Taxonomy & naming
Xenotoca lyonsi was described in 2016 by Omar Domínguez-Domínguez, Bernal-Zuñiga and Piller (Zootaxa 4189(1):81–98) as one of two species carved out of the long-recognised redtail splitfin, Xenotoca eiseni — the other being Xenotoca doadrioi. The three forms had previously been lumped together under the eiseni name, but morphological and geographic evidence supported recognising distinct species, with X. lyonsi occupying the Coahuayana drainage of Jalisco, Mexico. The holotype is an adult male of 2 in standard length, collected on 12 July 2015 and catalogued as CPUM-9590, from the Río Tamazula near Tamazula de Gordiano.
The genus name Xenotoca combines the Greek xenos, "strange," with tokos, "birth" or "offspring" — "strange birth" — a reference to the unusual viviparity of these goodeids, whose embryos are fed internally rather than developing from yolk alone. The species epithet lyonsi honours Dr. John D. Lyons, the American ichthyologist of the University of Wisconsin Zoological Museum, for his contributions to the understanding of the distribution, ecology and conservation of Mexican goodeids.
The species sits in the subfamily Goodeinae (the splitfins) within Goodeidae, order Cyprinodontiformes. The Goodeid Working Group notes that X. lyonsi may in fact belong to an undescribed genus and is retained in Xenotoca pending a fuller revision; for now Eschmeyer's Catalog of Fishes governs the valid name as Xenotoca lyonsi.
Morphology
Xenotoca lyonsi is a small, stout-bodied splitfin reaching about 2 in standard length — FishBase gives a maximum of 2 in SL for males and 2 in SL for females, while the original description cites a maximum of 2 in SL. The body is the typical robust, slightly deep goodeid shape, well suited to life in current. Meristic counts include 12–14 dorsal soft rays and 13–14 anal soft rays; females have been recorded with 13 dorsal and 14 anal rays, 12 pectoral rays, 8 caudal-peduncle scales and 30–31 scales in the lateral series.
Colouration is the species' most striking feature and the source of its "redtail" common name. Males carry an orange-to-red caudal peduncle and caudal fin, often set off by iridescent blue scales over the front of the body and a whitish belly that contrasts sharply with a darker back. Females are far plainer: brownish overall, with a dark midlateral stripe, a gravid spot when carrying young, and clear, unmarked fins.
Sexual dimorphism is pronounced. Beyond the male's red tail and brighter iridescence, males develop the goodeine "splitfin" — an andropodium formed from the anal fin, in which the first roughly seven rays are shortened and separated by a notch from the rest of the fin and used in mating. Females tend to be larger-bodied and duller, lacking the andropodium and the vivid tail.
Habitat
Xenotoca lyonsi is endemic to Jalisco, Mexico, in the upper Coahuayana basin. Historically it occurred in the Río Tuxpán drainage — including the Río Tamazula and Río San Gregorio — in the Río El Terrero (a tributary of the Río Naranjo in the mid Coahuayana), and probably in a section of the Río Ayuquila in the neighbouring Río Armería drainage. The type locality is the Río Tamazula, about 3 mi north-east of Tamazula de Gordiano. Three subpopulations corresponding to two evolutionarily significant units have been recognised, one of which — the Río Ayuquila unit — is probably already extinct.
The streams it inhabits are strongly seasonal. At the type locality the water swings from turbid, deep, high-flow conditions in the rainy season to clear, low-flow water in the dry season, when the channel can shrink to shallow pools. The bottom is mud and gravel with only sparse aquatic plants, and the surrounding land is dominated by sugar-cane plantations.
Its native fish community includes Ilyodon whitei, Poecilia butleri, Allodontichthys tamazulae and Astyanax anaeus, alongside the now locally extirpated Xenotoca melanosoma, plus introduced common carp (Cyprinus carpio) and tilapia (Oreochromis sp.). The species is a freshwater, benthopelagic, tropical fish of low trophic level (estimated around 2.0), reflecting a herbivore-leaning omnivorous diet.
Feeding
Xenotoca lyonsi is an omnivore with a herbivorous lean, consistent with its low estimated trophic level. In the wild its diet is built mainly around algae and aufwuchs — the film of algae, micro-organisms and detritus that coats submerged surfaces — supplemented by small crustaceans and insect larvae taken from the stream bed and water column.
In captivity it is undemanding and accepts a wide range of prepared and frozen foods: flake, granules, tablets and freeze-dried brine shrimp are all taken readily. Because the wild diet is algae-rich, a good share of vegetable or algal matter benefits the fish and helps maintain colour and condition.
The Goodeid Working Group emphasises that splitfins are active grazers that do best with a varied diet rather than a single staple, and that conditioning on richer foods supports their seasonal breeding cycle.
Mating
Like all goodeines, Xenotoca lyonsi practises internal fertilisation, but its mating apparatus differs from that of the New World poeciliids. Where a guppy or molly male uses a gonopodium formed from elongated anal-fin rays, a male splitfin uses an andropodium: the leading roughly seven rays of the anal fin are shortened and set off by a notch from the rest of the fin, producing the characteristic "split" appearance and serving to transfer sperm to the female. This structure, present only in mature males, is one of the clearest marks of sex in the species.
Reproduction is seasonal and temperature-cued. As in the closely related Xenotoca eiseni, breeding is triggered when water temperatures rise to about 68–70 °F and shuts down above roughly 77 °F or during the cool winter rest, so that young appear from late spring onward. Males show mild intraspecific aggression, with occasional male-male sparring, but courtship does not involve nest-building or territory defence.
This seasonal rhythm is important in husbandry as well as in the wild: keepers deliberately impose a cool winter rest below 68 °F for two to three months to halt fry production and keep breeding stock healthy, mirroring the natural pause in reproduction during the cooler months.
Breeding
Xenotoca lyonsi is a true matrotrophic livebearer. Rather than simply retaining yolked eggs, the developing embryos are nourished directly by the mother through trophotaeniae — ribbon- or rosette-like extensions that emerge from the embryo's vent and absorb nutrients from the maternal ovarian fluid, functioning as a placenta-analogue. This mode of internal nourishment is the defining feature of the Goodeinae and one of the reasons the genus name means "strange birth." After gestation the female gives birth to comparatively large, fully formed, free-swimming young.
The reproductive period mirrors that of Xenotoca eiseni, with broods produced from late spring on once temperatures rise into the low 20s Celsius. The cache does not record a specific brood size for X. lyonsi; goodeids in general produce modest broods of relatively well-developed fry, smaller in number but larger and more advanced than the broods of poeciliids, so figures should not be assumed here (data sparse).
In the aquarium the fry are large enough to take finely powdered foods and newly hatched brine shrimp from birth and grow steadily. Encouragingly for breeders, the Goodeid Working Group reports that the fry are generally not predated by the adults, so they can often be raised in the same tank — a contrast with the cannibalistic poeciliids — provided the colony is well fed and has structure for the young to use as cover.
In the aquarium
Xenotoca lyonsi is a fish for the dedicated splitfin keeper rather than the casual community aquarist, and much of its care is shaped by the seasonal, fast-flowing streams it comes from. It prefers moderate-to-swift current, plenty of structure such as rocks and roots, patches of dense submerged vegetation, and high oxygen levels — ideally at or above 8 mg/l. A recommended minimum tank is around 40 US gal with a low, wide footprint; height matters little (about 10 in is fine), but surface area and flow do. Large weekly water changes of 60–80 % keep the fish in good health.
Temperature management is the crux of long-term success. The species should be given a cool winter rest below 68 °F for two to three months each year, both to pause fry production and to maintain the breeding stock; conversely, holding it at constant temperatures above 77 °F causes sickness and accelerated ageing. This seasonal cycling, unfamiliar to keepers used to tropical community fish kept at a flat warm temperature, is essential rather than optional.
Behaviourally the fish shows only mild intraspecific aggression, mostly male-male, and the generally low predation on fry means a colony can be self-sustaining in a well-planted tank. Given how few wild populations survive, maintaining captive colonies through coordinated efforts such as the Goodeid Working Group is itself a meaningful conservation contribution.
Conservation
Xenotoca lyonsi is assessed as Critically Endangered on the IUCN Red List (criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv); assessed 14 March 2018), reflecting a very small, severely fragmented range with continuing decline in extent of occurrence, area of occupancy, habitat quality and number of locations. It is a fish clinging to the upper reaches of a single basin, and one of its two evolutionarily significant units — the Río Ayuquila population — is thought to be already extinct.
The threats are local and severe. The species has been extirpated from roughly 60 % of its historical localities (Pedraza-Marrón, 2011), and it is conspicuously absent downstream of village wastewater discharges. Its remaining streams are surrounded by sugar-cane plantations, and the combination of water extraction, habitat modification and untreated urban sewage has eliminated most populations; Lyons et al. (2019) likewise characterise it as declining and largely confined to the upper Coahuayana basin. Curiously, Mexico's national wildlife list, NOM-059-SEMARNAT-2010, assigns it no risk category, a gap between national and international assessment.
Given this trajectory, conservation effort focuses both on protecting the surviving wild habitat and on maintaining genetically managed captive populations. Organisations such as the Goodeid Working Group treat the species as one that should plainly be considered endangered and coordinate ex-situ stocks as insurance against further losses in the wild.