Taxonomy & naming
Cynodonichthys tenuis was described by the American ichthyologist Seth Eugene Meek from material collected at El Hule, Veracruz (and more precisely near the Oaxacan portion of that collecting trip on 22 April 1903), published in his landmark 1904 account of the freshwater fishes of Mexico north of the Isthmus of Tehuantepec. GBIF records the formal citation year as 1904 for the original description, while the parenthetical authority on the valid name is rendered as (Meek, 1907) in Eschmeyer's Catalog of Fishes — reflecting a secondary publication date sometimes associated with Meek's volumes; Catalog of Fishes governs the accepted treatment.
The species was long placed in the large, paraphyletic genus Rivulus, and most older hobbyist and scientific literature refer to it as Rivulus tenuis. Following the comprehensive molecular and morphological revision by Costa (2011 onward) that dismembered Rivulus sensu lato into numerous smaller genera, the species was reassigned to Cynodonichthys, a genus of Neotropical non-annual rivulines. The common name Dogtooth Rivulus, recorded on iNaturalist, is seldom used in the hobby, where the fish is still widely known under its former Rivulus name. No subspecies are currently recognised.
Cynodonichthys tenuis belongs to the family Rivulidae (South American killifishes) and is grouped among the South American rivulines — a broad informal assemblage of non-annual, permanent-water killis that radiated from South America into Central America.
Morphology
Adults reach approximately 2.5 in total length, with the British Killifish Association citing males to around 2.5 in and females slightly smaller. The body is slender and slightly laterally compressed — the specific epithet tenuis is Latin for 'slender', capturing the fish's trim, elongate profile in contrast to more robust rivulines.
Sexual dichromatism is pronounced. Males are tan to light brown dorsally with a yellowed to pale-orange ventral surface; the posterior half of the body takes on a subtle bluish iridescent sheen. The caudal fin is brown with a yellow band, a sky-blue band, and a purple submarginal stripe — a restrained but striking combination. The anal fin is pale green and the dorsal fin green with a yellow tip. Females are comparatively plain: overall tan, cream on the underside, with unmarked, tan-coloured caudal fins. The most diagnostic feature of the female is the well-defined 'Rivulus spot' — a dark, ocellus-like marking at the base of the caudal peduncle — a character retained from the ancestral Rivulus body plan and used by taxonomists and hobbyists alike to identify rivuline females.
Habitat
Cynodonichthys tenuis inhabits permanent and semi-permanent lowland freshwater habitats along the Gulf and Caribbean drainages of Central America. Its documented range spans from the Papaloapan river system of Veracruz and Oaxaca in southern Mexico through Belize, Guatemala, and Honduras — coastal-plain streams, roadside ditches, swampy forest pools, and slow-moving reaches of small rivers at low elevation. The type locality at El Hule, in the coastal lowlands of Veracruz, is a typical habitat: shallow, vegetated, permanent-water bodies in lowland tropical forest or forest-edge settings.
FishBase records the species as a permanent-water dweller, not subject to the dry-season desiccation that drives the annual life history in other killifish. Water chemistry across this range tends toward soft and slightly acidic to neutral conditions. The It Rains Fishes reference documents a preferred pH of approximately 6.2–7.4, with FishBase noting approximately 6.8 as a representative value; temperatures in the lowland range run through the mid-20s Celsius in cooler months, with summer highs into the upper 20s. The British Killifish Association notes that a small amount of salt (roughly one teaspoon per gallon) is tolerated and may be beneficial, hinting at mild estuarine influence along some portions of the coastal range. Dense aquatic and emergent vegetation is characteristic of occupied habitat.
Feeding
In the wild, Cynodonichthys tenuis feeds on small invertebrates, zooplankton, and surface-drift insects — the typical diet of a small, opportunistic predator inhabiting densely vegetated lowland pools. No detailed stomach-content studies are cited in the available literature for this species specifically, but the feeding ecology is consistent with that of other small Neotropical rivulines.
In captivity the fish is easily maintained on a varied diet. The British Killifish Association and It Rains Fishes both report that it accepts dried foods (flake, micro-pellet) alongside live and frozen offerings. Small live foods — newly hatched brine shrimp nauplii, microworms, grindal worms, and Daphnia — are eagerly taken and support good colour and condition. Adults can handle progressively larger food items as they grow. Feeding once or twice daily with appropriately sized items keeps the fish in optimal condition; overfeeding is a more common error than underfeeding with this species.
Mating
Cynodonichthys tenuis is a non-annual, plant-spawning killifish. Rather than congregating for a brief explosive spawning event tied to seasonal rains, pairs or small groups engage in extended, low-intensity courtship and spawning over weeks and months in permanent water — a reproductive strategy well suited to habitats that do not dry out.
Male courtship involves lateral displays that show off the iridescent flank colouration and the banded caudal fin. The male manoeuvres alongside or below the female, and the pair align briefly in a spawning embrace near the surface or within fine vegetation. Spawning is a quiet, repeated affair: It Rains Fishes documents production of 5–15 eggs per day during active spawning periods. The eggs are adhesive, amber-coloured, and approximately 0.5 in in diameter — laid individually or in small clusters on fine-leaved plants, Riccia (floating liverwort), or the fibres of a floating spawning mop near the surface or among mid-water cover.
Breeding
Eggs incubate in water without diapause — there is no requirement for a dry or 'resting' phase, which is the key practical distinction from annual killifish kept on peat. Incubation runs approximately 12–21 days depending on temperature, with the It Rains Fishes source citing roughly 14 days as the typical period and the British Killifish Association giving a range of 12–21 days correlated with temperature (warmer water accelerates development). Eggs can be picked from the mop or plants and incubated in a small container of clean water, or left in situ if the tank is densely planted — the British Killifish Association notes that adults in a well-planted setup generally ignore the fry.
Fry are large at hatch relative to the egg size and are immediately mobile. They accept newly hatched brine shrimp nauplii and microworms as first foods from day one. Growth is steady: It Rains Fishes reports juveniles reaching around 1 in within four to five weeks and sexual maturity at approximately five months. Full adult size is attained by seven to eight months. Captive lifespan reaches up to three years — long for a killifish — consistent with the non-annual life-history strategy of a fish whose native habitats are not eliminated by dry-season desiccation. A breeding pair or trio in a small dedicated tank (around 5.5–10 US gal) with a floating spawning mop and good water quality will produce fry consistently.
In the aquarium
Cynodonichthys tenuis is regarded as one of the more straightforward non-annual killifish for aquarists new to the group. Both the British Killifish Association and It Rains Fishes describe it as hardy and adaptable, tolerating a range of water chemistries (pH 6.2–7.4, soft to medium-hard water) and stable temperatures in the mid-20s Celsius — roughly 72–79 °F suits it well, though the species handles temperatures into the upper 20s without difficulty.
A dedicated breeding tank of 5.5–10 US gal works well for a pair or small trio. Dense floating plant cover — Riccia, frogbit, or Indian fern — along with a floating nylon mop provides spawning sites and gives the fish the visual security of a vegetated environment. Filtration should be gentle (a sponge filter is ideal) and the tank covered securely, as small killifish are accomplished jumpers. The species can also be kept in a well-planted community aquarium with peaceful, similarly sized tankmates, provided they do not outcompete it at feeding time.
Feeding is undemanding: the fish accepts dried foods readily, with live or frozen small invertebrates welcomed as supplements. Water changes of roughly 20–30 % weekly maintain water quality without the large, dramatic changes that can stress permanent-water lowland fish accustomed to relatively stable conditions. The Dogtooth Rivulus is a rewarding fish for the killifish enthusiast — active, visually attractive in the male, and productive enough to maintain a small colony without significant effort.
Conservation
Cynodonichthys tenuis is assessed as Data Deficient (DD) on the IUCN Red List, based on a 2018 assessment by Maiz-Tome. The DD status signals that insufficient data exist to determine population trends or assign a threat category — not that the species is secure. Its range across the coastal lowlands of southern Mexico and Central America places it squarely within some of the most heavily deforested and agriculturally developed landscapes in the Neotropics. The Gulf lowlands of Veracruz and Oaxaca, where the type locality El Hule is situated, have experienced substantial wetland drainage, conversion of seasonal floodplain habitats to cattle pasture and intensive agriculture, and pollution from agrochemical run-off — pressures that affect permanent-water killifish and many other lowland stream fishes throughout the region.
The species does not appear on any national Red List of imminent concern at the time of the IUCN assessment, and its distribution across multiple countries (Mexico, Belize, Guatemala, Honduras) suggests it is not a single-locality endemic. Nevertheless, the lack of population-trend data means that ongoing habitat loss in the lowland coastal drainages could be reducing populations without triggering formal conservation attention. The nine iNaturalist observation records, while limited, confirm the species is still being encountered across its range. Given the general trajectory of lowland tropical freshwater habitats in Central America, a future reassessment with adequate survey data would be valuable.