Taxonomy & naming
Allodontichthys zonistius was first described by Carl Leavitt Hubbs in 1932 (Copeia 1932, no. 2: 68–71) as Zoogoneticus zonistius, on the basis of an adult male holotype (MCZ 32818, 1.5 in SL) and a paratype (UMMZ 97316, 2 in SL) collected by Gustav Glueckert in 1913 from a tributary of the Río Colima near the city of Colima, Mexico. Turner later transferred the species to Alloophorus in 1937, before Hubbs and Turner erected the genus Allodontichthys in 1939 to hold this and its relatives, separating them from Zoogoneticus on the distinctive form of the jaw teeth — compressed, shouldered and keeled rather than simply conical. Eschmeyer's Catalog of Fishes treats the combination Allodontichthys zonistius (Hubbs, 1932) as the valid name, with Zoogoneticus zonistius and Alloophorus zonistius as historical synonyms.
The species sits in the family Goodeidae, subfamily Goodeinae — the viviparous goodeids endemic to the Mesa Central and Pacific slope drainages of Mexico. The genus name combines Greek allos (other, different) with odous (tooth) and ichthys (fish), 'fish with a different tooth,' a direct reference to the unusual dentition that justified the genus. The specific epithet zonistius derives from Greek zona (band) and istion (sail), meaning 'with a banded sail' — a reference to the black-banded dorsal fin of the male. Ichthyological work by Turner in 1946 on the ovarian and trophotaenial characters of unborn fry helped confirm the genus's relationship to Ilyodon within the Goodeinae.
Morphology
Allodontichthys zonistius is a small, elongate, robustly built goodeid adapted to life in current. Reported maximum size is modest: about 2 in standard length for males and around 2 in for females in the older aquarium literature (Wischnath 1993), with the largest known wild specimen recorded at 2.5 in SL (Miller et al. 2005). The body is somewhat streamlined and the fish is a strong, darter-like swimmer that hugs the substrate among rocks.
Colouration and patterning are diagnostic. The body carries dark-brown spotting above the lateral line and a characteristic black, comma-shaped 'Allodontichthys spot' immediately behind the operculum. The dorsal fin bears about five black bars edged in white — the banded 'sail' that gives the species its epithet. Males in condition take on a turquoise body broken by vertical brown blotches that read as a striped pattern, while females show roughly ten to twelve dark vertical bars and are altogether duller. The caudal fin carries a dark basal blotch followed by a light yellowish crescent. The jaw teeth are compressed, shouldered and keeled — the feature that defines the genus and reflects a carnivorous habit.
Habitat
The bandfin splitfin is endemic to west-central Mexico, in the states of Jalisco and Colima. Its core range is the Río Armería drainage — including the Río Ayuquila, the upper Río Armería down to its junction with the Río de la Lumbre, the Río Tuxcacuesco, the arroyos Ahuacapán and Manantlán, and the headwaters of the Río Colima near Comala. A second, smaller subpopulation occupies the lower Río Coahuayana drainage (the Río Salado system, including arroyos Cardona, Hondo and Grande and the Río El Cobaño near Cuauhtémoc), which the species is thought to have reached via a recent stream capture associated with the Colima volcano. The two subpopulations together form a single evolutionarily significant unit.
This is a strictly rheophilic, riffle-dwelling fish. It lives in clear, fast water over gravel, rocks and boulders, usually at depths under half a metre but occasionally to about a metre, where current is slight to moderate in the dry season and turbulent for the rest of the year. The fish shelters deep beneath large rocks, boulders and dead foliage; the only plant life in these habitats is typically green algae. Ecologically it is the Mexican analogue of the North American darters (Etheostoma, Percidae), occupying the same fast, rocky, oxygen-rich microhabitat. It co-occurs with the cichlid Amphilophus istlanus and, in places, with the goodeid Ilyodon furcidens.
Feeding
Allodontichthys zonistius is a carnivore, a habit reflected in both its anatomy and its environment. Its compressed, keeled teeth and short gut are those of a predator rather than a grazer, and in its swift, rocky habitat the available prey is dominated by benthic invertebrates. Gut contents of its congener A. tamazulae have been found to contain large insect larvae, and A. zonistius is understood to feed similarly — taking aquatic insect larvae and other small invertebrates from among the algae-coated rocks and gravel of the riffles it patrols. Its low trophic level estimate of roughly 2.1 reflects a diet of small animal prey rather than large or fish prey.
In the aquarium the species accepts a range of meaty foods in keeping with its wild diet. Keepers report success with bloodworms, insect larvae, Daphnia, Mysis and newly hatched brine shrimp, and the fish will also take good-quality flake, granulate and tablet foods. A varied, protein-rich diet supports both colour and breeding condition.
Mating
Like all Goodeinae, Allodontichthys zonistius reproduces by internal fertilisation, but the male's copulatory apparatus differs from the gonopodium of the New World poeciliids. Goodeid males do not fuse the anal-fin rays into a tube; instead the first several anal rays (about the first seven) are shortened and partly separated from the rest of the fin by a notch, forming the structure that gives the family its English name 'splitfins.' This modified region, the andropodium, acts as a conveyor that guides sperm toward the female during a brief copulation rather than as a long intromittent organ. FishBase notes the andropodium as a structureless lobule of the anal fin serving to assure fertilisation.
Sexual dimorphism in the species is pronounced and presumably tied to mate choice. Males develop a larger, more intensely coloured dorsal fin — the banded 'sail' — and a turquoise body overlaid with brown vertical blotches, while females remain duller and more heavily barred. Courtship and pairing in this rheophilic, rock-sheltering fish are not described in detail in the literature, and beyond the structural basis of mating the behavioural particulars are data sparse.
Breeding
Allodontichthys zonistius is viviparous and, characteristically of the Goodeinae, matrotrophic: the developing embryos are nourished by the mother throughout gestation through trophotaeniae — ribbon-like extensions of the gut that function as a placental analogue, absorbing nutrients from ovarian fluid. This contrasts sharply with the lecithotrophic (yolk-fed) live-bearing of poeciliids such as the guppy, and goodeids accordingly produce comparatively small broods of large, well-developed fry. Turner's 1946 study of the ovarian characters and trophotaenial features of unborn A. zonistius fry was important in establishing the species' affinities within the subfamily.
Gravid females have been collected in early April (Turner 1946) and in March and June (Goodeid Working Group records), indicating a breeding season running roughly from winter into early spring and on into summer. In aquaria the species is reported to time its reproduction to temperature: a cool winter rest below about 68 °F (ideally under 63 °F for two to three months) halts fry production, and spawning resumes as water warms to around 68–70 °F in spring, ceasing again above about 77 °F. Precise brood sizes for this species are not well documented in the available sources; goodeids in general produce modest broods, and exact figures here are data sparse. Superfetation — the carrying of multiple broods at different developmental stages simultaneously — is known in some poeciliids but is not a documented feature of Allodontichthys.
In the aquarium
This is a fish for keepers prepared to recreate a swift, cool, well-oxygenated stream rather than a still tropical tank. Successful husbandry calls for moderate to swift current, high dissolved oxygen (around 8 mg/l or more), and a hardscape of rocks and boulders supplemented with wood and roots that provide the deep crevices the fish naturally shelters in. A long, low aquarium of at least about 40 US gal suits it — height matters less than length, with around 10 in of depth being ample. Adults can be aggressive, so breaking up sightlines with structure helps reduce conflict. Large weekly water changes (60–80%) keep the water clean and oxygenated as the fish prefers.
Water chemistry should reflect its native riffles: pH around 7.0–7.7, moderate hardness (roughly 5–20 dH), and a tropical-to-subtropical temperature regime of about 70–77 °F in the active season, with a deliberate cool winter rest below 68 °F to cue breeding. Because day temperatures above about 63 °F suit it, the species can be kept outdoors from spring through autumn in suitable climates. A carnivorous menu of bloodworm, insect larvae, Daphnia, Mysis and brine shrimp, rounded out with prepared foods, maintains condition. This is a specialist's fish — rewarding to a conservation-minded keeper, but not a casual community species.
Conservation
Allodontichthys zonistius is assessed as Vulnerable on the IUCN Red List (assessment by Michael Köck, assessed 7 March 2018), under criteria B1ab(i,ii,iii)+2ab(i,ii,iii). The listing reflects a very restricted and fragmented range — the Río Armería basin plus the adjacent lower Río Coahuayana — combined with continuing pressures on the small, clear-water streams it depends on. Lyons et al. (2019) characterised the species as 'vulnerable but stable,' known from about twelve locations in the Armería basin and two in the lower Coahuayana. Under Mexico's NOM-059-SEMARNAT-2010 the species carries no formal risk category.
The principal threats are non-native fishes and water-quality degradation. Introduced largemouth bass, Lepomis sunfishes, tilapia and the poeciliid Poeciliopsis gracilis have been recorded at sites the species formerly occupied, and predation and competition from these invaders are a recurring concern in goodeid declines generally. Water quality is a mixed picture: improved treatment of sugar-mill discharge raised the fish's abundance in the Río Ayuquila near Autlán, but that gain has been offset by declines downstream near Colima. As with most narrowly endemic Mexican goodeids, the species' future depends on protecting the headwater stream habitat it requires and on controlling the spread of introduced fishes — and captive maintenance by groups such as the Goodeid Working Group provides an additional safeguard.