Taxonomy & naming
Maratecoara gesmonei belongs to the family Rivulidae (South American annual killifishes), order Cyprinodontiformes. The genus Maratecoara currently comprises four valid species — M. lacortei, M. formosa, M. splendida, and M. gesmonei — all restricted to Brazil. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) is the authority for valid names in this group.
The genus Maratecoara was erected for small, substrate-spawning rivulids of central and northeastern Brazil characterised by their compact body form, annual life history, and distinctive pigmentation patterns. Within the genus, M. gesmonei is distinguished from congeners by the absence of the horizontal dark orange spots that appear on M. lacortei (two to three spots) and on M. formosa and M. splendida (three spots), and by the lack of oblique orange bars on the ventral trunk — a feature present in the other three species. Meristic differences also separate it: vertebral counts run to 25–26 (versus 27–28 in M. lacortei; 26–27 in M. formosa; 27 in M. splendida), and both body depth and caudal peduncle depth are comparatively lower. These distinctions, drawn from the original species description literature, place M. gesmonei as the least ornately patterned member of an otherwise colourful genus.
This fish was long listed as Maratecoara gessneri; Eschmeyer's Catalog of Fishes now treats that name as a synonym of Maratecoara gesmonei, the valid name followed here.
Morphology
Maratecoara gesmonei is a small fish; FishBase records males at approximately 0.9 in standard length and females slightly larger at around 1 in SL. This places it among the most compact members of the genus, in keeping with the general pattern of annual killifishes investing in rapid maturation rather than sustained growth. The body is moderately compressed and elongate, with a rounded snout and a superior or terminal mouth suited to surface and mid-water feeding in shallow pools.
Colouration in males is subdued relative to congeners: the ground colour of the flanks is a warm olive to tan, crossed by indistinct vertical dark bars or mottling but conspicuously lacking the bold horizontal orange spots and ventral orange bars that ornament M. lacortei, M. formosa, and M. splendida. Females are plainer, typically a uniform olive-buff without the males' residual pattern. The fins are rounded, as is typical for rivulids occupying still, dense-vegetated microhabitats. Sexual dimorphism in size and colour is moderate, with males showing the more defined pigmentation.
Habitat
The species is native to the Rio Xingu system, Amazon basin, Pará state, Brazil, where it has been recorded from temporary pools on fluvial islands — shallow, sediment-rich depressions that form on river islands during flood recession and fill from direct rainfall during the wet season. These habitats are characterised by warm, soft to moderately soft water, fine organic substrate (mud, detritus, and leaf litter), and dense marginal or floating aquatic vegetation during the wet phase.
Water conditions in such pools shift substantially over the seasonal cycle: during the wet phase temperatures are moderate and oxygen levels reasonable; as pools contract and warm in the dry season, conditions become extreme before the pool disappears entirely. The fish must complete its reproductive cycle — grow from hatched fry to reproductively active adult — within the window of pool availability, which may be as short as a few months. The congener M. lacortei, from the Aruana region of Goiás, occupies similar seasonal savannah pools and lagoons; the habits of M. gesmonei are inferred to follow the same pattern given the shared genus ecology and body form.
Based on data from congeners and the broader Amazonian lowland seasonal-pool context, aquarium water should be kept soft and slightly acidic to neutral (pH around 6.5–7.0) and warm (75–81 °F), though precise field chemistry for this species has not been published in detail.
Feeding
Maratecoara gesmonei is a microcarnivore, as is typical for small rivulids in seasonal pools. The diet in the wild consists of small invertebrates — microcrustaceans, insect larvae, worms, and other zooplankton that colonise and proliferate in temporary pools during the wet season. The upturned to terminal mouth and small gape are consistent with a diet of invertebrates taken from the water column and surface.
In aquarium conditions the species accepts a range of small live and frozen invertebrate foods: Daphnia, Artemia nauplii, micro-worms, Grindal worms, and chironomid (bloodworm) larvae. Like other small annual killifishes, it benefits from varied live and frozen prey items that match the natural diet in particle size; dry foods may be accepted but should not form the bulk of the diet. Feeding twice daily in modest amounts suits the species' fast metabolism and short natural lifespan.
Mating
Maratecoara gesmonei reproduces by the annual killifish strategy: pairs court and spawn by driving into and depositing eggs directly in the substrate — soft mud, peat, or detritus on the pool floor. This soil-spawning mode is the defining reproductive trait of the South American annual killifishes and is fundamentally different from the plant-spawning strategy of non-annual rivulids. The eggs are not adhesive and are not attached to vegetation; they are instead pressed into the substrate and left buried.
Courting males display to females with fin-spreading and lateral posturing, likely involving colour intensification of the subdued pigmentation visible under good light. Spawning dives are brief; the pair press side by side into the substrate, release eggs and sperm, and separate. Multiple spawning acts occur over a session. In the wild, mating takes place during the wet-season pool phase when the fish are at or approaching peak adult condition, since the approach of dry-season pool contraction provides an urgent reproductive deadline.
Breeding
After the spawning dives, the fertilised eggs are left buried in the substrate and the adult fish play no further parental role. As the pool dries, the exposed substrate — and the eggs within it — enters a dormant phase. The eggs undergo embryonic diapause: development arrests at specific developmental stages (there may be multiple diapause phases, as documented for annual rivulids generally), a drought-resistant strategy that allows the embryo to survive weeks to months of desiccation. The wet-season return triggers resumption of development and synchronised hatching, seeding the re-filled pool with fry.
For hobbyists, the breeding cycle is managed by maintaining a peat or coconut-coir spawning medium in the aquarium floor, harvesting the medium at regular intervals, pressing it to a slightly moist (not wet) consistency, sealing it in a bag or container, and storing it for a minimum of six to eight weeks — sometimes longer, depending on the species — before re-wetting to induce hatching. Fry are small and require appropriately sized first foods (infusoria or commercial fry powder) before graduating to micro-worms and Artemia nauplii. Growth is rapid given the species' evolutionary pressure to mature within a single wet season.
In the aquarium
Maratecoara gesmonei is a specialist fish for hobbyists interested in annual killifishes. It is best kept as a species aquarium or in a killifish-focused setup, as its small size (under 1 in SL), specific substrate requirements, and short lifespan make it unsuitable as a general community fish. A small, well-planted tank of 8–16 US gal, with a soft peat or fine sand substrate suitable for spawning dives, a dark background, and subdued lighting, provides an appropriate environment. Floating plants reduce surface light and help the fish feel secure.
Water parameters should follow the warm, soft, slightly acidic to neutral profile of the species' Amazonian seasonal-pool habitat: temperature 75–81 °F, pH approximately 6.5–7.0, soft to medium-soft water. Filtration should be gentle — sponge filters or air-driven corner filters prevent fry and small adults from being drawn into flow. The tank must be covered, as killies of all kinds are capable jumpers.
The genus Maratecoara is uncommon in the hobby relative to more widely distributed annual killifishes, but it is occasionally maintained and bred by specialists affiliated with killifish societies. The primary appeal is the biological fascination of the annual lifecycle — the diapause egg storage, the synchronised hatching, the compressed adult lifespan — rather than the species' modest colouration. Exchange of eggs through killifish societies is the primary route to obtaining the species; it is rarely available in general aquarium retail.
Conservation
No formal IUCN Red List assessment is available for Maratecoara gesmonei. FishBase assigns the species an LC (Least Concern) inference, but this reflects the absence of documented population declines rather than a completed threat analysis. The species is a narrow-range endemic of the Rio Xingu drainage in Pará state, Brazil, and its habitat — temporary pools on fluvial islands and adjacent floodplains — is vulnerable to the full suite of pressures affecting the central Amazon basin: deforestation and land clearing for agriculture and cattle ranching, hydrological alteration from upstream damming (the broader Xingu system was severely impacted by the Belo Monte dam complex), and the drainage or modification of small seasonal waterbodies.
Annual killifishes are inherently fragile from a conservation standpoint: obligate use of temporary habitats that may occupy only a few hectares, a population structure that bottlenecks through the egg bank every dry season, and a life cycle with zero resilience to a missed wet season or permanent pool loss. A single habitat destruction event can extirpate a local population entirely, and because eggs do not survive indefinitely in the soil, a pool that dries without refilling for multiple seasons loses its egg bank. The Maratecoara genus as a whole is considered endemic to Brazil, and the stability of M. gesmonei populations depends entirely on the persistence of undisturbed seasonal pool habitats within its Xingu-drainage range.