Taxonomy & naming
Girardinus metallicus was described by the Cuban naturalist Felipe Poey in 1854, and Eschmeyer's Catalog of Fishes treats Girardinus metallicus Poey, 1854 as the valid name. It is the type species of the genus Girardinus, a small group of livebearing toothcarps largely endemic to Cuba. The species sits in the family Poeciliidae, subfamily Poeciliinae — the New World livebearers that include the guppies, mollies, platies and swordtails — within the order Cyprinodontiformes.
The genus name honours Charles Frédéric Girard (1822–1895), a French-American herpetologist and ichthyologist who worked as an assistant to Louis Agassiz, first at Neuchâtel and later in the United States. The specific epithet metallicus refers to the metallic sheen of the fish's flanks. In the aquarium hobby the species is best known by the English common names "metallic livebearer" and "black-chin livebearer," the latter describing the dark ventral marking of breeding males that is the fish's signature feature. The name Girardinus cubensis is treated as a synonym of this species in Eschmeyer's Catalog of Fishes.
Morphology
The metallic livebearer is a small, fairly stocky poeciliid with the pronounced sexual size dimorphism typical of the family. Females are the larger sex, reaching roughly 3.5 in in total length, while males stay much smaller at around 2 in; aquarium specimens often run smaller still, with kept males commonly around 2 in and females not always reaching their full wild maximum. The body is olive to grey-tan with a metallic sheen along the flanks that gives the species its name, and the scaling can show a faint reticulated pattern in good light.
The diagnostic feature is found in mature males: a band of jet-black pigment that runs from the lower jaw and chin back along the throat and belly to the tip of the gonopodium — the "black-chin" marking. This trait occurs naturally in many wild Cuban populations, but in the hobby it has been treated as a selected, cultivated character; lines are maintained for it, and males lacking the strong black mark are harder to obtain because they are usually culled to keep the trait expressed. As in all poeciliids, the male's anal fin is modified into the gonopodium, a slender rod-like intromittent organ; females retain a normal fan-shaped anal fin and, when gravid, show a darkened patch toward the rear of the abdomen.
Habitat
Girardinus metallicus is endemic to Cuba, occurring across the island's fresh waters; FishBase records it from Central America and Cuba, with Cuba as the endemic core of the range. It has also been introduced to several localities in the United States through the aquarium trade. Within its native range it occupies a variety of small, slow-water habitats — ponds, lakes, ditches and the quieter reaches of streams.
The species shows a clear preference for clear, stagnant or very sluggishly flowing water rather than swift current. It is a demersal, non-migratory freshwater fish that tolerates a broad range of conditions: FishBase gives a pH span of about 6.0 to 8.0 and a hardness range of roughly 9 to 19 dH, in tropical water around 72–77 °F. This tolerance of warm, still, moderately hard water — combined with its live-bearing reproduction — is consistent with its hardiness in captivity and with its ability to establish where it has been introduced outside Cuba.
Feeding
In the wild the metallic livebearer is a low-trophic-level grazer and omnivore, feeding heavily on diatoms, algae and detritus and supplementing this with insect larvae and aquatic vegetation. FishBase places it at a trophic level of about 2.3, reflecting a diet dominated by plant and detrital matter rather than animal prey. This grazing habit suits the clear, vegetated, slow waters it favours, where biofilm and algal growth are reliably available.
In the aquarium the species is undemanding and accepts the usual prepared foods, but it specifically benefits from a vegetable component in the diet — specialist keepers note that it appreciates plant and algal matter alongside flake and small live or frozen foods. A planted tank that supports some algal grazing matches the fish's natural feeding mode, and a varied diet that includes vegetable matter helps maintain condition and colour. As with most small livebearers, frequent modest feedings suit it better than infrequent large meals.
Mating
Like all poeciliids, Girardinus metallicus reproduces by internal fertilisation. The male transfers sperm to the female using the gonopodium, the rod-shaped intromittent organ formed from modified rays of the anal fin. Males pursue females actively rather than holding territory or building any structure, and the black-chin marking that runs to the tip of the gonopodium is displayed prominently during this courtship — the dark ventral band frames the very organ used in mating.
As in related livebearers, females can store viable sperm internally and produce successive broods from a single mating, so a male is not required for every brood once a female has been fertilised. There is no documented superfetation in this species in the way it is established for some poeciliids such as Heterandria and Poeciliopsis; the available husbandry data describe a single developing brood carried at a time. Where the precise courtship behaviour and any female mate preferences are concerned, the published detail is sparse, and beyond the general poeciliid pattern of gonopodial insemination and sperm storage the species-specific behaviour is not well documented.
Breeding
The metallic livebearer is viviparous and breeds readily in captivity. After internal fertilisation the female carries the developing young and gives birth to free-swimming fry rather than laying eggs. Specialist livebearer keepers report a gestation of roughly 25 to 30 days, after which the female drops a brood of about 10 to 30 young; brood size scales with the size and condition of the female, as is usual for the family.
The fry are comparatively small for a livebearer and, as with most poeciliids, are vulnerable to predation — including by other fish and by adults of their own kind. A practical breeding routine used by keepers is to move a gravid female into a separate, well-planted tank with fine-leaved cover such as Java moss or guppy grass (Najas), let her drop the brood, then remove her so she does not eat the young. The small fry are raised on newly hatched brine shrimp and finely powdered flake and can be returned to the main tank at around a month old once they are large enough not to be eaten. There is no parental care; protection of fry depends on dense cover or separation rather than on the parents.
In the aquarium
Girardinus metallicus is a hardy, good-natured aquarium fish and a popular subject among specialist livebearer keepers. Water conditions are not critical: it does well across a broad range of moderately hard, neutral-to-alkaline water and tolerates temperatures from roughly 70–79 °F (about 70 to 78 °F). It appreciates living plants, some water movement, and the opportunity to graze, and a mature, planted aquarium suits it well. It makes a good, peaceful community fish.
The main husbandry caveats are the usual livebearer ones. The species breeds freely, so population control becomes the practical management task in a settled tank, and fry survival in a community is low because small young are eaten by tankmates and adults alike — keepers who want to raise numbers separate gravid females rather than relying on the main tank. For aquarists who value the black-chin trait, sourcing and maintaining the line takes deliberate selection, since non-marked males turn up and are typically culled to keep the marking strong. Otherwise this is an easy, forgiving fish well suited to community and species tanks alike.
Conservation
Girardinus metallicus is assessed as Least Concern on the IUCN Red List, in an assessment dated 1 May 2020. The species is widespread across Cuba, occupies a range of common small-water habitats, and tolerates a broad span of conditions, so it faces no significant threat at the species level. It is neither CITES-listed nor covered by the CMS.
As with other hardy, prolific livebearers, the conservation note that matters most for aquarists runs the other way: the species has been introduced to several localities in the United States via the aquarium trade, where its tolerance and live-bearing fecundity let it establish outside its native Cuban range. The responsible practice is therefore never to release surplus fish or fry into local waters. Within its homeland the metallic livebearer remains a common, secure species.