Taxonomy & naming
Scleromystax salmacis was described in 2005 by Marcelo R. Britto and Roberto E. Reis, from coastal river systems in Santa Catarina, Brazil. It was placed directly in Scleromystax Günther, 1864 — the distinctive south-eastern and southern Brazilian genus of corydoradines whose males typically grow patches of erectile cheek odontodes and diverge sharply from females in colour and finnage. The species epithet is deliberately ironic: it refers to Salmacis, the nymph of Ovid's Metamorphoses who fused with Hermaphroditus into a single dual-sexed being, alluding to this catfish's unusually subtle sexual dimorphism.
Because it was described straight into Scleromystax rather than Corydoras, the 2024 revision of the Corydoradinae by Dias, Tencatt and colleagues — which split the old Corydoras into Brochis, Hoplisoma, Osteogaster, Gastrodermus and others — simply upheld Scleromystax as a valid lineage and left this species untouched. The Catalog of Fishes and FishBase list it as Scleromystax salmacis. It is very close to S. macropterus, differing in pattern, in lacking a black spot at the base of the median caudal-fin rays, in having more pectoral-fin rays, and above all in its near-absent dimorphism.
Morphology
This is a small Scleromystax, reaching about 2 in in standard length (the largest known specimen measured 2 in). It has the compact, deep-bodied, bony-plated form of the corys, a blunt head and three pairs of barbels. The flanks carry irregular, transverse dark markings over a pale ground, somewhat finer and more broken than in its close relative S. macropterus.
Its defining feature is what it lacks: the dramatic sexual dimorphism of the rest of the genus. Whereas in other Scleromystax the male's dorsal and pectoral fins may be two to three times as long as the female's, in S. salmacis the dorsal and pectoral fins are essentially equal in length in both sexes, and the pattern differences are slight — males are at most a touch more slender. The cheek odontodes characteristic of the genus are correspondingly modest. This understated, near-monomorphic appearance is exactly what the name commemorates.
Habitat
Scleromystax salmacis is known from coastal river systems in the state of Santa Catarina, southern Brazil — specifically the rios Mampituba and Araranguá — giving it, at the time of its description, the most southerly range of any Scleromystax. These are clear coastal streams of the southern Atlantic Forest, running over sand and gravel.
This is a subtropical, comparatively cool-water fish. Aquarium experience points to moderate temperatures around 73–82 °F at the warm end of its tolerance, with soft to medium, acidic-to-neutral water (about pH 5.5–7.5). As with the rest of the genus, clean, well-oxygenated flowing water suits it best, reflecting its origin in clear coastal streams rather than warm, still lowland waters.
Feeding
Scleromystax salmacis is a benthic foraging omnivore, like all its relatives. In its coastal streams it sifts sand and litter for small invertebrates, insect larvae, micro-crustaceans and organic detritus, working the bottom with its barbels and venting cleaned grit through the gills. In aquaria it readily takes sinking dried foods and accepts small live and frozen items — chironomid larvae (bloodworm), Tubifex and similar — and does best on a varied diet.
Mating
Reproduction follows the corydoradine norm: an egg-scattering shoaler with no pair bond and the 'T-position' spawning embrace, in which a male aligns at right angles to a ripe female's head, a few eggs are fertilised, and the female carries them in the basket of her pelvic fins to a cleaned surface. What is unusual here is the context rather than the mechanics: because the sexes are so alike, the visual signalling that drives courtship in flashier Scleromystax appears muted, and males lack the exaggerated fins and bristles that advertise condition in their congeners — a striking natural-history counterpoint within the genus.
Breeding
Captive breeding of S. salmacis is little documented, the species being uncommon in the hobby. It can be expected to spawn much as other Scleromystax do — conditioned on a rich, varied diet and triggered by cool, fresh water changes that imitate seasonal rain, laying small batches of adhesive eggs on firm surfaces with no parental care, the eggs best removed from the adults to develop. Cool, clean, well-oxygenated water and small live foods for the fry would be the keys to success, as with its relatives. Detailed, repeatable accounts are still lacking.
In the aquarium
This is an uncommon, specialist's cory rather than a commonly traded one. Keep it as a group of six or more over soft sand, with leaf litter, wood and gentle, well-oxygenated flow, at the cooler, subtropical end of community temperatures. Soft, near-neutral water suits it. It is small, peaceful and unobtrusive, and is best housed in a species set-up or with other small, calm fish that share its preference for cooler, clean water; it should not be combined with boisterous or large tankmates. The usual cory cautions apply: smooth substrate to protect the barbels, and a cup rather than a net for handling. Its appeal is partly biological curiosity — the near-monomorphic 'bristle-cheek' that barely bristles.
Conservation
Scleromystax salmacis has not been assessed by the IUCN and has no Red List status. It is a narrow-range endemic, recorded from just a couple of coastal river systems in Santa Catarina, and such restricted distributions are inherently vulnerable. Its home waters lie in the southern Atlantic Forest, a much-reduced and fragmented biome where coastal and restinga streams are under pressure from clearance, development and water use. The lack of an assessment reflects limited data rather than demonstrated security; a formal evaluation of its range and population status would be valuable.