Taxonomy & naming
Cynodonichthys magdalenae was originally described by Carl H. Eigenmann and Arthur Henn in 1916 from material collected in the Magdalena River basin of Colombia. The specific epithet magdalenae is a Latin genitive honouring the Río Magdalena, the great westward-draining river of Colombia's Andean interior and the species' primary type locality. The authority is presented in parentheses because the original description did not place this fish in Cynodonichthys — the species was subsequently transferred to its current genus, following the convention of Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong), which governs valid names for the site.
The genus Cynodonichthys belongs to the subfamily Cynolebiasinae (or its near equivalent in various phylogenetic treatments) within the family Rivulidae, the South American killifishes or rivulines. Unlike many rivuline genera that have been revised repeatedly through the twentieth century, Cynodonichthys is a relatively stable grouping of small, stream-dwelling non-annuals from the Neotropical lowlands. FishBase recognises the species under this combination and records it from both the Magdalena and Cauca basins, which share a tectonic and hydrological history that explains the overlap in faunal composition.
Morphology
Cynodonichthys magdalenae is a slender, elongate killifish reaching a maximum recorded total length of 2.8 in (FishBase). The body form is typical of stream-dwelling rivulines: laterally compressed, with a pointed snout, terminal mouth, and relatively large eyes positioned high on the head. The dorsal fin is set well back toward the caudal peduncle, which is a characteristic feature that distinguishes rivulines from many other small cyprinodontiform groups.
Colouration in males is more vivid than in females, displaying the iridescent lateral banding or spotting common to the genus against a brownish-olive base. Females are cryptically coloured, tending toward a uniform olive-tan with reduced patterning — a common dimorphism in plant-spawning killifishes where males compete visually for female attention while females rely on camouflage for predator avoidance. The fins of males may carry extended rays or more intense pigmentation at spawning readiness. Detailed pigment descriptions require examination of freshly preserved or live material; hobbyist accounts confirm the males are noticeably more colourful than females.
Habitat
The Magdalena and Cauca River basins together drain the inter-Andean valley of Colombia, running northward through some of the country's most densely inhabited lowlands before emptying into the Caribbean. Within this system, Cynodonichthys magdalenae occupies freshwater habitats described by FishBase as benthopelagic and non-migratory — it lives and feeds near the bottom and lower water column of streams and pools rather than undertaking seasonal migrations.
The species is not a seasonal-pond dweller and does not depend on temporary habitats that dry out annually. Its waters are permanent or near-permanent streams and slower reaches of the Colombian interior. Water parameters recorded by hobbyists who maintain the species suggest soft to moderately hard water with a near-neutral to mildly acidic pH (6.4–7.4), hardness of 2–10 °dGH, and temperatures in the range of 68–79 °F, consistent with the warm lowland-to-submontane streams of the Magdalena valley. These are not extreme blackwater conditions; the species tolerates and performs well in moderate water chemistry.
Feeding
Like most small rivulines, Cynodonichthys magdalenae is an opportunistic micro-predator in the wild, feeding on small invertebrates, aquatic insects, and crustaceans that are available in its stream habitats. The relatively small terminal mouth and active swimming behaviour are consistent with a diet of small live prey items rather than algae or detritus.
In the aquarium, the species responds well to a varied diet of live and frozen foods. Micro worms, Artemia nauplii, Daphnia, bloodworm, and similar small invertebrate-based foods maintain condition and encourage regular spawning. British Killifish Association keepers report that a rich live-food diet is the single most effective trigger for sustained egg production — pairs fed generously produce five to fifteen eggs per day continuously throughout their adult lives. Dry foods can be used supplementally but should not form the bulk of the diet; the species performs best on invertebrate-based feeding.
Mating
Cynodonichthys magdalenae is a non-annual killifish, and its mating system reflects that biology entirely. There is no diapause, no peat-spawning, and no seasonal burst followed by death. Instead, pairs form loose associations and spawn repeatedly and continuously throughout their adult lives — a sustained, low-intensity reproductive strategy suited to permanent stream habitats where eggs can be deposited and left to incubate in water year-round.
Spawning behaviour centres on fine-leaved aquatic plants or floating mosses. In the aquarium the pair approach a clump of Riccia, a floating spawning mop, or fine-leaved plants near the surface, and the male courts the female with lateral displays. The pair press side by side and deposit a small number of adhesive eggs — five to fifteen per day under good feeding conditions — which adhere to the plant material or mop fibres. There is no nest construction and no parental guarding; once eggs are placed the pair move on and the process repeats. A well-conditioned trio (one male, two females) can produce ninety to one hundred eggs across a three-day period.
Breeding
Eggs of Cynodonichthys magdalenae are adhesive and incubate submerged in water — there is no requirement for damp peat or a dry diapause period, distinguishing this species sharply from annual killies. At approximately 75 °F (75 °F) the eggs hatch in fourteen days; some batches may extend to twenty-one days. The fry emerge large relative to those of annual species and are dark in colouration. They are able to take micro worms and newly hatched brine shrimp from the first days of free swimming, which simplifies rearing considerably.
The British Killifish Association notes that Cynodonichthys magdalenae reaches sexual maturity at around six months of age and achieves full size by twelve to fifteen months — a lifespan and growth trajectory typical of non-annual rivulines living in permanent waters. Because spawning is continuous rather than seasonal, hobbyists typically run a standing mop in the breeding tank and remove it weekly, hatching eggs separately in shallow dishes. A small addition of salt (approximately 1 teaspoon per 3 US gal) has been reported to improve egg viability and fry robustness, though the species does not require brackish conditions.
In the aquarium
Cynodonichthys magdalenae is assessed by hobbyists as easy to maintain and an excellent introduction to South American rivulines. FishBase records it as straightforward to keep, and British Killifish Association members describe newly acquired fish as initially timid but becoming increasingly bold and confident as they settle into a tank. The species is not aggressive toward other fish of comparable size and can be maintained in a community setting with other small, peaceful species — though dedicated killifish pairs or trios in a species tank will spawn most reliably.
A tank of 11–16 US gal provides adequate space for a breeding pair or trio. Subdued lighting and a dark substrate reduce stress and bring out male colouration. Floating plants or spawning mops at the surface serve both as decor and as the primary spawning substrate. Water conditions of pH 6.4–7.4, hardness 2–10 °dGH, and temperature 72–79 °F suit the species well, with the upper range of that temperature bracket promoting the most active spawning. Regular partial water changes maintain water quality and appear to stimulate spawning activity. The species is commercially fished and exported from Colombia for the aquarium trade, and it appears periodically in specialist killifish society lists.
Conservation
The IUCN Red List assessed Cynodonichthys magdalenae as Least Concern in October 2014, reflecting a range that spans two major Colombian river systems — the Magdalena and the Cauca — broad enough that the species is not considered at immediate risk of range-wide population collapse. However, a Least Concern designation for a Colombian endemic is not an assurance of ecological security.
The Magdalena basin is one of the most modified river systems in South America. Agriculture, cattle ranching, hydroelectric impoundment, gold mining, and expanding urban and industrial water demand have degraded water quality and altered hydrology throughout the valley. Riparian deforestation reduces the shading, leaf-litter input, and bank stability that structure the microhabitats used by small stream fish. The Cauca, a major Magdalena tributary, passes through Colombia's most intensive agricultural zone and carries heavy sediment and agrochemical loads. For a non-annual killifish dependent on permanent, clear-flowing stream habitats, degradation of these systems is a meaningful long-term threat even where range size currently buffers against extinction risk. Future reassessments should track whether increasing hydrological alteration in the Magdalena-Cauca system produces detectable population declines in small endemic stream fish.