Taxonomy & naming
Ancistomus micrommatos was originally described as Hemiancistrus micrommatos by Cardoso and Lucinda in 2003, in Ichthyological Exploration of Freshwaters (volume 14), in a paper that simultaneously described two sister species — H. spilomma and H. spinosissimus — all from the rio Tocantins basin in Brazil. The type locality is within the Tocantins drainage; precise coordinates for the holotype locality have not been published in sources reviewed here.
In 2015, Armbruster and colleagues resurrected the genus Ancistomus and transferred all three of these Tocantins Hemiancistrus into it, resulting in the current combination Ancistomus micrommatos (Cardoso & Lucinda, 2003). The Catalog of Fishes (Eschmeyer, CAS) and FishBase currently list A. micrommatos as a valid species.
However, a significant caveat applies. Armbruster et al. (2015) explicitly noted that they could find no morphological characters sufficient to distinguish A. micrommatos from A. spilomma and A. spinosissimus. The three names were retained as valid rather than formally synonymised, but the authors' own data raise the real possibility that all three represent one variable species. This question has not been resolved by subsequent taxonomic work as of the sources reviewed here; aquarists and researchers should treat A. micrommatos as a provisionally valid name pending formal revision.
The specific epithet micrommatos comes from Greek: mikrós (small) and ómma (eye), referring to the small, dorsolaterally positioned eyes of this species.
Morphology
Ancistomus micrommatos reaches approximately 4.5 in standard length (SL), making it the smallest of the three species in the micrommatos–spilomma–spinosissimus complex by maximum recorded length, though the biological significance of this difference is uncertain given the potential synonymy.
The body is covered in the loricariid armour of interlocking bony scutes. The eyes are notably small and positioned high on the head in a dorsolateral placement — the defining character acknowledged in the species name. This eye position is characteristic of species that rest on or cling to hard substrates in the water column with their dorsal surface facing upward.
Sexual dimorphism in Ancistomus follows the pattern common to Hypostominae: males develop interopercular odontodes (cheek bristles) and additional odontodes on the pectoral-fin spines, especially during the breeding season. Females in breeding condition are noticeably fuller-bodied, particularly when viewed from above. These dimorphic traits are documented for the related A. spilomma complex and are presumed to apply to A. micrommatos.
Habitat
The known range of Ancistomus micrommatos is within the rio Tocantins basin in central Brazil. The Tocantins is one of Brazil's major rivers, draining a large part of the Cerrado and transitional Cerrado–Amazon zones from Goiás and Tocantins states northward to Pará. The related A. spilomma and A. spinosissimus (which Armbruster et al. consider potentially conspecific with A. micrommatos) are also recorded from the Tocantins and Araguaia systems.
Water chemistry data specific to the collection localities of A. micrommatos are not available in the published literature reviewed here. Based on related species in the same Tocantins tributaries (care notes from Scotcat for A. spilomma), appropriate water parameters are approximately 73–81 °F and pH 6.0–7.5. The Tocantins is subject to significant flow alteration from a cascade of hydroelectric dams, which has modified thermal and hydrological conditions along much of the main channel.
Microhabitat is consistent with the loricariid riffle and rapid guild: rocky substrate, moderate to swift current, well-oxygenated water.
Feeding
Ancistomus micrommatos is assumed to be an omnivorous grazer consistent with the feeding ecology of Hypostominae from rocky Tocantins tributaries. Care notes for the closely related and possibly conspecific A. spilomma describe a diet including vegetarian foods, sinking pellets and tablets, insect larvae, and zooplankton — a broad omnivorous spectrum typical of the Hemiancistrus-alliance loricariids.
In the aquarium, the related species complex accepts high-quality algae wafers, spirulina discs, blanched vegetables (courgette, peas, cucumber, sweet potato), soft wood for grazing, and occasional invertebrate supplements. No specific dietary study exists for A. micrommatos. Given the taxonomic uncertainty around the species' distinctness from A. spilomma, care notes for the latter are likely applicable.
Mating
Reproductive behaviour has not been described specifically for Ancistomus micrommatos. As in other Hypostominae, males presumably establish and defend crevice or cave territories, display interopercular odontodes to rival males and prospective mates, and may engage in brief territorial skirmishes.
Based on the closely related A. spilomma group, males develop prominent odontodes on the pectoral fins and posterior body in breeding condition. A conditioned female is visibly wider than the male when viewed from above and shows a rounded, swollen genital papilla. Peaceful cohabitation with dissimilar species is generally the rule for this size class of Hypostominae; intraspecific male–male conflict can arise in insufficiently furnished tanks.
Breeding
No captive breeding records specific to Ancistomus micrommatos have been located in the available literature. The species is not established in the hobby to a degree that has generated documented spawning accounts.
By analogy with the probable conspecifics A. spilomma and A. spinosissimus — and more broadly with Hypostominae reproductive biology — cave spawning with paternal brood care is the most probable mode. The male would be expected to guard a clutch of adhesive eggs in a narrow crevice or cave, fanning them until hatching. Fry development details and clutch size are undescribed for A. micrommatos specifically.
In the aquarium
Ancistomus micrommatos is not commonly available through the ornamental trade and is unlikely to be encountered by most aquarists under this name. The taxonomic confusion between A. micrommatos, A. spilomma, and A. spinosissimus means that any of these names could in principle apply to the same fish encountered in trade, though all three remain rare imports.
Care guidance from Scotcat for A. spilomma — the most documented of the three putative species — describes a peaceful fish suitable for a community aquarium, with temperature 73–81 °F and pH 6.0–7.5. Given a maximum size of approximately 4.5 in SL for A. micrommatos, a tank of at least 25–30 US gal with rocky structure, hiding places, and moderate current is appropriate.
Diet should be primarily plant-based: algae wafers, spirulina, blanched vegetables, and tablet foods. Soft wood in the aquarium is beneficial both for grazing and shelter. The species is peaceful toward dissimilar tankmates and is compatible with similarly sized, non-aggressive species that share its water chemistry requirements. Multiple individuals can be kept if sufficient territory and hiding spots are provided, though male–male interactions should be monitored.
Conservation
The IUCN Red List assessed Ancistomus micrommatos as Near Threatened in November 2018. This status reflects concern about the species' range within the rio Tocantins basin, which has been heavily modified by hydroelectric development. The Tucuruí Dam, one of the largest in the world, inundated a substantial portion of the lower Tocantins, and numerous additional dams further upstream have altered flow regimes, temperature profiles, and sediment dynamics across the system.
Species from restricted tributaries within such a heavily dammed river basin are at elevated risk even when not yet classified as Threatened. The Near Threatened assessment for A. micrommatos specifically (versus Least Concern for the potentially conspecific A. spilomma and A. spinosissimus) likely reflects differences in the distribution data available for each name at the time of assessment, and the unresolved taxonomy complicates interpretation of these different status designations.
If future taxonomic revision concludes that A. micrommatos, A. spilomma, and A. spinosissimus represent a single species with broader Tocantins–Araguaia distribution, the combined range would present a different conservation picture than the current narrow-range assessment for A. micrommatos alone. Pending that revision, the Near Threatened designation highlights real and ongoing habitat pressure in the Tocantins system.