Taxonomy & naming
Hisonotus megaloplax was described by Carvalho and Reis in 2009 in Zootaxa (number 2113), in a paper revising the genus in the rio Uruguay basin: 'Four new species of Hisonotus from the upper rio Uruguay, with a review of the genus in the rio Uruguay basin'. The species is known only from the headwaters of the rio Passo Fundo drainage, a tributary of the upper rio Uruguay basin in southern Brazil.
The genus Hisonotus sits within the subfamily Hypoptopomatinae, the dwarf armoured catfishes. FishBase and the Catalog of Fishes (Eschmeyer, CAS) list Hisonotus megaloplax as a valid species. The 2009 revision in which it appeared was part of a broader effort to clarify the many small, similar-looking Hisonotus populations of southern South America.
Morphology
Hisonotus megaloplax reaches about 2 in standard length and has the elongate, bony-plated body typical of the genus. The dorsal fin has two spines and seven branched rays; the anal fin has one spine and five branched rays. Its defining feature, and the source of its name, is a series of greatly enlarged lateral abdominal plates that contact one another at the midline of the belly, with the usual median abdominal plate series absent or reduced to a single posterior triangular plate.
The species can be separated from the similar Hisonotus montanus by its nearly uniformly dark caudal fin, which lacks the vertical dark stripes seen in that species, and by the infraorbital sensory canal entering the skull through the sphenotic rather than through a compound pterotic. Sexual dimorphism is not documented in the available data.
Habitat
The species is restricted to the headwaters of the rio Passo Fundo drainage, a tributary system of the upper rio Uruguay basin, at roughly 28 to 29 degrees south in southern Brazil. It is a benthopelagic, tropical freshwater fish of headwater streams. As with other Hisonotus, it is capable of facultative air-breathing, which helps it tolerate the shallow, variable conditions of small upland watercourses. Detailed microhabitat data beyond its headwater association are limited.
Feeding
No species-specific dietary study has been published for Hisonotus megaloplax. The genus comprises aufwuchs grazers that rasp the biofilm of algae, diatoms, bacteria and fine organic matter coating submerged stones, wood and vegetation. The ventral sucker-mouth and fine teeth of these fishes are adapted to scraping this film rather than to wood-eating. Specific dietary information for this species remains sparse.
Mating
No mating or courtship behaviour has been described for Hisonotus megaloplax. The species is known from museum material and field collections, with no documented aquarium observations of reproduction.
Breeding
There are no published breeding records for Hisonotus megaloplax. Its reproductive mode, fecundity, spawning sites and any parental care are undocumented. Small hypoptopomatines often attach adhesive eggs to plant surfaces, but no such data are reported for this species, so no breeding claims can responsibly be made.
In the aquarium
Hisonotus megaloplax is not an established aquarium species. It is a narrow-range endemic that is not part of the ornamental trade, and no husbandry data exist for it. Hobbyists drawn to small grazing loricariids of this kind typically keep Otocinclus or more widely traded congeners, which need mature, well-oxygenated, biofilm-rich tanks with stable water chemistry.
Conservation
The IUCN Red List assessed Hisonotus megaloplax as Near Threatened (NT) under criterion B1b(iii) on 7 November 2018. The assessment reflects its narrow distribution within the headwaters of a single drainage and an ongoing decline in habitat quality, while falling just short of the thresholds for a threatened category. Headwater endemics in the upper rio Uruguay basin face pressures from agriculture, deforestation, dam construction and water-quality decline, any of which could push a restricted species like this toward a higher risk category. CITES and CMS have not evaluated it.