Taxonomy & naming
Moenkhausia guaruba was formally described by Flávio C.T. Lima, Vitor A. Vita, Guilherme O. Dutra, Willian M. Ohara, and Manoela M.F. Pastana in 2023, with the description published in Zootaxa. The holotype, 4 in standard length, was collected from the rio Braço Norte, a tributary of the rio Teles Pires in the rio Tapajós basin, on the Serra do Cachimbo plateau of Pará state, Brazil.
The species belongs to the genus Moenkhausia, a large and taxonomically complex assemblage within the family Characidae containing approximately 100 described species distributed across tropical South America. The new species is diagnosed within the genus by a combination of characters unusual in its congeners: 43–46 scales in the longitudinal series (higher than the 23–41 reported for most congeners), 21–25 branched anal-fin rays, a single distinct humeral blotch, a rounded and symmetrical caudal-peduncle blotch, and conspicuous yellow coloration on all fins. No synonyms are currently recorded and the name remains as originally proposed.
Morphology
M. guaruba is among the largest members of its genus, with the holotype measuring 4 in standard length. The body form is typical for larger Moenkhausia: moderately elongate and compressed, with a relatively large eye set in a blunt head. The diagnostic scale count of 43–46 in the longitudinal series and the high branched anal-fin ray count of 21–25 distinguish it metrically from most congeners.
The most immediately striking feature is the coloration. All fins — dorsal, caudal, pectoral, pelvic, and anal — carry an intense yellow pigment, giving the fish an overall golden-finned appearance in life. The body itself bears the typical Moenkhausia markings: a single humeral blotch on the shoulder region and a rounded, symmetrical blotch on the caudal peduncle. The combination of large size, high lateral-line scale count, and saturated yellow fins sets M. guaruba apart from all other described species in the genus.
Habitat
The known range of M. guaruba is restricted to the upper rio Braço Norte headwaters on the Serra do Cachimbo plateau in southern Pará, Brazil, within the broader rio Tapajós drainage. Type-locality records describe the habitat as crystal-clear water, with the upper reaches characterised by rocky substrates and waterfalls, while areas further downstream shift to sandy beaches.
The Serra do Cachimbo plateau is recognised as an endemism hotspot for freshwater fishes, and the physical isolation of its headwater streams — cut off from lowland drainages by escarpments and rapids — promotes the evolution and retention of endemic taxa. All available evidence suggests M. guaruba is restricted to this small headwater system, making it highly sensitive to any disruption of its limited habitat.
Feeding
No published dietary data specific to M. guaruba have been recorded in the source literature available at the time of writing. Moenkhausia species are generally omnivorous opportunists, consuming small aquatic invertebrates, insects, plant material, and particulate organic matter in the water column and at the surface; the diet reflects what is seasonally available in the Neotropical stream habitats they occupy.
Given M. guaruba's relatively large body size compared with many congeners, it can be expected to take larger prey items than the smallest Moenkhausia species. In the clear, rocky, and sandy headwaters of the Braço Norte, invertebrates associated with rock surfaces and leaf litter are likely important food sources. Aquarium observations on the diet of captive individuals are not yet available in the published literature.
Mating
No specific reproductive observations for M. guaruba in the wild or in captivity have been published. Members of the family Characidae are overwhelmingly egg-scattering open spawners with no parental care, depositing small adhesive eggs among fine-leaved aquatic plants, roots, or over the substrate.
By analogy with other Moenkhausia species, spawning likely involves a brief pairing or group event, with eggs released and fertilised in the water column or among vegetation, after which the adults take no further interest in the eggs. Whether M. guaruba shows any unusual reproductive behaviour associated with its headwater, high-gradient habitat — such as preferences for rocky substrate or faster flow — is not yet known.
Breeding
M. guaruba has not been reported in the aquarium hobby or in captive breeding programmes at the time of its description; practical breeding information is therefore unavailable. The general Moenkhausia and characid pattern involves adhesive eggs deposited among fine-leaved plants or spawning mops, an incubation period of several days at tropical temperatures, and very small fry that initially require infusoria or similarly fine first foods before accepting larger live and prepared items.
For reference, captive breeding of related Moenkhausia species is typically achieved in soft, slightly acidic water (pH 5.5–6.5, hardness 1–5 °dGH) at temperatures around 79–84 °F (79–84 °F), with parents removed after spawning to prevent egg predation. Whether M. guaruba requires replication of the harder, cleaner headwater conditions of its Serra do Cachimbo home has not been tested.
In the aquarium
Moenkhausia guaruba is not established in the aquarium hobby. It was described from museum and freshly collected specimens, and no captive populations or import records are known as of its 2023 description. Its endemism to a restricted headwater system and its likely sensitivity to collection pressure make it an unlikely candidate for the general trade in the near term.
Based on its ecology in clear, well-oxygenated Cerrado headwater streams, any future aquarium maintenance would likely benefit from clean, well-circulated water with good oxygen levels, a temperature in the mid-to-upper 20s Celsius, and a diet appropriate for a large omnivorous characid — varied live and frozen invertebrates alongside quality prepared foods. Its large adult size (approaching or exceeding 4 in SL) means it would require a spacious aquarium. General aquarium parameters suggested for related Moenkhausia species — neutral to slightly acidic pH, soft to moderately hard water, temperature 75–82 °F — provide a reasonable starting point until species-specific data become available.
Conservation
Moenkhausia guaruba was described in 2023 and has not yet been assessed by the IUCN Red List. Its conservation situation is a matter of genuine concern despite the lack of a formal assessment: the species appears to be a strict endemic of the upper rio Braço Norte headwaters on the Serra do Cachimbo plateau, a geographically isolated system with no known satellite populations.
The Serra do Cachimbo region has been identified as a freshwater endemism hotspot, and narrow-range endemics in Neotropical headwaters are inherently vulnerable to deforestation, changes in water quality, and infrastructure projects that alter stream hydrology. Shoal Conservation's 2023 new-species roundup highlighted M. guaruba as a notable discovery, in part because of the conservation implications of undescribed endemics in threatened Amazonian headwater systems. A formal IUCN assessment — most likely resulting in a Vulnerable or higher category given the restricted range — would be a valuable next step for this species.