Taxonomy & naming
Charax caudimaculatus was described by Carlos A. S. de Lucena in 1987, with the type locality recorded as Laguna Chica, Reserva Nacional de Tambopata, Madre de Dios, Peru (12°50'30"S 69°17'30"W). The valid name as listed in Eschmeyer's Catalog of Fishes (CAS) is Charax caudimaculatus Lucena, 1987. The species belongs to the subfamily Characinae — the glass-chars and their allies — a group of predatory characins characterised by specialised dentition and a compressed body form.
A revisionary treatment by Menezes and de Lucena (2014) examined the species in detail, reconfirming its meristic and morphometric diagnosis and extending its known range to include other sites in the Madre de Dios and Madeira basin. Charax is the largest genus in Characinae, and C. caudimaculatus is diagnosed from its congeners by a combination of characters: orbital diameter 25.0–28.5% of head length, snout length 20.3–22.8% of head length, lateral line complete with 51–56 perforated scales, scale rows from dorsum to lateral line 16–17, scale rows from anal fin to lateral line 9–13, and the anal-fin origin positioned anterior to the dorsal-fin origin — along with the absence of ectopterygoid teeth.
The Lucena and Menezes 2003 checklist (in Reis, Kullander and Ferraris, eds., Freshwater Fishes of South and Central America) confirmed maximum length and distributional data for the species within the Madeira basin context.
Morphology
Charax caudimaculatus reaches a maximum recorded standard length of about 4 in (approximately 3.8 in), though the type series examined by Menezes and de Lucena (2014) included specimens from 1–5 in SL, suggesting growth in excess of 4.5 in SL in the largest individuals. The body is elongate and moderately deep, strongly compressed laterally — a build shared across Charax and adapted for swift, precision strikes at prey.
The fin formula is consistent with the genus: dorsal fin ii,9; anal fin with iv or v (usually iv) unbranched rays followed by 46–54 branched rays — the long anal fin being a Characinae hallmark; pectoral fin i,13–15; pelvic fin i,7; caudal fin 10/9. Dentition is the species' defining feature: the premaxillary carries canine-like teeth followed by conical teeth (9–12 total), the maxillary bears conical teeth (58–78), and the dentary echoes the premaxillary pattern with anterior canines and posterior conical teeth — all adapted for gripping slippery prey.
Coloration in preserved specimens is pale to light yellow, darker along the dorsum. A dark blotch marks the humeral region, and a bold, diamond-shaped dark blotch on the caudal-fin base gives the species its name (Latin: cauda, tail; macula, spot). Pectoral and pelvic fins are hyaline with scattered dark chromatophores; caudal and anal fins are hyaline. Live coloration likely includes a silvery or yellowish flush, as seen in related congeners.
Habitat
Charax caudimaculatus is native to the Madeira River basin of western South America, with the type locality in the Madre de Dios drainage of southeastern Peru and additional records from Parque Nacional Manu (also Madre de Dios). FishBase records the species' distribution as the Madeira River basin broadly. The Madre de Dios is a major Amazonian tributary draining the eastern slopes of the Andes in Peru and Bolivia before entering Brazil, where it joins the Madeira.
As a benthopelagic freshwater species, C. caudimaculatus inhabits the main channel and flooded-forest margins typical of lowland Amazonian drainages, frequenting the middle and lower water column where it can pursue prey. Specific microhabitat data are limited for this species; by analogy with congeners in similar Amazonian drainages, it likely favours areas with moderate cover — submerged wood, root mats, and vegetated margins — that afford concealment for an ambush predator. The Tambopata type locality in southwestern Amazonian Peru sits within tropical rainforest at relatively low elevation.
Feeding
Charax caudimaculatus is a piscivorous and invertivore predator. The dentition — prominent canine-like teeth on both premaxillary and dentary, backed by numerous conical teeth on the maxillary — is clearly adapted for seizing and holding prey: smaller fish, large aquatic invertebrates, and terrestrial insects that fall onto the surface. This is the feeding guild shared across Characinae, where species in the genus Charax are regarded as sit-and-lunge predators rather than active pursuit hunters.
No specific gut-content studies have been published for C. caudimaculatus, but the general Charax feeding mode is well characterised in the broader literature on Characinae. In aquaria, similar species in the genus accept live small fish and large invertebrates; frozen food and pellets may be taken after acclimation, though individuals vary.
Mating
No published observations of mating behaviour exist specifically for Charax caudimaculatus. By inference from the reproductive biology of Characinae and closely related Charax species, the species is almost certainly an egg-scattering open spawner with no parental care — the reproductive mode that predominates across Characidae. Males and females likely aggregate during seasonal spawning peaks tied to rising water levels and the associated environmental cues of the Amazonian flood pulse.
Courtship in predatory characins typically involves lateral displays between males competing for female access, with spawning occurring among fine-leaved vegetation or in open water over the substrate. Adhesive or semi-adhesive eggs would be scattered and then abandoned. Data on clutch size, egg diameter, incubation time, and larval development are not available in the published literature for this species.
Breeding
Charax caudimaculatus has not been bred in the aquarium under documented conditions, and specific breeding data do not appear in the accessible literature. Based on the biology of related Characinae, captive spawning would likely require a large aquarium with soft, slightly acidic to neutral water, a significant water-change trigger to simulate the onset of the rainy season, and live feeder fish or large invertebrates to bring the adults into breeding condition.
Egg-scattering characins typically require removal of adults after spawning to prevent egg predation. Fry of predatory characins are themselves predatory from an early stage and require live food — small nauplii or micro-worms initially, progressing quickly to larger live prey. Until the species is more commonly kept and bred, keepers would need to rely on the broader Charax and Characinae literature for guidance.
In the aquarium
Charax caudimaculatus is essentially unknown in the ornamental fish trade and is not a commercially available aquarium fish. It is encountered only rarely in ichthyological collections and has not been the subject of any published husbandry accounts. Any specimens reaching aquarists would almost certainly arrive as wild-caught bycatch in broader South American shipments.
Should specimens become available, the keeper would need to cater to the species' piscivorous nature: a large aquarium (at minimum 47 in), warm tropical temperatures (around 75–82 °F), and a diet of live or frozen meaty foods. The prominent canine dentition means it should not be housed with any fish small enough to be prey. Water chemistry in the Madre de Dios drainage is typically soft and slightly acidic, consistent with Amazonian lowland conditions. Given the size of the largest recorded specimens (to at least 4.5 in SL), a 50–55 US gal aquarium would be a minimum for a single adult.
Conservation
Charax caudimaculatus was assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed on 15 January 2021. The LC listing reflects a distribution across the Madeira River basin large enough to buffer against localised threats, with no evidence of rapid population decline meeting threshold criteria for a threatened category.
The Madre de Dios and broader Madeira basin face ongoing pressures from deforestation, agricultural expansion, and artisanal gold mining, which increases turbidity and mercury contamination in river systems. However, the species' benthopelagic, mid-basin lifestyle and occurrence across a large drainage likely confer resilience. It is not a target species for the ornamental trade, removing that pressure. Continued monitoring of Amazon-basin ichthyofaunas and habitat condition across the Madeira watershed remains important for detecting any future changes in status.