather than netting it directly. Reportedly hardy and peaceful with similarly sized tankmates, it is not a plant-eater and does well with other calm bottom- and mid-water fish that will not out-compete it for food after dark.
Conservation
The IUCN Red List assesses Microglanis eurystoma as Least Concern. It appears to be reasonably common within its native rio Uruguay drainage, and no specific threats to the species were identified in the sources consulted.
As with many South American stream and river catfish, ongoing pressures on the rio Uruguay basin — including water-quality changes and habitat modification associated with agriculture and development — are the general long-term concerns for species tied to this drainage, even where a Least Concern rating currently applies.