Taxonomy & naming
Poeciliopsis scarlli was described in 1985 by Manfred K. Meyer, Rüdiger Riehl, J. A. Dawes and Ivan Dibble, in the French aquarium-science journal Revue française d'Aquariologie, from material collected in Michoacán, on Mexico's Pacific slope. The original description (Rev. Fr. Aquariol. 12(1): 23–26) established it as a new taxon within Poeciliopsis, the large genus of New World livebearers commonly called guatopotes in Mexico. Eschmeyer's Catalog of Fishes governs the valid name, and the species is placed in the family Poeciliidae, subfamily Poeciliinae, order Cyprinodontiformes.
Its taxonomic standing is not settled. Some authorities — notably Miller and colleagues in the standard reference Freshwater Fishes of México (2005) — treat Poeciliopsis scarlli as a junior synonym of the wide-ranging Poeciliopsis turrubarensis, and the IUCN assessment explicitly flags that the taxonomy needs resolution. The species is retained here as a valid name pending that resolution, but the reader should understand that its distinctness from neighbouring guatopotes is genuinely in question rather than merely a matter of nomenclatural bookkeeping.
The genus name Poeciliopsis combines the Greek poikilos, "many-coloured" or "varied," with opsis, "appearance" — literally a fish resembling Poecilia. The species epithet scarlli is an eponym honouring John Richard Scarll (died 1945) of Doncaster, England, one of the collectors associated with the holotype material from Mexico.
Morphology
Poeciliopsis scarlli is a small livebearer. The maximum recorded length is about 1 in total length for males, after Wischnath's Atlas of Livebearers of the World (1993), with females reaching a larger size of roughly 2 in — the size disparity between the sexes being typical of the family, where females are the larger, deeper-bodied sex.
Detailed morphological data specific to this species are sparse in the cached sources, and beyond overall size and the standard poeciliid body plan little is firmly documented. In general form it is a slender, streamlined guatopote of the kind found in shallow coastal streams: a benthopelagic fish of modest proportions without the exaggerated finnage of domesticated livebearers. As in all poeciliids, the most diagnostic external structure is the male gonopodium — the modified anal fin used in internal fertilisation — but fine details of its ray morphology, the feature on which species separation in this genus often turns, are not reproduced in the available references. Given the unresolved question of whether scarlli is separable from Poeciliopsis turrubarensis, these are exactly the characters that would settle the matter, and the present account does not assert distinguishing colour or fin features that the cached sources do not support.
Habitat
The species is endemic to Mexico, restricted to the Pacific-slope drainages of the states of Guerrero and Michoacán. FishBase and the IUCN both characterise it as a fish of streams near the coast, and a documented record places it at a canal at El Bordqueral, roughly 25 mi north along highway 200 from the Guerrero–Michoacán state boundary (Espinosa-Pérez et al. 1993; Page et al. 2013).
Its habitat is freshwater and tropical — permanent rivers, streams and creeks of the inland wetlands, together with coastal freshwater situations near the sea. FishBase records a temperature range of about 73–86 °F, consistent with the warm lowland coastal streams of western Mexico in which the species lives. As a benthopelagic fish it occupies the shallow water column of these small flowing waters rather than open lakes or deep channels.
Detailed water-chemistry data — pH, hardness, conductivity — are not given in the cached sources for this species specifically, and are not asserted here. What is clear is that this is a narrow-range coastal endemic of two adjacent Mexican states, a distribution small enough that its conservation outlook depends heavily on the condition of a limited set of Pacific-slope streams.
Feeding
No diet study specific to Poeciliopsis scarlli appears in the cached sources, so its feeding ecology is inferred from its body plan and from what is known of small stream-dwelling guatopotes generally. Like its congeners it is best understood as a micro-omnivore, taking a mixed diet of algae and biofilm grazed from surfaces, organic detritus, and small aquatic invertebrates and insect larvae picked from the substrate and water column of its coastal streams.
This opportunistic, generalist feeding is characteristic of the small Poeciliidae, which exploit whatever drifting and benthic food their shallow waters offer and shift the animal-to-plant balance with availability. In the absence of a published gut-content analysis for this species, the present account treats its diet as that of a typical small omnivorous livebearer rather than asserting a specialised feeding mode the sources do not support.
Mating
Poeciliopsis scarlli reproduces by internal fertilisation, the defining feature of the poeciliid livebearers. Males possess a gonopodium — a slender intromittent organ formed from modified rays of the anal fin — through which sperm is transferred directly to the female. This dispenses with the egg-scattering and external fertilisation of egg-laying fishes: mating is a matter of the male manoeuvring to bring the gonopodium into contact with the female, often combining display with persistent pursuit in the manner typical of small livebearers.
As in many poeciliids, females are able to store viable sperm internally after mating, allowing successive broods to be produced from a single insemination. Superfetation — the simultaneous carrying of more than one brood at different stages of development — is known in some members of the genus Poeciliopsis, but it is not documented specifically for P. scarlli in the cached sources and is therefore not asserted here for this species. The detailed courtship behaviour of scarlli has not been described in the available references, and the account is kept to the reproductive mechanics that are firmly established for the genus.
Breeding
Poeciliopsis scarlli is viviparous: after internal fertilisation the female carries the developing young and gives birth to free-swimming fry rather than laying eggs. FishBase records a gestation period of about 28 days, after which the female produces a brood of roughly 10 to 20 young — a modest brood size in keeping with the small size of the species.
The newborn are fully formed, independent miniatures able to swim and feed from birth, as is the rule among livebearers; there is no parental care, and in the confined conditions of captivity adults of small poeciliids will readily take their own fry, so cover or separation is the usual route to raising numbers. FishBase rates the species' resilience as high, with a minimum population-doubling time of under 15 months, reflecting the rapid maturation and continuous reproduction characteristic of small livebearers. Brood frequency, the role of stored sperm in successive broods, and any occurrence of superfetation in this particular species are not documented in the cached sources and are not stated here beyond what the genus reliably supports.
In the aquarium
Poeciliopsis scarlli is essentially absent from the ornamental aquarium hobby. No dedicated American Livebearer Association, Goodeid Working Group, or reputable biotope species page for this fish could be located in the cached research, and what husbandry guidance can be offered rests entirely on the species' biology rather than on a body of keeping experience. It is a true poeciliid livebearer — not a goodeid — and would be expected to behave as a small, hardy, undemanding guatopote.
On the basis of its natural conditions, a keeper would aim for warm water in the range of about 73–86 °F recorded by FishBase, with the clean, stable, gently flowing conditions of a coastal stream and a varied omnivorous diet of small flake, micro-foods, and live or frozen invertebrates. As a small, prolific livebearer it would likely breed readily in captivity and require fry cover to raise good numbers. These notes are extrapolations from the species' ecology and from general livebearer husbandry; precise tankmate, hardness and stocking recommendations specific to scarlli are not available in the sources and are not invented here.
Conservation
Poeciliopsis scarlli is assessed as Data Deficient (DD) on the IUCN Red List (version 3.1). The assessment was carried out on 23 March 2018 and published in 2019 (Espinosa-Pérez, H. 2019; assessment e.T191766A2002518). The Data Deficient listing reflects exactly the gaps that run through this whole account: more information is needed on the species' population size and trend, its habitat requirements, and the threats it faces before it could be placed in any category other than DD. The population trend is recorded as unknown and threats as unknown.
Compounding the difficulty is the unresolved taxonomy. Because some authorities regard scarlli as a synonym of the more widespread Poeciliopsis turrubarensis, the very unit being assessed is uncertain — a species cannot be confidently conserved, or confidently judged secure, while it is unclear whether it is a distinct entity at all. No conservation actions are currently in place for it.
For a narrow-range endemic of coastal Guerrero and Michoacán, the responsible reading of a Data Deficient status is caution rather than reassurance: the absence of evidence of decline is not evidence of safety. Resolving the taxonomy and surveying the small set of Pacific-slope streams the species occupies are the prerequisites to knowing whether it needs protection.