Taxonomy & naming
Ameca splendens was described by the American ichthyologists Robert Rush Miller and J. Michael Fitzsimons in 1971. It is the only species placed in the genus Ameca, which takes its name from the Río Ameca, the river system to which the fish is endemic. Eschmeyer's Catalog of Fishes treats Ameca splendens Miller & Fitzsimons, 1971 as the valid name, within the family Goodeidae.
The Goodeidae are a distinctive family of New World livebearers. The familiar subfamily, the Goodeinae or splitfins, is a radiation almost entirely endemic to the highlands of west-central Mexico, especially the Mesa Central and the upper Río Lerma–Santiago and neighbouring basins; a second, much smaller subfamily occurs in the south-western United States. The goodeids are set apart from the poeciliid livebearers (guppies, mollies, swordtails) by a fundamentally different mode of live-bearing: they nourish their developing young internally through specialised structures rather than relying on a yolk supply, and the males lack a true gonopodium, instead using a notched, shortened front portion of the anal fin — the andropodium — to transfer sperm. The split, divided look of that modified anal fin gives the splitfins their common name.
Morphology
Ameca splendens is a robust, deep-bodied, moderately large goodeid. Females are the larger sex, reaching around 4–4.5 in in total length, while males are smaller, more slender and more colourful, generally up to roughly 3 in. The body is solid and somewhat chunky, grey-green to olive over the back and silvery on the flanks, often with a scattering of dark flecks or spots.
The species' signature is the male's tail. The caudal fin carries a bold dark subterminal band followed by a bright yellow or gold terminal edge — the "butterfly" pattern that gives the fish its common name — and a displaying male in good colour is a striking sight. Males also show the family's diagnostic andropodium, the notched anterior portion of the anal fin used in mating, in place of the rod-like gonopodium of poeciliids. Females are plainer and heavier-bodied, lacking the bright caudal banding. As a goodeid, the species does not produce the elaborate domestic colour strains of the poeciliid livebearers; the fish kept in the hobby are essentially the wild-type animal, which is part of why captive stocks have conservation value.
Habitat
Ameca splendens is endemic to the Río Ameca basin of west-central Mexico, in the states of Jalisco and adjacent Nayarit, where it was historically found in a small number of springs, spring-fed pools, streams and associated waters on the Mexican plateau. The classic locality associated with the species is a spring system near the town of Ameca, and the fish's natural distribution has always been extremely restricted — a handful of upland water bodies rather than a broad river network.
The habitat is warm, hard, alkaline, often spring-fed water, characteristic of the volcanic highland streams of the region, with temperatures broadly in the low-to-mid 20s Celsius and a tendency toward clear, mineral-rich conditions. The fish favoured shallow, vegetated and current-bearing reaches where it could graze. This naturally tiny range is the heart of the species' predicament: a fish confined to a few highland springs has essentially no buffer against habitat loss, water extraction, pollution and introduced species — all of which have pressed hard on the Mesa Central goodeids as a group.
Feeding
Ameca splendens is strongly herbivorous, more so than most of the poeciliid livebearers. In the wild it grazes extensively on algae and the aufwuchs film of algae and micro-organisms coating submerged surfaces, supplementing this with detritus and small invertebrates. The fish has the gut and feeding behaviour of a grazer, working over plant and rock surfaces through the day.
In the aquarium this dietary leaning matters: the species does best on a vegetable-rich diet. Spirulina- and algae-based flakes and pellets, blanched vegetables, and a generous share of plant matter should form the bulk of the menu, with live and frozen foods such as Daphnia and brine shrimp offered for variety and conditioning rather than as a staple. A diet too heavy in rich protein and too poor in vegetable matter is a common cause of poor condition in captive goodeids, so a planted, algae-bearing tank that lets the fish graze naturally suits it well. Notably, butterfly splitfins will also nip and graze soft aquarium plants and have a reputation for browsing on filamentous algae, which can make them useful as well as decorative.
Mating
Goodeid mating differs fundamentally from that of the poeciliid livebearers. The male Ameca splendens uses the andropodium — the shortened, notched front of the anal fin — rather than a gonopodium to transfer sperm, and goodeids do not store sperm for long-deferred multiple broods in the way guppies and swordtails do, so successful reproduction depends on the regular presence of both sexes in breeding condition.
Males are active and display vigorously, courting females with lateral displays that show off the banded, gold-edged tail, and they spar with rival males. The species has a reputation among keepers for being bold, alert and even semi-aggressive, with a degree of personality unusual in a small livebearer; males in particular can be pushy toward one another and toward females, and may nip fins. Courtship is most intense when fish are well fed and warm, and the combination of the male's display behaviour and the distinctive andropodium-based insemination makes the splitfins a rewarding group for aquarists interested in livebearer biology beyond the familiar poeciliids.
Breeding
Ameca splendens is viviparous in the specialised goodeid manner, and breeding it is both straightforward and remarkable. Rather than provisioning eggs with yolk, the female nourishes her developing embryos internally; the young are connected to the mother by trophotaeniae — ribbon-like external nutritive structures, a kind of placental analogue, that absorb nourishment during development. As a result, gestation is relatively long for a livebearer, on the order of seven to eight weeks, and the young are born unusually large, well-developed and few in number compared with the prolific poeciliids.
Brood sizes are modest — often roughly a dozen to a few dozen large fry — and newborn goodeid fry frequently carry a temporary trailing remnant of the trophotaeniae, which is soon shed. The fry are big enough to take substantial first foods at once, including powdered flake, newly hatched brine shrimp and finely chopped foods, and they grow steadily. As with other livebearers, adults will eat fry, so cover or separation improves survival, though the larger fry are less vulnerable than tiny guppy young. The species' ease of captive reproduction is precisely what has made hobbyist and institutional breeding programmes the mainstay of its survival.
In the aquarium
The butterfly splitfin is a hardy, long-kept aquarium fish with a loyal following, valued for its intelligence, activity and the male's flashing tail — but it is not a fish for a delicate community. It is bold, busy and can be nippy, particularly the males, and it does best in a group in a roomy tank where aggression is spread out, ideally with a majority of females. A larger tank with open swimming space, some current and robust planting suits its grazing, restless nature; soft-leaved plants may be browsed.
Water should match the species' hard, alkaline, spring-fed origins: neutral to alkaline, medium-hard to hard water, in temperatures through the low-to-mid 20s Celsius. Many keepers find the species appreciates a slightly cooler, well-oxygenated, current-bearing setup rather than the warm, still tank suited to tropical poeciliids, and it tolerates a seasonal dip in temperature. Tankmates should be similarly sturdy and not long-finned or timid; other robust goodeids and hardy community fishes that share the hard-water preference are sensible companions. Beyond its qualities as an aquarium fish, keeping Ameca splendens carries a conservation dimension: because the wild population has collapsed, the captive-bred fish in hobbyist tanks form part of the species' surviving genetic reservoir, and responsible keepers maintain and distribute pure lines through livebearer associations and the Goodeid Working Group.
Conservation
Ameca splendens is one of the iconic conservation cases among aquarium fishes. Its wild range — a few springs and streams in the Río Ameca basin — has been devastated by water extraction, pollution, habitat destruction and the introduction of non-native fishes, and surviving wild populations are confined to a handful of precarious remnant sites. The IUCN Red List currently assesses the butterfly splitfin as Critically Endangered, the highest category short of presumed loss from the wild, reflecting the severe collapse of its natural populations.
What keeps the species from outright extinction is captivity. Ameca splendens has been bred by aquarists for decades and is maintained by livebearer societies, public aquaria and dedicated conservation breeding efforts — most notably the Goodeid Working Group, an international initiative devoted to preserving the threatened splitfins through coordinated captive husbandry. For this fish, the aquarium is not a luxury but a lifeboat, and the central conservation message for keepers is to maintain genetically sound, locality-true captive lines and to share them through these networks, so that a fish all but gone from its Mexican springs persists in the care of the hobby that first prized it.