Taxonomy & naming
The species was first described by Tarleton Hoffman Bean in 1887 as Characodon bilineatus, in the Proceedings of the United States National Museum, from specimens that the French-Mexican naturalist Alfredo Dugès had sent from Guanajuato. The holotype is an adult female of about 1.77 in total length (USNM 37832); no precise type locality is recorded, though Bean supposed it came from the Pacific slope of Guanajuato. The fish has since been shuffled through several genera: Frank Meek moved it to Skiffia in 1902, Regan treated it as Goodea bilineata in 1907, and in 1937 Hubbs and Turner erected the monotypic genus Neotoca for it.
This split persists today. Eschmeyer's Catalog of Fishes — the authority for valid names — retains the species in Skiffia as Neotoca bilineata, and that is the name used here. The Goodeid Working Group and much of the recent conservation literature, however, follow Neotoca bilineata, citing morphological and molecular distinctness; phylogenetic work (e.g. Ornelas-García et al. 2012) has recovered it as basal to or sister to the Girardinichthys–Skiffia group rather than nested firmly within Skiffia. Aquarists will encounter both names, and FishBase itself files the species under the slug "neotoca-bilineata" while still listing the genus as Skiffia.
The two name-fragments encode the fish's biology and history. The epithet bilineata is Latin for "two-lined," from bi- ("two") and lineatus ("marked with a line") — a reference to longitudinal markings that are most evident in preserved specimens. The competing genus name Neotoca, from the Greek neos ("new" or "young") and tokos ("offspring" or "birth"), was coined for the distinctively shaped trophotaeniae — the ribbon-like feeding structures of goodeid embryos — that nourish the developing young.
This fish was long listed as Skiffia bilineata; Eschmeyer's Catalog of Fishes places the species in Neotoca, and the current combination Neotoca bilineata is followed here.
Morphology
Neotoca bilineata is a small, deep-bodied splitfin. Females are the larger sex, reaching about 2.4 in total length in the wild, with a recorded maximum standard length of roughly 1.65 in (Miller et al. 2005); males are smaller and slighter. In warm aquaria the fish tends to stay considerably smaller, with females sometimes under 1.2 in and males under 0.8 in total length — a plasticity that aquarists should keep in mind when judging condition.
Sexual dimorphism is marked, especially in breeding fish. Courting males develop blackened dorsal and anal fins, a yellow-and-black wash over the flanks, and an enlarged dorsal fin set on a daintier, more compact body. Females are subtler — bluish-grey with a silvery longitudinal stripe, frequently a yellowish belly, and a dark gravidity blotch at the rear of the abdomen when carrying young; their sides may show ten to fifteen thin, regular vertical bars. The dentition is typical of a small omnivorous goodeid: bicuspid teeth arranged in two rows.
The single most diagnostic feature is the male anal fin. Like all male goodeids, N. bilineata bears not a true gonopodium but an andropodium — the first six or seven anal-fin rays are shortened and partly separated from the rest of the fin by a notch, giving the family its English name, the splitfins. This structure is used to deliver sperm during internal fertilisation but is not the elongated, mobile intromittent organ seen in poeciliids.
Habitat
The species is endemic to the highlands of west-central Mexico, spanning the states of Jalisco, Guanajuato and Michoacán. Historically it occupied around fourteen localities across three drainage systems: the upper Río Grande de Santiago and Lake Chapala, the middle Río Lerma (including the Laja and Turbio rivers and Lake Yuriria), and the endorheic Lake Cuitzeo / Grande de Morelia basin. FishBase records it specifically from the Río Lerma basin and the Río Grande de Santiago.
It is a fish of quiet, often turbid or muddy waters: lakes, slough-like channels, ponds, ditches and irrigation canals, typically less than a metre deep over bottoms of mud, silt, sand, clay or rock, with little to no current. Surveys have turned it up in water only a few centimetres to about 11.8 in deep, in flooded grass and among dense aquatic vegetation — green algae, water hyacinth (Eichhornia), Potamogeton, Chara, Typha, Scirpus and Azolla. It is notably cold-tolerant for a Mexican livebearer, capable of producing young at water temperatures as low as about 50 °F, an adaptation to the cool highland winters of the Mexican Plateau. FishBase gives a broad tropical range of 72–82 °F, pH 7.0–7.5 and hardness up to 20 dH.
Much of the original range is now lost. The species persists chiefly in the Lake Cuitzeo / Grande de Morelia drainage — Lake Cuitzeo itself, Cointzio Reservoir, the Queréndaro River and the Borbollón springs — plus a few sites on the Turbio River such as Presa Corralejo and Lago El Pedregal near Pénjamo and Abasolo.
Feeding
Neotoca bilineata is a microphagous omnivore. Its gut is relatively short for a goodeid — about one and a half to two times body length — pointing to a diet weighted toward animal matter rather than the long-gut herbivory of some relatives. In the wild it takes tiny invertebrates such as copepods and insect larvae, along with aufwuchs (the film of algae and micro-organisms on submerged surfaces) and detritus. Its low trophic level (about 2.1 in FishBase) reflects a diet built largely from the bottom of the food web.
In the aquarium it is undemanding and accepts a wide range of small foods. A varied menu — quality flake and micro-pellets supplemented with small live and frozen items such as Daphnia, cyclops, brine shrimp and finely chopped bloodworm — keeps the fish in colour and breeding condition, and a vegetable or algal component suits its partly herbivorous habits. As with most small, constantly grazing livebearers, several modest feedings serve it better than one heavy meal, and overfeeding is the more likely husbandry error.
Mating
Reproduction in N. bilineata is internal and viviparous, as in all goodeids, but the family departs sharply from the more familiar poeciliid livebearers. The male carries no elongated gonopodium; instead, fertilisation is achieved with the andropodium, the notched front portion of the anal fin formed from a handful of shortened rays. Sperm transfer is a brief, close-quarters affair rather than the persistent harassment-and-sneaking of guppies, and goodeids generally do not store sperm for long periods the way poeciliids do, so each brood tends to follow its own mating.
Courtship is driven by the colour change of the male. As he comes into condition, his dorsal and anal fins darken, the flanks flush yellow and black, and the dorsal fin enlarges; he displays this finery to grey, silver-striped females. The seasonal nature of breeding in the wild — concentrated in spring — means males invest in this display over a defined window rather than year-round.
The most consequential mating behaviour for this species, however, involves another fish entirely. Where the invasive guppy Poecilia reticulata has colonised its waters, female N. bilineata superficially resemble female guppies, and male guppies pursue and attempt to mate with them; the hooked guppy gonopodium can physically injure the goodeid females (Valero et al. 2008). This reproductive interference is one of the documented threats to wild populations and a stark illustration of how an introduced livebearer can harm a native one through mating behaviour alone.
Breeding
Goodeids practise a placental form of viviparity that is among the most elaborate in fishes. After internal fertilisation the embryos develop inside the female and are nourished not from a yolk but through trophotaeniae — ribbon- or rosette-like extensions of the gut that grow out through the vent and absorb maternal nutrients, functioning much like a placenta. This matrotrophic mode (the genus name Neotoca was coined for the distinctive trophotaeniae of these fry) means the young are born large, fully formed and well developed, and newborns often bear a remnant of the trophotaenial tissue for a short while after birth.
In the wild the breeding season runs from early March to late May, when gravid females and young are found together. Brood sizes are modest by livebearer standards: Meek (1902) recorded 27 young in a single female collected on 26 May near Huingo in the Cuitzeo basin, a figure consistent with the small body size of the species. Gestation, as in other goodeids, lasts several weeks.
The species is a tractable colony breeder in captivity. The fry are large and, helpfully, not usually cannibalised by the adults, so a planted tank will often raise young without intervention. Breeders exploit the fish's cold tolerance to regulate spawning: a winter cool rest below about 68 °F for two to three months pauses reproduction, which then resumes when temperatures rise to around 63–64 °F and tapers off above roughly 75 °F. Keeping the fish permanently warm suppresses breeding and shortens life, so the seasonal temperature swing is part of responsible husbandry.
In the aquarium
Neotoca bilineata is one of the oldest goodeids in the hobby — first imported to Europe around 1935, lost during the Second World War, and re-established in later decades — and it remains a rewarding subject for aquarists interested in conservation-oriented fishkeeping. A tank of about 16 US gal or more suits a group, and because the fish lives in shallow highland waters a relatively low water column (5.9–7.9 in) is perfectly acceptable. It appreciates dense vegetation, leaf litter, and structure of rocks and roots, with anything from no current to a moderate flow.
Water quality matters more than precise chemistry. The species does well in neutral to slightly alkaline, moderately hard water (pH around 7.0–7.5, up to about 20 dH) and tolerates lower dissolved oxygen than many fishes (down to roughly 6 mg/l), but it responds to clean, stable conditions; a 20–30% water change every couple of weeks keeps it in good order. The single most important husbandry point is temperature cycling: give the colony a cool winter rest below about 68 °F and avoid holding it constantly above 75 °F, which suppresses breeding and stresses the fish.
For temperament it is a peaceful, sociable species best kept in a group, and one with real significance beyond the tank — because wild populations are collapsing, hobbyist colonies maintained through networks such as the Goodeid Working Group function as a genetic ark. Keeping it well, breeding it, and passing on fry is itself a small act of conservation.
Conservation
Neotoca bilineata is assessed as Endangered (EN) on the IUCN Red List (species 191713), under criteria A2ac and B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv); the assessment dates to March 2018. In Mexico it is likewise listed by SEMARNAT under NOM-059 as "P — En Peligro de Extinción" (in danger of extinction). The trajectory behind these listings is steep: a 2000–2001 survey (De La Vega-Salazar et al. 2003) relocated the species at only about two of its fourteen historical sites, and Lyons et al. (2019) classify it as Endangered and still declining, with the Lake Chapala lineage regarded as extinct and a second evolutionarily significant unit considered critically endangered.
The threats are the familiar combination that has driven the broader collapse of the Mexican Goodeidae: water pollution, habitat modification and loss across the heavily populated Lerma–Santiago and Cuitzeo basins, and the spread of invasive species. Among the invasives, the introduced guppy is singled out as a specific menace — not only as a competitor but through reproductive interference, as male guppies harass and physically injure the similar-looking female goodeids (Valero et al. 2008).
The long-term survival of N. bilineata in the wild is genuinely uncertain. That makes the captive populations maintained by aquarists and by coordinated efforts such as the Goodeid Working Group disproportionately important: for a number of Mexican goodeids, hobby and conservation breeding stocks may already represent the most secure refuge of the species. Anyone keeping this fish is, in effect, a custodian of an endangered genome.