Taxonomy & naming
Moenkhausia chlorophthalma was described by Sousa, Netto-Ferreira & Birindelli in 2010, with the type locality recorded as the Rio Treze de Maio, a tributary of the rio Curuá in the rio Iriri–Xingu drainage near Altamira, Pará, Brazil (approximately 08°45'S 55°02'W). The original description was published in Neotropical Ichthyology. Eschmeyer's Catalog of Fishes treats Moenkhausia chlorophthalma Sousa, Netto-Ferreira & Birindelli, 2010 as the valid name.
The species belongs to the large genus Moenkhausia, which is placed within the family Characidae and encompasses several dozen species distributed across drainages from the Guianas south to the Río de la Plata basin. The genus was named in honour of William J. Moenkhaus (1871–1947), an American geneticist and ichthyologist who served as Professor of Physiology at Indiana University Medical School. Within Moenkhausia, M. chlorophthalma is set apart by the combination of its adipose-fin pigmentation pattern and vivid eye colour, though a full phylogenetic placement within the genus awaits comprehensive revision.
Morphology
Moenkhausia chlorophthalma is a small, compact characid reaching a maximum recorded standard length of about 2.5 in (approximately 2.7 in), with the largest specimen in the type series measuring 2.5 in SL. The body is short and relatively deep for the genus, laterally compressed in the typical characid fashion, and the scales bear dark blotches that produce a slightly speckled appearance.
The species' most conspicuous diagnostic feature is a well-delimited black pigmented area on the proximal portion of the adipose fin — a character useful for separating it from related Moenkhausia in the Xingu drainage. In life, the eyes are a striking iridescent green, a feature unusual enough within the genus to serve as the basis for the species name. Beyond these key characters, the fin and body proportions are broadly typical of small members of the genus, and the fish has the general jizz of a benthopelagic characid suited to clear, flowing water in shallow creeks.
Habitat
The known range of Moenkhausia chlorophthalma is confined to small creeks near the headwaters of the rio Curuá, a tributary system within the rio Iriri arm of the broader Xingu drainage in the state of Pará, Brazil, at roughly 8°S–9°S, 55°W–56°W. Crucially, these headwater creeks are isolated from the lower Xingu by large waterfalls, which explains the species' apparent endemism to the Serra do Cachimbo upland region — fish cannot move past the falls in either direction.
Within those creeks the species is described as benthopelagic, inhabiting the water column close to the substrate in clear, shallow, flowing water characteristic of upland headwater systems. The isolation imposed by the waterfalls means the population has no connectivity to larger river sections and is effectively marooned in a small island of suitable habitat, a situation that carries significant implications for both its ecology and its conservation.
Feeding
No detailed dietary study of Moenkhausia chlorophthalma in the wild has been published. Like other small Moenkhausia in similar habitats, it is expected to be an opportunistic omnivore, taking small aquatic invertebrates — insect larvae, microcrustaceans, and other benthic and water-column prey — along with algae and organic detritus.
In the aquarium, related Moenkhausia species accept a wide range of foods without difficulty, including good-quality flake and small pellets supplemented with frozen or live foods such as Daphnia, Artemia nauplii, and bloodworm. A varied diet maintains condition and colour best. The species' small maximum size means it requires appropriately sized food items, and overfeeding in the confined space of a small aquarium should be avoided.
Mating
No direct observations of mating behaviour in Moenkhausia chlorophthalma have been published. Characids in this genus are egg-scattering open spawners: males typically display to females in the water column or among fine-leaved vegetation, driving spawning over plants, algal mats, or open substrate. No pair bonding or territory defence is involved, and adults provide no care once eggs are released.
As with other small Moenkhausia, spawning in captivity would be expected to occur during morning hours after appropriate conditioning, with eggs distributed among fine-leaved plants or spawning mops. The absence of parental care means eggs and fry are at risk from tankmates and from the spawning adults themselves, and a separate rearing vessel is the standard approach for breeders wishing to raise a cohort.
Breeding
Moenkhausia chlorophthalma has not been documented in the aquarium hobby to a degree that permits detailed breeding notes. By analogy with congeners, it would be expected to deposit small, adhesive or semi-adhesive eggs among fine-leaved aquatic plants or synthetic mops, with no parental care whatsoever. Incubation at warm temperatures would be brief — typically a few days for small characids — and fry would be tiny at hatching, requiring infusoria or similarly fine first foods before graduating to newly hatched brine shrimp.
Given the species' restricted wild distribution and the waterfalls that isolate its population, it is unlikely to be available in the commercial hobby at present. Any aquarium observations would be scientifically valuable and should be reported to ichthyological journals or fish-society publications.
In the aquarium
Moenkhausia chlorophthalma is not currently established in the ornamental fish trade, and there is no body of hobbyist experience with the species to draw on. Its origin in clear upland creeks of the Xingu drainage suggests it would require clean, well-oxygenated water with moderate flow, temperatures in the range typical for small Neotropical characids (approximately 72–82 °F), and a pH tending toward neutral or slightly acidic, soft-to-moderate hardness.
Should the species become available, its small size and typical Moenkhausia body form suggest it would suit a well-planted aquarium of moderate size, kept in a group with other peaceful small tetras or similarly scaled fish. The iridescent green eye would make it an attractive display fish, but prospective keepers should ensure any specimens are captive-bred rather than wild-caught, given the species' apparently narrow natural range.
Conservation
Moenkhausia chlorophthalma is assessed as Least Concern (LC) on the IUCN Red List (2018). While the Least Concern designation reflects the assessment at the time, it is worth noting that the species appears to be endemic to a small cluster of isolated headwater creeks separated from the lower Xingu by large waterfalls. Such biogeographic isolation, combined with the pressures facing the greater Xingu basin — including large-scale hydroelectric development, deforestation in the catchment, and agricultural runoff — places the species in a potentially vulnerable position regardless of its current listing.
The Serra do Cachimbo region, while remote, is not immune to broader Amazon basin pressures. The species has a restricted range by any measure, and ongoing monitoring of its population status would be warranted. No specific conservation actions targeting M. chlorophthalma are known to be in place.