Taxonomy & naming
Girardinus denticulatus was described by the American ichthyologist Samuel Garman in 1895. It belongs to the genus Girardinus, a small group of livebearing toothcarps endemic to Cuba, placed in the subfamily Poeciliinae of the family Poeciliidae within the order Cyprinodontiformes — the New World livebearers that also include the guppies, mollies, platies and swordtails. Eschmeyer's Catalog of Fishes governs the valid name; the combination Girardinus denticulatus Garman, 1895 is the accepted name for the species.
The genus Girardinus honours Charles Frédéric Girard (1822–1895), the French-American naturalist who worked as an assistant to Louis Agassiz first at Neuchâtel in Switzerland and later in the United States, and who contributed extensively to North American ichthyology and herpetology. The species epithet denticulatus is Latin for "finely toothed" or "furnished with small teeth," and it is the source of the English common name "toothy topminnow."
Girardinus is a genuinely Cuban radiation of small topminnows, and G. denticulatus is one of several congeners on the island. The hobby literature tends to treat the Cuban Girardinus collectively as small, undemanding community livebearers rather than documenting each species in detail, so species-level published natural history for G. denticulatus specifically is comparatively sparse.
Morphology
Girardinus denticulatus is a small fish with marked sexual size dimorphism, the females being substantially the larger sex. Reported maximum standard length is about 2 in for males and unsexed fish, while females reach roughly 3.5 in SL — a near-doubling of length in the female that is among the more pronounced size differences in the family and immediately useful for sexing adults.
As in all poeciliids, the most reliable sexing character is the male's anal fin, which is modified into a gonopodium — a slender, forward-set intromittent organ built from elongated and specialised anal-fin rays and used to transfer sperm to the female. Females retain a conventional, fan-shaped anal fin, are deeper-bodied, and develop a darkened gravid region at the rear of the abdomen when carrying young.
Beyond size and the gonopodium, G. denticulatus is a typical small Cuban topminnow in build — an elongate, somewhat cylindrical body with a flattened dorsal profile and an upturned mouth suited to feeding at and below the surface as well as over the bottom. Detailed colour and meristic descriptions are not widely circulated in the hobby literature, so this profile keeps to the well-attested characters rather than guessing at pattern.
Habitat
Girardinus denticulatus is endemic to Cuba, where it is a strictly freshwater, demersal, non-migratory fish of tropical lowland waters. It occurs in ponds, lakes and streams but shows a clear preference for clear, sluggish, slow-flowing streams, where it forages over the bottom rather than holding station in current.
Recorded environmental tolerances place it firmly in the warm, near-neutral to slightly alkaline range typical of Cuban lowland streams: water temperatures of about 72–82 °F, pH around 6.8–7.5, and a moderate hardness of roughly 5–10 dH. These are unremarkable tropical-stream conditions, and the species' tolerance of them is part of why Cuban Girardinus adapt readily to aquaria.
As an island endemic, its entire natural range is confined to Cuban inland waters, and it has not spread or been introduced in the way the more notorious poeciliids have. That endemism is the single most important thing to understand about its distribution: the species exists in the wild only on Cuba.
Feeding
Girardinus denticulatus is a bottom-feeder with a largely herbivorous, detritivorous diet. In the wild it feeds at the bottom on aquatic vegetation, diatoms and algae, supplemented by insect larvae — a feeding mode reflected in its low trophic level of about 2.3, which places it well toward the plant-and-detritus end of the spectrum for a poeciliid.
This grazing habit means the species is well served in captivity by foods with a substantial vegetable or algal component — quality flake with plant matter, algae-based pellets and the biofilm that develops on surfaces in a mature aquarium — rounded out with small live or frozen invertebrate foods to cover the insect-larva part of its natural intake.
Like most small livebearers it is a near-continuous grazer rather than a fish that takes one large meal, so several modest feedings suit it better than a single heavy one. Beyond this, species-specific dietary detail is sparse; the wild diet above is the firm basis for feeding it well.
Mating
Reproduction in Girardinus denticulatus is internal, as in all Poeciliidae. Fertilisation takes place inside the female: the male uses his gonopodium, the modified anal fin, to transfer sperm during a brief copulation, and there is no spawning of eggs into the open water. This is the defining reproductive feature of the family and the reason the gonopodium is such a reliable sexing character.
Male poeciliids generally do not build nests or hold territories; instead they court and pursue females, and in many species males alternate between display and surreptitious mating attempts. Females of the family commonly retain viable sperm internally for an extended period after mating, allowing successive broods to be produced from a single insemination — a trait that should be assumed for G. denticulatus on family grounds even though species-specific behavioural studies are not readily available.
Given the limited published behavioural detail for this particular species, the safest framing is the well-established poeciliid pattern: internal fertilisation via the gonopodium, no parental nest, and the capacity for sperm storage to fuel repeated broods.
Breeding
Girardinus denticulatus is viviparous — a true livebearer that gives birth to free-swimming young rather than laying eggs. After internal fertilisation the female carries the developing brood and delivers fully formed fry. As in the rest of the family, the young are lecithotrophic, nourished chiefly by yolk through development, with the modest matrotrophy typical of poeciliids.
Precise gestation length, brood size and birth interval for this species are not well documented in the accessible literature, so this profile does not assign exact figures. What can be stated with confidence from the family pattern is that broods consist of free-swimming, immediately independent fry, that successive broods can follow from stored sperm, and that — as is usual among livebearers — the fry need cover from the outset because adults, including the mother, will eat them.
FishBase notes a medium population resilience for the species, with an estimated population doubling time of roughly 1.4 to 4.4 years, consistent with a small, regularly reproducing livebearer. For breeding in the aquarium, the standard livebearer approach applies: dense planting or floating cover to shelter newborn fry, and small powdered or newly hatched live foods to raise them.
In the aquarium
Cuban Girardinus, including G. denticulatus, are kept in the hobby as small, peaceful livebearers well suited to small and medium community aquaria. They are undemanding fish that adapt to ordinary tropical conditions, and their care follows the general livebearer template rather than anything specialised.
Water should reflect the species' recorded preferences: warm, around 72–82 °F, near-neutral to slightly alkaline pH in the region of 6.8–7.5, and moderate hardness of about 5–10 dH. As grazing, bottom-oriented fish they appreciate a mature aquarium with established algae and biofilm and some planted cover, which also gives newborn fry a refuge from hungry adults. Because females grow much larger than males, a group will show an obvious size split between the sexes.
As with other livebearers, persistent males can harass females, so keeping more females than males spreads attention and reduces stress on any single fish, and breeding tends to take care of itself in suitable conditions. Detailed temperament notes specific to this species are limited, but nothing in its biology suggests it is anything other than a small, peaceable community fish.
Conservation
Girardinus denticulatus is assessed as Least Concern on the IUCN Red List, in an assessment dated 30 April 2020. The status reflects the species' situation across its Cuban range rather than any active threat process flagged at the species level, and it is not listed under CITES or CMS. It is harmless to humans.
That said, the species is a single-island endemic, and endemics of this kind are intrinsically more exposed to localised pressures — habitat alteration, pollution of small streams, and the spread of introduced fishes — than widely distributed species are, simply because their entire global range is so confined. A Least Concern listing is not a statement that nothing could affect Cuban freshwaters, only that the species is presently judged secure.
Unlike its invasive relatives the guppy and the western mosquitofish, G. denticulatus is not a fish that has been moved around the world or established feral populations. The conservation interest here is therefore the ordinary one for an island endemic: keeping its native Cuban stream habitats intact, and — as for any aquarium livebearer — never releasing captive fish into local waters.