Taxonomy & naming
Apteronotus caudimaculosus was described by de Santana in 2003 and belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New-World electric knifefishes. Like all Gymnotiformes it produces a weak electric organ discharge (EOD) used for electrolocation and communication; within the Apteronotidae that organ is built from modified nerve cells rather than muscle, giving the family the fastest, highest-frequency wave-type discharges of any knifefish group.
The original description places A. caudimaculosus within the Apteronotus albifrons species complex, a cluster that includes the familiar black ghost knifefish (A. albifrons) alongside at least six undescribed Apteronotus species sharing a two-band caudal colour pattern. A. caudimaculosus is distinguished from its relatives chiefly by that caudal pattern and by its restricted Pantanal distribution. The Catalog of Fishes lists no junior synonyms, and the name is not to be confused with the unrelated Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to an entirely different order and only resemble gymnotiforms by convergence.
Morphology
As with other Apteronotus, the body is deep and strongly compressed side to side, tapering to a slender tail, with no dorsal fin and no functional caudal fin for steady swimming; propulsion comes from a long anal fin — in this species carrying roughly 146 to 154 soft rays — thrown into travelling waves that drive the fish forwards or backwards with equal facility. The species is scaleless, as all ghost knifefishes are.
Its most distinctive external feature is the tail marking that gives it its name: two light bands encircle the caudal peduncle, and the first of these bands is broken up by irregular dark spots — caudi-maculosus, "spotted tail." The species is sexually dimorphic, with males developing an elongated snout, a trait shared with several other members of the albifrons complex. FishBase records a maximum total length of 11.5 in, notably smaller than the black ghost's near-19.5 in ceiling.
Habitat
Apteronotus caudimaculosus is known from the Pantanal wetlands of the Paraguay River basin in Mato Grosso do Sul, western Brazil, with records from the Rio Novo and Rio do Peixe. The type locality was a small swamp channel with turbid water, a mud-and-sand bottom, moderate current and thick aquatic vegetation — still, shaded, sediment-rich water rather than open river channel.
FishBase classifies the species as freshwater, benthopelagic and tropical. Exact temperature and pH figures have not been published for this species specifically; based on the Pantanal's typical swamp conditions and the tolerances of related ghost knifefishes, warm water in roughly the 73–82 °F range and mildly acidic to near-neutral pH is a reasonable inference, but this should be treated as data sparse rather than measured fact. As in other Apteronotidae, the turbid, dimly lit water suits a fish that navigates primarily by electrolocation rather than sight.
Feeding
FishBase places Apteronotus caudimaculosus at a trophic level of 3.3 ± 0.3, consistent with a carnivorous diet of small aquatic invertebrates and, likely, small fish — the same general feeding mode documented in other ghost knifefishes. As a benthopelagic species in shaded, sediment-heavy swamp channels, it almost certainly hunts by electrolocation, sensing prey hidden in mud or vegetation rather than by sight, though no species-specific feeding study has been published.
Because the species is essentially unknown in the aquarium trade, there is no established record of its feeding behaviour or diet preferences in captivity. Any aquarium account would have to be extrapolated from its better-studied relatives, and that extrapolation should be labelled as such rather than presented as observed fact for this species.
Mating
No courtship behaviour has been documented specifically for Apteronotus caudimaculosus. What is known about mating in the genus comes mostly from laboratory work on the brown ghost knifefish (A. leptorhynchus): the wave-type electric organ discharge carries information about identity, sex and spawning readiness, and males and females shift discharge frequency toward one another during courtship, effectively duetting electrically in the dark. Given that A. caudimaculosus shares the same electric-organ architecture and is sexually dimorphic in snout length, a broadly similar electric courtship is plausible, but this is inference from relatives, not direct observation of this species.
Data on this species' own mating behaviour is sparse to nonexistent in the literature reviewed, and no claim beyond that inference should be made.
Breeding
Apteronotus caudimaculosus is an egg-layer, as all Apteronotidae are, but its breeding biology has not been documented in detail. No spawning has been reported from the aquarium, and the species is essentially absent from the ornamental trade, so there is no hobbyist record to draw on either. Based on the family's general pattern, eggs are likely scattered among vegetation or other cover rather than guarded, with no substantial parental care — but this, too, is inference from related species rather than a confirmed account of A. caudimaculosus itself.
Given how little is published, the honest summary is that breeding in this species is rarely observed and poorly documented, and any aquarist attempting it would be working without an established protocol.
In the aquarium
Apteronotus caudimaculosus is rare to essentially unavailable in the aquarium trade; most sources treat it as a wild Pantanal specialist rather than a hobby fish, and there is no meaningful body of aquarium-keeping experience to report. Where it might appear, the general care needs of ghost knifefishes should apply: it is scaleless, so exceptional care is needed with medications — no copper-based treatments, and reduced doses of anything else — and with water quality, since ammonia and nitrate are poorly tolerated. As a nocturnal, electrolocating predator it would need a dim, well-covered tank with secure hides and should not be trusted with bite-size tankmates after dark.
Because it reaches a comparatively modest maximum length of under 12 in TL, it would in principle need less floor space than the black ghost, but with essentially no trade history, no aquarium water parameters, and no feeding or compatibility data specific to the species, any care sheet for A. caudimaculosus today would be more speculation than fact. This is a fish better understood through its Pantanal habitat than through the aquarium hobby at present.
Conservation
Apteronotus caudimaculosus is assessed by the IUCN as Least Concern, with the assessment dated 2018. FishBase records its fishing vulnerability as low. It is known only from the Pantanal wetlands within the Paraguay River basin of Mato Grosso do Sul, Brazil — a narrow range by comparison with the continent-spanning black ghost knifefish, though the Pantanal itself is one of the world's largest wetland systems and currently supports a stable assessment.
As with other narrow-range Neotropical endemics, the species' long-term security depends on the health of its home basin: the Pantanal faces pressures from agricultural runoff, wetland drainage and periodic severe wildfires, though no species-specific threat has been documented for A. caudimaculosus. It is not targeted by the aquarium trade in any meaningful volume, so collection pressure is not a concern for this species.