Taxonomy & naming
Allodontichthys polylepis was described by Mary Rauchenberger in 1988, in a Copeia paper that erected the species while revising the comparative morphometrics of the whole genus ("A new species of Allodontichthys (Cyprinodontiformes: Goodeidae), with comparative morphometrics for the genus," Copeia 1988(2): 433–441). The holotype is an adult male of 1.5 in standard length (UMMZ-213951), collected by R. R. Miller and colleagues in February 1976, with twenty paratypes also lodged at the University of Michigan Museum of Zoology. Eschmeyer's Catalog of Fishes treats Allodontichthys polylepis Rauchenberger, 1988 as the valid name, placing the species in the family Goodeidae, subfamily Goodeinae — the live-bearing splitfins endemic to the Mesa Central of Mexico.
The genus Allodontichthys was erected by Hubbs and Turner in 1939 and is distinguished from the related Zoogoneticus by its compressed, shouldered, keeled jaw teeth — the name itself, from Greek allos (other), odous (tooth) and ichthys (fish), translates roughly as "a fish with a different tooth." The species epithet polylepis, from Greek polys (many) and lepis (scale), refers to this fish carrying some 10–20% more lateral-line scales than its congeners, the trait that first marked it as distinct.
The English name in general use is Finescale splitfin (Spanish Mexcalpique de escama or escamitas). "Splitfin" is the family name for the goodeids, a nod to the notched anal fin of the male; "finescale" captures the same many-scaled character preserved in the Latin epithet.
Morphology
Allodontichthys polylepis is a small goodeid, reaching about 2 in standard length in males and up to 2 in in females, with the largest recorded specimen around 2 in SL (Miller et al. 2005). The body is the stout, slightly compressed shape typical of the genus, and the species' diagnostic feature is its scalation: it bears markedly more lateral-line scales than other Allodontichthys, the basis for the epithet polylepis. The mouth is armed with the genus' characteristic compressed, shouldered, keeled jaw teeth — in this species the outer-jaw teeth are shouldered and incipiently tricuspid — and the gut is notably short, only about 0.67–1.20% of total length, the proportions of a predator rather than a grazer.
This is a strongly dimorphic fish. Males are the more colourful sex: the upper body is separated from the lower by a series of black blotches, overlaid with shiny blue, green and turquoise areas, and a prominent black scapular blotch sits underlined by yellow and overlined by blue. Dominant males develop an orange to reddish caudal peduncle set against a dark grey caudal fin, and carry a larger dorsal fin. Females are duller, lacking the iridescent areas, but show a yellow zone behind the scapular blotch and a more intensely yellow anal fin.
The male's defining structure is the andropodium, the goodeine mating organ — formed by a notch following the first roughly seven shortened anal-fin rays. Unlike the elaborate intromittent gonopodium of the New World poeciliids, the goodeid andropodium is a simpler, structureless lobule of the anal fin that acts only as a conveyor to help direct sperm during internal fertilisation.
Habitat
Allodontichthys polylepis is endemic to the Pacific slope of Jalisco, in west-central Mexico, where it occupies clear creeks and small streams with riffles and pools. The substrate is a mix of sand, gravel, rock, boulders and some silty mud, often with green algae, and current ranges from swift over the riffles to slight or absent in the dry season; recorded depths are usually under half a metre, occasionally to about 4 ft. FishBase notes a tropical regime of roughly 70–77 °F, pH around 7.0–7.7 and hardness of 5–20 dH. Ecologically the species is a bottom-dwelling, riffle-inhabiting fish strikingly similar to the North American darters (Etheostoma) — an unusual niche for a goodeid, most of which favour quieter water.
The known range comprises just two areas, treated as a single evolutionarily significant unit. The type locality is the Arroyo Potrero Grande (Arroyo Grande), a short tributary of the Río Ameca west of Ameca; the second is the Arroyo Estanzuela (Río de las Bolas / de la Pola) and its tributary the Arroyo de Avalos (Arroyo Dávalos) in the Río Atenguillo subbasin. The two areas lie roughly 300 river kilometres apart yet are managed as one conservation unit.
The type-locality population in the Arroyo Potrero Grande had disappeared by around 2000 — possibly extinct in the wild there — attributed to competition from the introduced Green Swordtail (Xiphophorus hellerii) and to pollution. The surviving populations in the Atenguillo system persist only at very low density.
Feeding
Allodontichthys polylepis is a carnivore, a trait written into its anatomy: the short gut (well under 1.5% of total length) and the shouldered, incipiently tricuspid outer-jaw teeth are the equipment of a predator on invertebrates rather than a herbivore. Direct diet data for the species are sparse, but its feeding is inferred from its congeners — gut contents of the related A. tamazulae contained large insect larvae — and from the riffle habitat it shares with darter-like invertebrate hunters. In the wild it takes aquatic invertebrates: insect larvae, bloodworms, water fleas and similar small prey worked from the gravel and current.
In captivity it is readily fed a carnivore's menu — bloodworms, insect larvae, brine shrimp nauplii, and the occasional flake, tablet or granular food, even fragments of mussel flesh. Live and frozen invertebrate foods suit it best and are important for bringing fish into breeding condition. FishBase places the species at a trophic level of about 2.1.
Mating
Like all goodeids, Allodontichthys polylepis practises internal fertilisation, but it does so without the elaborate intromittent gonopodium of the New World poeciliids. The male's anal fin is modified into an andropodium — a notch following the first roughly seven shortened rays forming a structureless lobule that serves only to convey sperm to the female. There is no long, hooked, rod-like organ as in guppies and their kin; goodeine insemination is a briefer, close-contact affair, and the sexes pair by mutual approach rather than the male pursuit-and-sneak tactics of the poeciliids.
Sexual selection in this species runs along the lines visible in the male's bright finery. Dominant males develop the most intense colour — iridescent blue, green and turquoise flanks, the yellow-and-blue-bordered scapular blotch, and an orange-red caudal peduncle — and a larger dorsal fin, advertising condition and status to females and rivals alike. The fish are aggressive among themselves, and adult males in particular contest space, behaviour that aquarists must plan around when housing them.
Unlike many poeciliids, goodeids do not store sperm for successive broods and do not show superfetation (carrying overlapping broods at different stages); none has been reported in this species. Each brood follows a discrete mating, in keeping with the goodeine reproductive pattern.
Breeding
Allodontichthys polylepis is viviparous — a true livebearer that gives birth to free-swimming young rather than laying eggs. Goodeids differ fundamentally from poeciliid livebearers in the mechanics of gestation: the developing embryos are nourished not by a large yolk supply but matrotrophically, by the mother, via ribbon-like trophotaeniae — placenta-analogue structures that extend from the embryo's vent and absorb nutrients directly from the maternal ovarian fluid. The result is a small number of comparatively large, well-developed young.
Field observations are limited, but Miller (2005) reported gravid females and small young in this species from late February into early March, indicating a spring breeding season in the wild. Brood sizes in Allodontichthys are modest — a feature of the matrotrophic goodeine strategy, which trades many small fry for few large ones — and no superfetation has been recorded.
In captivity the species is bred under a seasonal regime that mimics its stream origins. Aquarists give the fish a cool winter rest below about 68 °F for two to three months; spawning then occurs at around 68–70 °F and ceases above 77 °F. The well-formed fry are large enough at birth to take substantial first foods, and because the species is scarce, every brood is significant — the captive lineage now stems from a tiny founding stock, which makes careful, generation-by-generation breeding management central to keeping it alive.
In the aquarium
Allodontichthys polylepis is a specialist's fish, not a community livebearer, and the husbandry that keeps it well is built around its riffle-stream origins. Keepers recommend a tank of at least 40 US gal with a low, wide footprint — a height of around 10 in is ample — well structured with rocks and roots and run with moderate to swift current and high oxygen (8 mg/l or more). The fish are aggressive among adults, so space and structure to break sight lines matter, and large weekly water changes of 60–80% keep conditions clean enough for them.
Temperature management is seasonal rather than static. A cool winter rest below about 68 °F for two to three months conditions the fish to breed; spawning follows at roughly 68–70 °F and stops above 77 °F. Where summers allow, the species can be kept in outdoor tubs or ponds from spring through autumn whenever daytime water temperatures stay above about 63 °F, which suits its temperate-stream physiology. Diet is straightforwardly carnivorous — bloodworms, insect larvae, brine shrimp, and supplementary prepared foods.
This is emphatically a fish for experienced keepers willing to take part in its survival rather than simply enjoy it. With perhaps only a few hundred specimens in existence at the low point, the captive population is coordinated through programmes such as the Goodeid Working Group, and responsible keeping means breeding it, sharing fry into the managed population, and never letting a hard-won lineage dwindle to nothing in a single tank.
Conservation
Allodontichthys polylepis is assessed as Critically Endangered on the IUCN Red List (criteria B1ab(i,ii,iv)+2ab(i,ii,iv); assessed 17 April 2018), and is listed under Mexico's NOM-059-SEMARNAT-2010 as P (En Peligro de Extinción — in danger of extinction). Its decline has been severe and well documented. By 2000 the type-locality population in the Arroyo Potrero Grande had gone, apparently driven out by the introduced Green Swordtail (Xiphophorus hellerii) and by pollution. The surviving Atenguillo-system populations in the Río de las Bolas (de la Pola) and the Arroyo de Avalos (Dávalos) persisted but kept declining — an intensive 2016 survey recovered only a single individual from each water body. Lyons et al. (2019) classed the species as Critically Endangered and declining.
The species was widely thought extinct in the wild until the 2016 rediscovery of those last few individuals. Its real lifeline has been captivity: it was first brought to Europe by A. Radda in 1987 and bred by H. Stefan, but by around 2011 only some 30–40 specimens survived in captivity, earning it a reputation as one of the rarest fishes in the world. A coordinated effort by the Goodeid Working Group and Aqualab Morelia, with European stewardship centred on the Haus des Meeres in Vienna, rebuilt the captive population to a few hundred fish by 2015–2017. (An earlier semi-captive Fish Ark Mexico programme begun in 2008 failed for this particular species.)
The conservation lesson here is the inverse of the usual aquarium-trade caution. For Allodontichthys polylepis the hobby is not a threat but a lifeboat: a fish that has all but vanished from its native creeks survives mainly in the tanks of organised breeders, and its future depends on keeping that managed captive lineage healthy, genetically diverse, and — wherever the streams can be secured — available for eventual reintroduction.