Taxonomy & naming
Allodontichthys tamazulae was described by Clarence L. Turner in 1946, in the Occasional Papers of the Museum of Zoology, University of Michigan (No. 495). The holotype, an adult female of about 1 in standard length, was collected by Turner himself on 3 April 1939 in the Río Tamazula just above the town of Tamazula de Gordiano, Jalisco, together with a small series of paratypes; the specimens are held at the University of Michigan Museum of Zoology (UMMZ-143022 and 143023). The species sits in the order Cyprinodontiformes, family Goodeidae (the splitfins), subfamily Goodeinae.
The genus Allodontichthys was erected by Hubbs and Turner in 1939 to separate these fishes from Zoogoneticus on the basis of their distinctive jaw teeth — compressed, shouldered and keeled rather than simply conical. The genus name reflects exactly this character: from the Greek allos ("other" or "different"), odous ("tooth") and ichthys ("fish"), it means roughly "fish with different teeth." The species epithet tamazulae is a toponym honouring the type locality at Tamazula de Gordiano, a town whose own name derives from the Nahuatl tamazullan, "place of toads."
Eschmeyer's Catalog of Fishes governs the valid name and treats Allodontichthys tamazulae Turner, 1946 as valid. In the hobby and conservation literature the fish goes by several common names, including Tuxpan splitfin, peppered splitfin, and the Spanish mexcalpique de Tamazula.
Morphology
Allodontichthys tamazulae is a small, elongate goodeid built for life in moving water, with a streamlined body and a somewhat flattened ventral profile suited to a bottom-dwelling existence among rocks and riffles. Reported maximum size differs between sources: FishBase lists a modest 2 in standard length, while the standard regional reference (Miller, Minckley & Norris, Freshwater Fishes of México, 2005) gives a considerably larger maximum of about 3 in SL. The discrepancy is worth noting, and the larger figure from the dedicated regional treatment is generally taken as the more complete account of the species' full size range.
The genus is defined by its teeth — compressed, shouldered and keeled jaw teeth that distinguish Allodontichthys from related goodeids and reflect a predatory feeding style. The gut is correspondingly short, only about two-thirds to three-quarters of the standard length, as is typical of a carnivore.
The species is sexually dimorphic. Males carry the modified anal fin that gives the splitfins their name: the first several anal-fin rays are shortened and slightly separated to form an andropodium, the goodeid mating organ. Males are also the brighter sex, with a larger and darker dorsal fin, a bluish cast to the back and a line of dark blotches along the flank, while females are duller and predominantly yellowish.
Habitat
The Tuxpan splitfin is endemic to Jalisco, Mexico, in the upper Río Coahuayana basin of the Pacific slope. Its range covers the Ríos Tamazula and Tuxpan (the Coahuayana takes the name Tuxpan along part of its course), reaching downstream to roughly 45 river-kilometres below the town of Tuxpan, the upper Río El Tule drainage (including the Ríos El Terrero and La Trampa), and the Río Pihuamo of the Río Ahuijullo drainage, where the species was confirmed as recently as 2019. Conservation assessments recognise three subpopulations — Río Tuxpan (the type drainage), Río Ahuijullo and Río El Tule — corresponding to a single accepted evolutionarily significant unit.
This is a fish of small, rocky streams and riffles, living over substrates of sand, rock and boulder with sparse or absent green-algal vegetation. It tolerates a range of conditions, from turbid to clear water and from slight to swift current, and is typically found in shallow water, usually no more than half a metre deep and rarely beyond a metre. It is fundamentally a bottom-dweller and riffle-inhabitant, often holding position afloat in the current or sheltering in dense root mats and riparian vegetation.
Notably, A. tamazulae is niche-separated from its congener A. hubbsi where the two occur together: the Tuxpan splitfin favours open current and root tangles, while A. hubbsi hides beneath rocks. This partitioning lets the two splitfins share the same streams without competing directly for the same microhabitat.
Feeding
Allodontichthys tamazulae is a carnivore, a fact written into both its anatomy and its gut contents. The compressed, shouldered teeth and the short intestine (about two-thirds to three-quarters of the standard length) are the equipment of a predator rather than a grazer, and FishBase places the species at a low trophic level of about 2.1.
In the wild the fish takes substantial invertebrate prey for its size, including large insect larvae up to around 0.5 in. Recorded and likely foods include bloodworms and other insect larvae, water fleas, mysids, and hellgrammites (the aquatic larvae of dobsonflies); juvenile Macrobrachium prawns have also been suggested as prey. As a riffle specialist it presumably picks much of this from the fast water and the crevices between rocks where stream invertebrates concentrate.
In captivity the species accepts a carnivorous diet of small live and frozen foods in keeping with its natural feeding — bloodworm, insect larvae and similar items — reflecting its predatory build rather than the omnivory typical of many aquarium livebearers.
Mating
As a member of the family Goodeidae, Allodontichthys tamazulae practises internal fertilisation, but its mating apparatus differs from that of the more familiar poeciliid livebearers such as guppies and platies. Rather than a true gonopodium formed from elongated anal-fin rays, male goodeids use an andropodium: in A. tamazulae the first several anal-fin rays (after roughly the first seven) are shortened and slightly separated, forming the notched "split fin" that gives the family its English name. This structure is used to transfer sperm to the female during mating.
Reproduction is seasonal in this species. The breeding period runs from early winter through spring, and field observers have found gravid females in a range of developmental stages alongside freshly born fry in March and June of 2019. Turner, collecting in April, found no gravid females at that time, consistent with a reproductive cycle that does not run year-round.
This seasonal rhythm is tightly linked to temperature, and it shapes how the fish must be kept and bred in aquaria (see below): spawning is triggered as cool winter water warms into spring, rather than proceeding continuously as it does in tropical poeciliids.
Breeding
Allodontichthys tamazulae is viviparous and, like all goodeids, matrotrophic: the developing embryos are nourished inside the mother rather than living off a yolk supply. Nutrition is delivered through trophotaeniae, ribbon- or rosette-like extensions that grow from the embryo's vent and act as an umbilical-like connection to the maternal tissue — the defining reproductive innovation of the family Goodeidae. The young are therefore born as comparatively large, fully formed free-swimming fish rather than hatching from eggs.
Breeding is seasonal and temperature-dependent. In the aquarium the species requires a genuine winter rest: a cool period below 68 °F, ideally maintained for two to three months, is needed to halt fry production and reset the cycle. Spawning then resumes as the water warms in spring, at around 68–70 °F, and stops again once temperatures climb above about 77 °F. Without this cool dormant phase the fish will not breed reliably, and sustained warmth is actively harmful (see In the aquarium).
Fry are vulnerable to predation by the adults, including their own parents, and are usually eaten unless the tank offers plentiful hiding places and ample food. Successful rearing therefore depends on heavily structured aquaria and, often, on separating or protecting the young — a contrast with the prolific, hands-off breeding of guppies, and a reflection of this species' slower, seasonal, specialist reproductive strategy.
In the aquarium
The Tuxpan splitfin is a fish for the dedicated coolwater keeper rather than the general community aquarist. It does best in a spacious tank of at least about 40 US gal with a low, wide footprint — height is not important, and around 10 in is fine — strongly structured with rocks and roots or wood to provide both current breaks and sight barriers. It wants moderate to swift flow and high oxygen levels (around 8 mg/l or more), mirroring its native riffles, and benefits from large weekly water changes on the order of 60–80%.
Temperature management is the crux of keeping this species well. It must be kept cool, with a winter rest below 68 °F for two to three months; it spawns at 68–70 °F as spring warmth arrives but stops above about 77 °F, and constant temperatures above roughly 75 °F cause sickness and shortened life. The species does well kept outdoors from spring to autumn in suitable climates — brought outside once daytime water temperatures exceed about 63 °F and returned indoors before they fall below that mark.
Behaviourally it is less aggressive than its congeners, but adults do fight, so the aquarium should be arranged so that fish cannot constantly see one another. Fry will usually be eaten unless there is dense cover and abundant food. None of this makes A. tamazulae a beginner's fish, but for goodeid specialists it is a rewarding, characterful riffle-dweller.
Conservation
Allodontichthys tamazulae is assessed as Vulnerable on the IUCN Red List, under criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv), with the assessment dated 7 March 2018. The listing reflects the species' restricted range within a single small basin and the ongoing degradation of its habitat, rather than evidence of steep recent decline; the underlying assessment work (Lyons and colleagues, 2019) found the fish at around ten localities, several with moderately large numbers, and judged the populations to be broadly stable.
The threats are largely those of a small endemic squeezed into developed watercourses. Pollution from a sugarcane mill at Tamazula has rendered the lower Tamazula River uninhabitable for the species since the 1970s; sewage from the town of Tuxpan and effluent from a paper mill at Atenquique add further pressure. These are localised, point-source impacts on a fish whose entire world is a handful of connected streams, which is precisely why so narrowly distributed a species is treated as Vulnerable.
Mexico affords the species additional national protection: under NOM-059-SEMARNAT-2010 it is listed as P (en peligro de extinción, "in danger of extinction"). The Goodeid Working Group and allied hobbyist conservation efforts also play a role here, maintaining captive populations of Mexican goodeids as a hedge against further habitat loss in the wild.