Taxonomy & naming
Xenotoca melanosoma was described by John R. Fitzsimons in 1972, in a revisionary paper in Copeia (4: 728–756) that sorted out several of the redtail-splitfin goodeids. The holotype — a mature male of 2.5 in standard length catalogued as UMMZ 189077 — was collected by Robert Rush Miller and H. L. Huddle on 3 April 1968 from the Río Tamazula below the Highway 110 bridge, about 3 mi south of the Guzmán turnoff in Jalisco, Mexico, within the Río Tuxpan (Coahuayana) drainage. That stream remains the type locality.
The genus name Xenotoca derives from the Greek xenos, "strange," and tokos, "birth" or "offspring" — "strange birth," a reference to the unusual viviparity of the goodeids. The specific epithet melanosoma combines the Greek melas, "black," with soma, "body," describing the near-black coloration that courting males develop and that distinguishes the species from its redtail relatives.
The placement of this fish in Xenotoca is provisional. Molecular and morphological work indicates that the redtail/black-splitfin clade — Xenotoca eiseni, X. doadrioi, X. lyonsi and X. melanosoma — does not belong with the true Xenotoca, and specialists such as the Goodeid Working Group write the name in quotation marks ("Xenotoca" melanosoma) pending a formal generic revision. The Catalog of Fishes (Eschmeyer, CAS) remains the authority for the valid name, currently Xenotoca melanosoma Fitzsimons, 1972, within the family Goodeidae, subfamily Goodeinae.
Morphology
Xenotoca melanosoma is a small, deep-bodied splitfin reaching about 3 in total length; FishBase records a maximum of 3 in TL, and Miller and colleagues (2005) cite specimens to roughly 3.5 in standard length. The body is robust and laterally compressed, in the typical splitfin mould, and the species is strongly sexually dimorphic in both shape and colour.
Males are the more striking sex. Outside of courtship they are a fairly uniform brownish-copper, but a displaying male darkens dramatically — the body turns nearly jet-black, the trait that gives the fish its name. Males also carry a larger, taller dorsal fin than females and bear the diagnostic goodeid mating structure: the first several anal-fin rays (about seven) are shortened and set off by a distinct notch, forming the andropodium, or "split fin," used in internal fertilisation. This notched anal fin is what separates the goodeid livebearers from the gonopodium-bearing poeciliids.
Females are plainer — whitish on the belly, brown over the back, with a dark midline and scattered spots or blotches along the flanks. In breeding condition courting females can flush a transient iridescent blue across the venter. As in most goodeids, females run somewhat larger and deeper-bodied than males, and a gravid female noticeably swells as the brood develops internally.
Habitat
The species is endemic to Jalisco, in the Pacific-slope drainages of west-central Mexico. It is documented from the Río Tuxpan (Coahuayana) system — including the type locality on the Río Tamazula and the Río El Terrero — and from the Río Ayuquila in the Armería basin, each population representing a distinct evolutionarily significant unit in the conservation literature.
Within these drainages the black splitfin occupies ponds, streams and small lakes ranging from clear to muddy water. Substrates are varied — mud, sand, gravel, rocks, boulders and bedrock — and the species is associated with habitats that carry little to no vegetation as well as those fringed with emergent and aquatic plants such as Cyperus, Eleocharis, Potamogeton, water hyacinth, Lemna, Nasturtium, Scirpus and Typha. Currents range from still to moderate, and the fish favours shallow water, generally less than a metre deep.
These are warm, low-elevation Pacific streams, and the fish is a tropical freshwater, demersal species. Its narrow distribution across only a few short stream reaches is central to its conservation plight: with so little occupied range, local degradation of any one site removes a large fraction of the whole.
Feeding
Xenotoca melanosoma is an omnivore that leans toward animal prey — the more carnivorous member of the black/redtail splitfin species pair. In the wild it takes predominantly aquatic animals: crustaceans, worms, insect larvae and other mid-sized invertebrates, with lesser amounts of algae and aufwuchs. Its low trophic level (estimated near 2.0 by FishBase) reflects a diet built on small invertebrates and some plant material rather than fish prey.
In the aquarium the species is undemanding at the table and accepts a broad range of foods. A varied diet weighted toward protein-rich items — small live and frozen foods such as Daphnia, brine shrimp, bloodworm and chopped invertebrates — suits its natural preferences, supplemented with quality flake or micro-pellets and some vegetable or algal content. Because the fish grazes and hunts more or less continuously, several modest feedings serve it better than one heavy meal, and well-fed adults come into breeding condition readily.
Mating
Like all goodeids, the black splitfin fertilises internally, but it does so without the rod-like gonopodium of the poeciliid livebearers. Instead the male uses the andropodium — the front rays of the anal fin, shortened and separated by a notch — to transfer sperm to the female. Courtship is bound up with the species' dramatic colour change: a male intent on spawning darkens to near-black and displays his enlarged dorsal fin to the female, who may herself flush an iridescent blue across the belly when receptive.
Reproduction is seasonal in the wild. Young have been captured in late spring, and breeding is documented at least from March through May, coinciding with warming water. Detailed behavioural data are sparse, in large part because the species is so rare and possibly already gone from its native streams; much of what is known comes from observation of aquarium stocks maintained by goodeid specialists.
Unlike some poeciliids, goodeids do not store sperm for long periods or practise superfetation, so each brood follows a discrete mating cycle rather than overlapping with the next. In captivity males can be intolerant of one another, and same-sized rival males will spar, so spawning groups are usually managed with this intraspecific aggression in mind.
Breeding
Xenotoca melanosoma is viviparous in the distinctive goodeid manner. After internal fertilisation the female carries the developing embryos and nourishes them directly through trophotaeniae — ribbon-like placental outgrowths that extend from the embryo and absorb nutrients from the mother, a matrotrophic strategy that sets the Goodeidae apart from the lecithotrophic (yolk-fed) livebearing of guppies and their kin. The young are therefore born comparatively large and well developed.
In the aquarium the species is bred at cool temperatures, around 68–70 °F; reproduction slows and stops above about 77 °F, so a seasonal rhythm is built into husbandry. Keepers commonly impose a winter cooling rest of two to three months below 68 °F to pause fry production and condition the breeders, then warm the tank to trigger spawning in spring. Broods are modest by livebearer standards, and the newborn fry — fully formed miniatures able to swim and feed at once — are readily eaten by adults, so dense planting or removal of the fry is needed to raise good numbers.
Because the wild population is so precarious, captive breeding is the species' main hold on survival. The Goodeid Working Group and allied hobbyists maintain studbook-style lines specifically to keep the genetic diversity of the surviving aquarium stock intact, treating each spawning as part of an ex-situ conservation effort rather than mere fishkeeping.
In the aquarium
This is a species for the dedicated keeper rather than the beginner — not because it is delicate in a tropical-fish sense, but because it needs cool, clean, well-oxygenated water and is increasingly a conservation responsibility. Goodeid specialists recommend a large, low and wide aquarium of at least 65 US gal; height is unimportant (10 in is ample) but footprint and water volume matter, structured with rocks, roots and patches of dense vegetation that give fry cover.
Water quality is the crux. The black splitfin wants moderate current, high dissolved oxygen (on the order of 8 mg/l or more), and large weekly water changes of 60–80 percent — goodeids are intolerant of the accumulated waste that easier livebearers shrug off. Temperatures should run cool, with spawning around 68–70 °F and reproduction ceasing above roughly 77 °F; a winter cool-down below 68 °F for a couple of months mimics the natural season and rests the breeders. Where summers are mild, the fish can be kept outdoors from spring to autumn while daytime water stays above about 63 °F.
Socially, males show intraspecific aggression toward same-sized rivals, so the group needs space and structure to break up sightlines, and fry are heavily predated by adults. None of this is exotic, but it adds up to a fish that rewards method and punishes neglect — and one whose continued existence may depend on keepers getting it right.
Conservation
Xenotoca melanosoma is assessed as Endangered on the IUCN Red List (species ID 191718), under criteria B2ab(i,ii,iii,iv), with a decreasing population trend. The assessment reflects a tiny occupied range — only a few short stream reaches — squeezed by water diversion, pollution and introduced species. In Mexico the fish is likewise listed as En Peligro de Extinción (P) under NOM-059-SEMARNAT-2010.
The two best-documented populations are both in deep trouble. The upper Río Tamazula stock survives at only two locations threatened by water diversions and invasive fishes, while the Río Ayuquila population — a single short reach downstream of El Grullo in the Armería basin — has suffered chronic water-quality degradation and had not been seen since around 2004, raising the prospect that it is already gone in the wild. Field surveys by the Goodeid Working Group in 2016, 2018 and 2019 were inconclusive, and parts of the historic range, such as the Río Tecalitlán, are heavily polluted; the species is now widely regarded as probably extinct in the wild.
That uncertainty makes the aquarium population unusually important. Much of the surviving genetic stock is held by hobbyists coordinating through the Goodeid Working Group, who maintain the species precisely so that, should suitable habitat be secured, reintroduction remains possible. For the black splitfin, responsible captive husbandry is not a hobby footnote but very likely the difference between persistence and extinction.