Taxonomy & naming
Rivulus cylindraceus was described by the Cuban naturalist Felipe Poey in his landmark work Poissons de Cuba, Espèces nouvelles, with the description effectively completed in 1860 and published in 1861. The type locality is a stream near Havana, Cuba. Poey simultaneously established the genus Rivulus, designating R. cylindraceus as the type species by original designation — making this species the nominal anchor of the entire South American rivuline assemblage.
Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid combination as Rivulus cylindraceus Poey, 1861. The authority is not parenthesised because Poey both erected the genus and described the species within it — the original combination holds. Within the family Rivulidae, the genus Rivulus encompasses a substantial number of South and Central American killifishes, though systematic revisions over the past two decades have transferred many former Rivulus species into other genera; R. cylindraceus itself retains placement in Rivulus sensu stricto.
No subspecies are currently recognised by the Catalog of Fishes, and the species shows limited geographic variation across its Cuban range. Its position as type species of Rivulus gives it special nomenclatural importance: any future generic revision affecting Rivulus must account for this nominal type.
Morphology
The Cuban rivulus is a small, cylindrical killifish — the specific epithet cylindraceus, from the Latin cylindrus (cylinder), refers directly to this elongated, rounded body form. FishBase records a maximum size of 2 in total length, making it a moderately small species within the family. The body is relatively slender and the cross-section nearly circular, well-suited to squeezing through dense aquatic vegetation and surface debris.
Colouration is subdued but characterful. Males are typically brownish-olive to greenish-brown on the dorsum, fading to a yellowish or tan ventrum, with irregular rows of small reddish-brown spots or dashes along the flanks that may coalesce into faint horizontal banding. A dark ocellus (the 'rivulus spot') at the base of the upper caudal peduncle is characteristic of the genus and is more distinct in females and juveniles. Females are plainer, lacking the body spotting of males and instead displaying the caudal ocellus more prominently. The fins are rounded and relatively plain.
Sexual dimorphism is modest but consistent: males are more intensely patterned with spotting on the body and fins, while females are cryptically coloured with the prominent caudal ocellus serving as the primary identification feature. Both sexes are small enough to inhabit very shallow water and negotiate narrow passages between aquatic stems.
Habitat
Rivulus cylindraceus is endemic to Cuba, where it inhabits small flowing streams, roadside ditches, and vegetated pools in the island's coastal river basins. It is the only killifish species native to Cuba and, as such, occupies a unique position in the island's freshwater fish fauna. The British Killifish Association records the species from small flowing streams in Cuba; reports of populations in Florida refer to introduced or historically introduced fish.
The habitat is typically shallow, well-vegetated freshwater with slow to moderate flow, offering dense marginal or floating vegetation that provides both shelter and spawning substrate. FishBase characterises the species as benthopelagic and non-migratory in freshwater tropical conditions. Water parameters measured at Cuban collection sites are generally neutral to slightly acidic: FishBase gives a pH around 7 and temperature of approximately 73 °F, with a moderate hardness around 8 dH. The British Killifish Association notes a temperature range of 70–82 °F (approximately 70–82 °F) in the hobby.
As a surface-oriented fish the Cuban rivulus is highly prone to jumping — a common trait across killifishes — and any collected water body with emergent vegetation or overhanging banks should be assumed to harbour individuals capable of brief excursions out of water. Cover is essential both in nature (floating leaf litter, dense Riccia mats, bladderwort) and in captivity.
Feeding
In the wild, the Cuban rivulus is an opportunistic carnivore that feeds predominantly on small invertebrates at or near the water surface. Small insects, insect larvae, worms, and microcrustaceans make up the core of its diet — the surface-hunting posture typical of the genus positions it well to take emerging insects and terrestrial invertebrates that fall onto the water film.
In captivity the species strongly prefers live or frozen invertebrate foods. The British Killifish Association records a preference for small live foods including white worm (Enchytraeus), tubifex, and daphnia, with dried and prepared foods accepted but less eagerly taken. It Rains Fishes recommends feeding appropriately sized live and frozen items to maintain condition. Artemia nauplii are the standard first food for newly hatched fry, with adults moving on to larger live and frozen invertebrates. The fish is not a large feeder by volume and a twice-daily schedule with small amounts is appropriate.
Mating
The Cuban rivulus is a non-annual killifish: it inhabits permanent or near-permanent water and does not follow the boom-and-bust annual life cycle of seasonal-pool killifishes. Pair bonding is temporary and spawning is a repeated, low-volume activity rather than a single mass event — the fish produce a small number of eggs each day over an extended period throughout their adult lives, rather than laying a large clutch all at once before dying.
Courtship involves the male driving the female actively, often vigorously — the British Killifish Association notes males can be persistent drivers, and adequate cover or a sufficiently spacious setup reduces stress on the female. The spawning position is typical of plant-spawners: the pair align side by side or in a T-position near the chosen spawning site, and eggs are deposited singly or in small numbers among fine vegetation, plant roots, or fibres of a floating spawning mop. Eggs are adhesive and cling individually to the spawning medium rather than being scattered or buried.
Breeding
Breeding the Cuban rivulus is considered straightforward, and the species is frequently recommended as a first killi for hobbyists unfamiliar with the family. The British Killifish Association recommends a minimum tank volume of around 2 gallons (approximately 2 US gal), while It Rains Fishes uses a 2.5 US gal setup, with nylon spawning mops or floating plants such as Riccia fluitans or bladderwort suspended at or near the surface to receive eggs. A tight-fitting cover is essential to prevent jumping.
It Rains Fishes reports a daily production of 5–15 adhesive eggs per pair or trio, with eggs measuring approximately 0.5 in in diameter and presenting an amber colouration. Incubation takes 10–14 days in water — no diapausing or peat burial is required, as the eggs develop continuously in aquatic conditions. Breeding water reported by the British Killifish Association centres around pH 6.8–7.2 and a moderate hardness of 80–100 ppm DH, with a water temperature of approximately 78 °F (78 °F) at spawning; It Rains Fishes uses somewhat softer, more acidic conditions (75 °F, pH 5.5) and finds these also successful. A small pinch of salt is optional.
Fry hatch as miniature adults and should immediately be offered artemia nauplii. Adults will consume both eggs and fry if given the opportunity, so routine collection of eggs from the mop — every few days — and hatching them separately in a small container is standard practice. Growth is steady at tropical temperatures and the fish reach sexual maturity within a few months.
In the aquarium
The Cuban rivulus is an excellent choice for the killifish newcomer. Its modest size (to 2 in), uncomplicated breeding method, tolerance of a reasonable range of water conditions, and resilient constitution make it one of the more accessible members of a family that can otherwise demand specialist conditions. A small, well-covered tank of 2–4 US gal suits a breeding pair or trio; a larger community of the species could be housed in a 10–15 US gal aquarium with dense planting.
Water quality should be clean and well-oxygenated but the species is not demanding about chemistry — neutral to slightly acidic pH (6.8–7.2), moderate hardness, and a temperature between 21 and 82 °F all suit it. Décor of dark substrate, floating vegetation (Riccia, bladderwort, or Indian fern), and subdued lighting creates conditions close to the natural habitat and encourages natural behaviour. The lid must be completely tight: like all rivulines, this fish can jump through a gap smaller than its body.
The Cuban rivulus can be kept with peaceful, similarly sized fish in a community setting provided water parameters overlap, but it is most reliably bred in a species-only setup. The male can be a persistent driver; providing visual breaks and ensuring the female has retreat space reduces wear. Long-lived for a killifish, individuals may live three to four years in good conditions — considerably longer than annual species of similar size.
Conservation
The IUCN Red List assessed Rivulus cylindraceus as Least Concern in May 2020, reflecting a distribution spanning coastal river basins across Cuba without evidence of rapid population decline. The species is not considered globally threatened at present, and its tolerance of modified habitats — roadside ditches and agricultural drainage channels as well as natural streams — gives it some resilience relative to more specialised freshwater endemics.
Nevertheless, as the only killifish native to Cuba, its status as an island endemic deserves ongoing attention. Freshwater habitats in Cuba face pressures common to Caribbean island ecosystems: agricultural run-off and pesticide contamination, water abstraction for irrigation, modification of stream channels, and the introduction of non-native fish species including mosquitofish (Gambusia spp.), which compete intensively with small surface-oriented native fish and consume their eggs and fry. The introduced tilapia is also present in many Cuban waterways and alters habitat structure.
Hobbyist breeding populations maintained under the auspices of killifish associations in Europe and North America represent a low-cost insurance against local population collapse, as the species breeds readily in captivity and can be distributed through studbook-like networks. The British Killifish Association and similar organisations actively maintain stocks of Cuban rivulus, and the species' ease of culture makes it a realistic candidate for reintroduction support should wild populations come under more serious pressure in future.