Taxonomy & naming
Girardinichthys multiradiatus was first described by Seth Eugene Meek in 1904 as Characodon multiradiatus, in the Field Columbian Museum's publication series, on material collected from the upper Río Lerma basin near Toluca by J. N. Rose around the turn of the century. The holotype, a specimen of just under 1.5 in total length, resides in the Field Museum collection. Over the following decades the species was shuffled through several genera and accrued a tangle of synonyms — Lermichthys multiradiatus Hubbs 1926 and Girardinichthys limnurgus Jordan & Evermann 1927 among them — before settling into Girardinichthys, where Eschmeyer's Catalog of Fishes treats Girardinichthys multiradiatus (Meek, 1904) as the valid name.
The species belongs to the family Goodeidae and its subfamily Goodeinae, the splitfins or mexclapiques — a group of viviparous toothcarps largely confined to the highlands of central Mexico. The genus name honours Charles Girard, the assistant of Louis Agassiz, combined with the Greek ichthys, "fish"; the epithet multiradiatus is Latin for "many-rayed," a reference to the relatively high count of rays in the dorsal and anal fins. In Mexican usage the fish is called the mexclapique de Zempoala, and in English the darkedged splitfin, after the dark margins of the males' unpaired fins.
Wild populations sort into two recognised evolutionarily significant units: an isolated, colder-water lineage in the Lagunas de Zempoala (protected within a national park) and a more widespread lineage spanning the Lerma, Pánuco and Balsas drainages. The distinction matters for conservation, since the Zempoala stock is geographically and genetically separate from the rest of the species.
Morphology
This is a small splitfin. Males reach about 1.5 in total length and females are larger, to roughly 2 in total length, with maximum standard lengths around 2 in reported in the literature. The body is fairly deep and laterally compressed in the goodeid mould, the ground colour ranging from greyish-yellow to a bright, almost golden yellow depending on condition and population.
Sexual dichromatism is marked. Males carry yellow to orange-golden unpaired fins finished with dark, near-black terminal bands — the "darkedged" character — and enlarged dorsal and anal fins, sometimes overlaid with faint stripes. Females are plainer, with clear fins, a distinct gravidity spot, and broad striping along the posterior flank. Beyond visible colour, courting males display ultraviolet-reflecting markings on the flanks; because females of the species can perceive ultraviolet light, these signals function in mate assessment in a channel invisible to the human eye.
The diagnostic male structure is the andropodium, the goodeid analogue of the poeciliid gonopodium. Rather than a fused intromittent tube, the first several anal-fin rays are shortened and set off from the rest of the fin by a notch — the "split" that gives the splitfins their name — forming the organ used to transfer sperm during internal fertilisation.
Habitat
Girardinichthys multiradiatus is endemic to the highlands of central Mexico, centred on the upper Río Lerma drainage and reaching across the States of Mexico, Michoacán, Querétaro, Hidalgo and Morelos. Beyond the Lerma it occurs in parts of the Balsas drainage (the Cutzamala basin and reservoirs such as Presa El Bosque), in the Río Pánuco drainage north of the Lerma divide, and in the endorheic Lagunas de Zempoala, the only Morelos habitats for the species. It is the highest-altitude goodeid known, the Zempoala lagoons sitting near 2,800 metres — a true "mountain climber" among splitfins.
The fish favours quiet or slowly moving small waters: spring-fed ponds, ditches, irrigation channels, dams, the still margins of cool streams, and shallow lakes. Depths are slight, generally under a metre and often half that, over substrates of silt, mud, clay, sand, gravel or rock, in water from clear to muddy and usually well vegetated with duckweed, watercress, Salvinia, Potamogeton, Scirpus and Juncus.
Among goodeids it shows unusually broad water-chemistry tolerance, occupying soft to hard water across a pH span of roughly 6.0 to 9.0. Field measurements at one dam recorded very soft water, stable pH between 7 and 9, dissolved oxygen of 7 to 10 ppm, and a strongly seasonal temperature regime — around 68–77 °F in the warm months and dropping to 50–64 °F in the cold season. FishBase summarises the species as temperate, with a 54–68 °F range.
Feeding
Girardinichthys multiradiatus is polytrophic but leans carnivorous, with a low trophic level near 2.2. Examination of stomach contents shows the diet dominated by insect larvae and small crustaceans: chironomid and other dipteran larvae make up close to half the intake, with detritus and cladocerans together accounting for much of the remainder, and mayfly nymphs and hymenopterans taken in smaller amounts. In sum, a handful of prey categories — midge larvae, organic detritus and water fleas — supply the great majority of the diet.
This bottom-and-margin foraging fits the fish's habitat of shallow, vegetated, invertebrate-rich waters, where it works the substrate and plant cover for larvae and small drifting animals.
In the aquarium it accepts a correspondingly varied menu. Small live and frozen invertebrates — Daphnia, insect larvae, newly hatched brine shrimp — suit it best and bring it into condition, supplemented by good-quality flake, granules and sinking tablets. As with most goodeids, a diet weighted toward animal foods, with some plant matter, maintains colour and breeding readiness.
Mating
Like all goodeines, Girardinichthys multiradiatus reproduces by internal fertilisation. The male's andropodium — formed from the shortened, notched leading rays of the anal fin — delivers sperm to the female; there is no true tubular gonopodium as in the poeciliids, and goodeids generally do not store sperm long-term the way guppies do, so successful broods depend on repeated mating through the season.
Courtship is visual and the setting for some of the species' most studied behaviour. Males display their enlarged, yellow, black-edged fins to females and present ultraviolet-reflecting flank markings; because females see into the ultraviolet, this UV signal forms part of how they assess potential mates. The same conspicuousness carries a cost: predators of the species, including the blackbelly garter snake (Thamnophis melanogaster), are themselves sensitive to these visual cues, setting up the familiar tension between attracting mates and avoiding being eaten.
Reproduction is strongly seasonal and temperature-cued. Spawning begins around March as the water warms, peaks across the May-to-September period of highest gonadal activity, and shuts down both in the coldest months and, at the other extreme, once temperatures climb above roughly 75 °F.
Breeding
Girardinichthys multiradiatus is viviparous and matrotrophic — the defining reproductive mode of the Goodeinae. After internal fertilisation the female does not simply carry yolk-provisioned eggs; instead the developing embryos are nourished directly by the mother through trophotaeniae, ribbon-like rosette structures extending from the embryo that function as a placental analogue, absorbing nutrients within the ovarian cavity. The young are therefore born comparatively large and well developed.
Gestation lasts roughly 55 days, and a female produces a modest brood of about 10 to 20 young, occasionally up to 30 — small numbers compared with the dozens a guppy may drop, but each fry is larger and more advanced at birth. Reproduction continues through most of the year except the coldest months (roughly November to February), tracking the seasonal temperature window. Unlike some poeciliids, goodeids of this kind do not exhibit superfetation — there is a single developing brood at a time rather than overlapping cohorts at different stages.
In captivity the species breeds readily when its seasonal rhythm is respected. A cool winter rest below about 64 °F, down toward 50 °F, for two to three months pauses fry production and conditions the fish; spawning then resumes at around 63–64 °F as temperatures rise, and stops again above about 75 °F. Newborn fry are large enough to take brine shrimp nauplii and finely prepared foods from the outset.
In the aquarium
Girardinichthys multiradiatus is a coolwater specialist, and keeping it well means resisting the tropical-aquarium default. It suits a tank of at least 20 US gal with a low, wide footprint — height of around 10 in is ample — moderate current, and high oxygenation (8 mg/L or better), reflecting the cool, well-aerated streams and ponds it comes from. Dense vegetation in the corners, together with roots and wood, gives cover and structure, while leaving open swimming space across the broad floor.
It is not a delicate fish chemically — its wide tolerance of hardness and pH (roughly 6.0 to 9.0) makes water chemistry undemanding — but it is sensitive to warmth and to poor water quality. Large weekly water changes of 60 to 80 percent keep conditions clean, and the fish positively benefits from a genuine cool winter period rather than a year-round constant temperature. In suitable climates it can be kept outdoors from spring through autumn, which encourages natural colour and condition.
Feeding follows the wild diet: a base of small invertebrates such as Daphnia and insect larvae, brine shrimp nauplii, and good-quality flake, granules or tablets. Kept cool, fed well and given a winter rest, it is a rewarding subject — and, as an endangered endemic, a species whose maintenance in dedicated aquarist hands has real conservation value through groups such as the Goodeid Working Group.
Conservation
Girardinichthys multiradiatus is assessed as Endangered on the IUCN Red List (criteria B2ab(i,ii,iii,iv)), with a decreasing population trend. The species declined substantially through the twentieth century, and what was once a wide distribution across the Mexican Plateau is now a set of remnant, often isolated stocks. The drivers are the familiar ones for highland Mexican fishes: drainage and pollution of springs, ponds and channels; water extraction; habitat modification; and the spread of introduced species. National assessments under Mexico's NOM-059-SEMARNAT-2010 have not formally categorised it as at risk, but the international assessment and the published literature both place it among the declining goodeids.
The two evolutionarily significant units differ in their prospects. The Zempoala lagoon population is isolated in colder, higher water and benefits from protection within a national park, while the more widespread Lerma–Pánuco–Balsas lineage is exposed to the heavier human pressures of the central plateau's agricultural and urban landscapes.
For a species in this position, aquarium populations are a meaningful insurance. Coordinated captive maintenance — notably through the Goodeid Working Group — holds genetically managed stocks against further wild decline, and the species' willingness to breed under the right cool, seasonal regime makes such ex-situ conservation realistic. The responsible aquarist keeps documented, locality-aware stock and never releases fish into the wild.