Taxonomy & naming
Xenurolebias myersi was originally described by Carvalho in 1959 as Cynolebias myersi, placed within the large catch-all genus Cynolebias that once housed most South American annual killifishes. The species was later transferred to the genus Xenurolebias by Wilson J. E. M. Costa in 2011, as part of a broader revision of the Cynolebiinae that dismantled the old Cynolebias aggregate into several more precisely circumscribed genera. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) recognises the current valid combination as Xenurolebias myersi (Carvalho, 1959), with the parenthetical authority indicating the generic transfer.
The genus Xenurolebias is a small group of annual rivulines within the family Rivulidae (subfamily Cynolebiinae), characterised by meristic and morphometric features including a relatively high anal fin ray count and a deep body. It rains fishes records meristic values for this species of approximately D 16.5, A 22.0, and lateral line scales around 29.5, with predorsal length at approximately 60.4% of standard length and body depth at approximately 31.6% of standard length. The species epithet myersi honours George S. Myers, the American ichthyologist who contributed extensively to the taxonomy of South American freshwater fishes. The name Hypsolebias myersi is treated as a synonym of this species in Eschmeyer's Catalog of Fishes.
Morphology
Xenurolebias myersi is a small, deep-bodied annual killifish reaching a maximum of approximately 2 in standard length, consistent with the compact proportions typical of the Cynolebiinae. The body is laterally compressed and notably deep relative to length, a profile shared by several genera in this subfamily. Fins are well developed: the dorsal fin is set far back on the body, beginning well posterior to the midpoint — a diagnostic character within the genus reflected in the predorsal measurement of roughly 60% of standard length.
Sexual dichromatism is pronounced, as in most annual rivulines. Males display vivid colouration — specific patterns and tones vary between individuals and populations — while females are typically cryptically coloured in drab browns with a less elaborate fin display. Males of Cynolebiinae generally develop iridescent scales and bold patterning that intensify during courtship. Females are rounder-bodied when gravid. Some males exhibit aggression toward conspecifics, particularly rival males in confined spaces.
Habitat
The species is known from a small number of localities concentrated in the coastal lowlands of Espírito Santo and southern Bahia, Brazil — a landscape dominated by restinga vegetation, coastal Atlantic Forest remnants, and the seasonal wetlands they frame. The type locality, documented by It Rains Fishes, lies approximately 11 mi from Conceição da Barra along the road to Itaúnas (approximately 39°46'W, 18°29'S). The American Killifish Association records three confirmed localities: Conceição da Barra, Conceição–Itaúnas at PK 18, and Prado in Bahia. The overall range, as characterised by the AKA, extends from the vicinity of Vitória in Espírito Santo north to Caravelas in Bahia.
Within these localities the fish occupies temporary bodies of water — shallow floodplain pools, coastal lowland swamps, and restinga wetland depressions — that fill during the wet season and dry completely during the dry months. These are soft-bottomed systems with accumulations of organic matter including peat and decomposing plant material. The water chemistry reflects the coastal lowland setting: likely soft and slightly acidic, consistent with the organic-rich substrates typical of restinga environments, though precise field measurements from this species' pools are not widely published. Research published in the MDPI journal Diversity (Gomes et al., 2024–2025) confirms the species' endemic association with the coastal floodplain and restinga biome in the Itaúnas and Riacho Doce area.
Feeding
Like most annual killifishes of the Cynolebiinae, Xenurolebias myersi is a small carnivore adapted to exploit the invertebrate productivity that characterises newly flooded seasonal pools. In its brief growing season the fish feeds opportunistically on microcrustaceans, insect larvae, worms, and other small invertebrates that become available as the pool fills and vegetation decomposes into the water column.
In the aquarium the species accepts live and frozen foods readily — Artemia nauplii, Daphnia, Tubifex, bloodworm, and Grindal worm are all appropriate. Prepared micro-pellets may be accepted once the fish is established, but live foods remain valuable for conditioning breeding pairs. The species' small size means that food items must be appropriately scaled; adult males at 2 in SL can take small Artemia and chopped worm, but fry require infusoria or commercial fry powder before graduating to nauplii.
Mating
Xenurolebias myersi is a true annual killifish, and its reproductive biology is organised entirely around the soil-spawning strategy that characterises the Cynolebiinae. In the wild, courtship and spawning occur during the wet season as the temporary pool fills and warms. Males are colourful and competitive; It Rains Fishes notes that some individuals are aggressive, and in confined aquaria rival males must be managed carefully. The male pursues the female with characteristic display behaviour — fin erection, lateral posturing, and colour intensification — before initiating spawning.
Spawning takes place by substrate diving: the pair align side by side and plunge together into the soft bottom material — peat, mud, or decomposing organic substrate — where eggs are deposited individually or in small batches. The act is repeated across many dives over the course of a spawning session. The eggs are left entirely buried in the substrate without any form of parental attendance. It Rains Fishes records that eggs become visibly eyed after approximately 3.5 months of incubation, with the embryos appearing darkly pigmented at that stage.
Breeding
After the breeding season ends and the pool dries, the eggs remain buried in the substrate in a state of developmental arrest (diapause), surviving the dry months protected by a water-resistant chorion while the adults die. When seasonal rains return and the pool refills, the now-developed embryos hatch — a response triggered by rehydration and temperature cues. The embryos undergo multiple diapause phases (DI, DII, and DIII) typical of annual killifish, allowing flexible hatching timing that reduces the risk of all fry emerging simultaneously in a pool that may still be marginal.
In the aquarium, the standard approach for annual killifishes applies: the spawning medium — peat moss or coconut coir — is collected periodically from the tank, squeezed to remove excess water, sealed in a plastic bag labelled with the collection date, and stored in a warm, dark location for the incubation period. It Rains Fishes gives an egg incubation period of approximately 3.5 months before the embryos are eyed and ready to hatch. To hatch, the stored peat is placed in shallow, soft, slightly acidic water; fry that fail to emerge on the first wetting can often be successfully hatched on a second or third attempt, as staggered diapause completion is normal. Juveniles reach sexual maturity quickly — It Rains Fishes records maturity at approximately 5–6 weeks — reflecting the extreme brevity of the wild growing season.
In the aquarium
Xenurolebias myersi is suited to the specialist annual killifish keeper rather than the general aquarist. A species tank of modest size — 10–15 US gal is adequate for a small group — is appropriate; the species should not be kept with significantly larger or more aggressive fish. A tight-fitting lid is essential, as killifishes are accomplished jumpers. Water should be soft and slightly acidic in keeping with the restinga environment: a pH in the range of 5.5–7.0 and low to moderate hardness is recommended, with temperature maintained at around 72–79 °F.
The substrate should include a soft peat or coconut coir layer at least 2 in deep to allow the fish to perform their natural spawning dives — without appropriate substrate the fish cannot breed and will show frustration behaviour. Gentle filtration and partial water changes maintain quality without disrupting the spawning medium. Décor should be uncluttered to allow observation of courtship, with floating plants providing surface cover and reducing the intensity of male-to-male aggression. Multiple females per male reduces the stress on any individual female during spawning periods.
Because this is a Critically threatened single-region endemic with a very restricted wild population, responsible husbandry includes maintaining breeding records and exchanging stock through killifish societies to preserve genetic diversity in captivity. The American Killifish Association lists the species in its studbook programme.
Conservation
The IUCN Red List assessed Xenurolebias myersi as Endangered in 2018, a status attributed by the American Killifish Association to evaluation carried out under ICMBio (Instituto Chico Mendes de Conservação da Biodiversidade) in 2023. The Endangered classification reflects the species' extremely restricted range — only three confirmed localities are known — and the severe fragmentation and ongoing degradation of its coastal lowland wetland habitat in Espírito Santo and Bahia.
The restinga and coastal Atlantic Forest floodplains that Xenurolebias myersi depends on are among the most threatened ecosystems in Brazil. Drainage for agriculture, urban expansion in the coastal zone, road construction, and the alteration of hydrology in seasonal floodplains all destroy or permanently modify the temporary pools the species requires. Research published in the MDPI journal Diversity (Gomes, Sarmento-Soares, Martins-Pinheiro & Leite, 2024–2025) specifically assessed habitat and conservation status for Xenurolebias species in the coastal floodplains, confirming the vulnerability of restinga environments and the very limited spatial extent of suitable habitat. A preprint from the same research group (Preprints.org, 2025) places the genus within the broader context of annual killifish conservation in riparian wetlands, noting that the four species of Xenurolebias collectively occupy a small, threatened corridor of the Brazilian coast.
Because this species completes its entire life cycle within seasonal pools that can be eliminated by a single drainage event, even localised habitat disturbance near a known locality constitutes a meaningful threat to the population. The hobby plays a genuine role in maintaining insurance populations: captive-bred stock distributed through killifish societies represents a safeguard against local extirpation, and the species' ease of captive breeding relative to the cost of habitat protection makes killifish organisations a practical conservation partner.