Taxonomy & naming
Gymnotus capitimaculatus was described by Rangel-Pereira & Müller in 2014 in the journal Vertebrate Zoology, part of a wave of species-level splitting within the genus Gymnotus over the past two decades. It belongs to the Gymnotus pantherinus species-group, a cluster of small-bodied, boldly spotted or barred banded knifefishes from coastal drainages of eastern Brazil. The genus sits in the family Gymnotidae, within the order Gymnotiformes — the New-World electric knifefishes, a group of bony fish defined by an electric organ and a single long anal fin used for locomotion, and only distantly related to any other aquarium fish. The Asian featherback and clown knifefishes (Chitala, Notopterus) share the common name "knifefish" but belong to a wholly different order; the resemblance is convergent, not a sign of kinship.
The species epithet, capitimaculatus, combines the Latin capitis (of the head) and maculatus (spotted, blotchy), referring to the diagnostic pair of round blotches on the underside of the head, beneath the eyes, that separates this species from its close relatives in the pantherinus group. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
Gymnotus capitimaculatus is a slender, elongated banded knifefish, with a body depth of only about 7–9% of its total length — noticeably slimmer than many other Gymnotus. It reaches a modest maximum size of around 6 in total length. The original description separates it from its relatives on a combination of fine anatomical characters: a pair of round blotches on the ventral head surface beneath the eyes (the trait behind its name), a tail patterned with a series of bands or round spots, and a precaudal trunk that lacks bands but carries rounded or vermiculated spots instead. Internally it has 45 precaudal vertebrae and nine scales above the lateral line at mid-body.
Like every Gymnotus, it has no dorsal fin and no true tail fin; movement is powered almost entirely by a long anal fin running beneath the body, undulated in travelling waves that let the fish move forwards or backwards with equal ease. It is scaleless over most functional purposes despite bearing small scales (a Gymnotiformes-wide trait of a smooth, unarmoured skin), and an electric organ runs down the body generating the weak pulse discharge the species uses to sense its surroundings. As in other Gymnotus, this is a pulse-type electric organ discharge (EOD) — distinct from the continuous wave-type discharge of ghost knifefishes (Apteronotidae) and glass knifefishes (Sternopygidae) — used for electrolocation of prey and obstacles in the dark and for intraspecific communication, with each pulse's timing and shape carrying information about species, sex and individual identity.
Habitat
Gymnotus capitimaculatus is known from a single population in the Rio do Ouro, part of the rio Jucuruçu basin, a coastal Atlantic Forest drainage in Bahia state, northeastern Brazil. No other member of the G. pantherinus group has been recorded in that basin or in adjacent drainages, and no other populations of this species are currently known — a genuinely narrow-range endemic. FishBase notes the distribution as a relict, a pattern attributed to historical deforestation across the rio Jucuruçu basin that has fragmented and reduced the freshwater habitat available to it.
As in other small-stream Gymnotus of coastal Brazil, the species likely occupies warm, soft, acidic tropical stream water, sheltering by day among roots, leaf litter and marginal vegetation and becoming active after dark, when its electric sense takes over from vision in the dim, often tannin-stained water typical of these forest streams.
Feeding
Detailed dietary data for Gymnotus capitimaculatus itself has not been published, but the genus is consistently one of nocturnal micropredators, and there is no reason to expect this species differs. Related Gymnotus forage after dark over the substrate and among leaf litter for aquatic insect larvae, small crustaceans such as copepods and ostracods, worms and, in larger species, small fish, locating prey largely by electrolocation rather than sight.
Given its small adult size (under 6.5 in) and its stream habitat among leaf litter and root tangles, G. capitimaculatus most plausibly forages on insect larvae and other small invertebrates hidden in sediment and cover, in the manner documented for its better-studied relatives.
Mating
No courtship or spawning behaviour has been directly observed and published for Gymnotus capitimaculatus. Across the genus and the wider Gymnotidae, however, the electric organ discharge is known to double as a communication channel: its pulse pattern is thought to carry sex- and individual-specific information used in social and reproductive signalling, so courtship in this species almost certainly plays out at least partly through electric exchange in the dark stream water it inhabits, alongside close physical approach.
Work on the closely related G. refugio, another member of the pantherinus group, has documented male parental care around a guarded nest, fractional spawning, and one of the lowest relative fecundities recorded in the Gymnotiformes, with the reproductive period running from late winter through summer. G. capitimaculatus likely shares this general reproductive pattern, though it has not been directly confirmed for the species itself.
Breeding
Breeding has not been documented in aquaria for Gymnotus capitimaculatus — unsurprising for a species known from a single wild population and never established in the hobby. Parental care is, however, a defining trait of the family Gymnotidae, and the pattern seen across Gymnotus generally, and specifically reported for its close relative G. refugio, is a male-guarded nest: the male builds and defends a shelter, often a foam or bubble mass tucked among vegetation, the female spawns into it, and the male then guards the eggs and larvae through to independence. Spawning in these small pantherinus-group species is seasonal and tied to the wet season, with fractional spawning (repeated small batches) and unusually low fecundity for the order.
Given the complete absence of direct observation for this species, and its Critically Endangered status limiting any hands-on research, this account states the likely mode by inference from close relatives rather than as an established protocol — data sparse.
In the aquarium
Gymnotus capitimaculatus is not established in the aquarium trade and, given its Critically Endangered status and single-river distribution, should not be collected or kept outside conservation or research contexts. This account is included for completeness of the genus, not as a stocking recommendation.
Where other small Gymnotus are kept, the general husbandry principles that apply to the genus would apply here too: a scaleless, nocturnal predator needs dim lighting, secure cover such as pipes or root tangles to retreat into by day, and stable, clean, warm water, since scaleless fish are unusually sensitive to ammonia, nitrate and to copper-based medications. As territorial nocturnal hunters, Gymnotus are not safe with bite-sized tankmates and are often intolerant of their own kind in close quarters. None of this is fish available to hobbyists, however; the species' welfare is far better served left in its native Rio do Ouro than in any tank.
Conservation
Gymnotus capitimaculatus is assessed by the IUCN as Critically Endangered, one of the most severe threat categories, reflecting its restriction to a single known population in the Rio do Ouro within the rio Jucuruçu basin of Bahia, Brazil. FishBase attributes its relict, fragmented distribution to historical deforestation across the basin — a familiar pressure on the Atlantic Forest, one of the most reduced tropical forest biomes on Earth, where coastal drainages like the Jucuruçu have lost the great majority of their original forest cover.
With no other population currently known anywhere, the species has essentially no buffer against further habitat loss, water pollution or hydrological change in its one home stream. Its conservation case rests entirely on protecting the Rio do Ouro and the wider rio Jucuruçu basin; there is no aquarium-trade angle to its conservation story, since the fish is not collected or traded.