Taxonomy & naming
Poecilia vandepolli was described in 1887 by the Dutch zoologist Theodorus Willem van Lidth de Jeude, working on collections from the Netherlands Antilles. The species sits in the genus Poecilia within the family Poeciliidae — the New World livebearers that include the guppies, mollies, platies and their relatives — and in the subfamily Poeciliinae. Eschmeyer's Catalog of Fishes governs the valid name, treating Poecilia vandepolli Van Lidth de Jeude, 1887 as valid. Modern phylogenetic work on the tribe Poeciliini, notably Rodríguez's 1997 analysis (Copeia 1997(4):663–679), placed the species within the broader Poecilia radiation.
The epithet is an eponym honouring Jacob Rudolph Hendrick Neervoort van de Poll (1862–1925), a Dutch entomologist and collector active in the Netherlands Antilles, whose material underpinned the original description. The genus name Poecilia derives from the Greek poikilos, "many-coloured" or "varied," a reference to the variegated patterning typical of these fishes — and apt here, given how variable P. vandepolli is across its range.
FishBase lists the species under the single territory of Curaçao, but iNaturalist and broader regional records treat it as native across the ABC islands of the Lesser Antilles — Aruba, Bonaire and Curaçao — which better reflects its actual distribution. The two leading English common names, "Vandepoll's molly" and "island molly," both trace to this island endemism.
Morphology
Poecilia vandepolli is a small molly. FishBase records a maximum length of about 2 in TL for the available (male or unsexed) reference material, placing it among the more modestly sized members of the genus. In overall build it is a typical Poecilia: a compact, somewhat deep body, a flattened dorsal profile suited to surface and shallow-water life, and an upturned mouth for taking food from the film and water column.
The most striking feature of the species is how plastic its appearance is. Body size, robustness and colour all vary significantly with the salinity of the water an individual grows up in: saltwater specimens tend to grow faster, become more robust, and develop more colour than their freshwater counterparts. This environmentally driven variation means that populations from different habitats on the same island can look noticeably different, which historically complicated the species' description and identification.
As in all poeciliids, the sexes differ. Males carry the gonopodium, a slender intromittent organ formed from modified rays of the anal fin and used to deliver sperm to the female. Males are the smaller, more colourful sex; females are larger, plainer and deeper-bodied, developing the dark gravid spot near the vent when carrying young. Detailed published colour descriptions for this species are sparse, and finer points of male pattern are best treated as variable rather than fixed.
Habitat
Poecilia vandepolli is an island fish of the southern Caribbean, native to the arid ABC islands — Aruba, Bonaire and Curaçao — off the Venezuelan coast. These are dry, low islands with limited and seasonal fresh water, and the species' defining trait is that it copes with this by being thoroughly euryhaline: it inhabits fresh, brackish, salt and even hypersaline waters, and is one of the most common fishes wherever it is found.
This salinity tolerance shapes its ecology. The islands' freshwater habitats dry out seasonally, and when they refill, freshwater populations are recolonised from the sea — the marine and brackish reaches act as a refuge and a source from which inland pools are repopulated. A benthopelagic fish of shallow water, it occupies pools, ditches, coastal lagoons and salinas, moving readily between water bodies of very different salinity.
The FishBase environmental envelope for the species is freshwater and tropical, with pH around 7.5–8.5, hardness from roughly 10 dH upward, and a temperature band of about 79–84 °F — alkaline, hard, warm water consistent with its arid limestone-island setting. In practice the salt and brackish habitats it also occupies extend well beyond these freshwater figures, and the species should be understood as a salinity generalist rather than a strict freshwater fish.
Feeding
Detailed diet studies specific to Poecilia vandepolli are sparse, but its trophic level of about 2.7 (FishBase) and its body plan mark it as a small omnivore and micro-grazer in the mould of other mollies. Mollies of the genus Poecilia characteristically rasp algae and biofilm (aufwuchs) from surfaces and take small invertebrates, detritus and whatever drifting items they can capture, and P. vandepolli's upturned mouth and shallow-water habits fit this generalist surface-and-substrate feeding mode.
Its presence across fresh, brackish and hypersaline waters implies a correspondingly flexible diet, drawing on the algal mats, microfauna and organic material available in each habitat type. In nutrient-rich coastal lagoons and salinas, productive algal and microbial films likely form a substantial part of the intake; in transient freshwater pools, small invertebrates and detritus would feature more.
The species is classed as harmless and of low fishing vulnerability, with no commercial significance. As an island generalist it occupies the same broad ecological role as other small poeciliids — a hardy, opportunistic feeder near the base of the food web rather than a specialist.
Mating
Like all poeciliids, Poecilia vandepolli reproduces by internal fertilisation rather than external spawning. The male uses the gonopodium — the modified, forward-rotating anal fin — to transfer sperm directly to the female. There is no nest and no spawning site; instead males pursue and court females, and mating is internal, so reproduction can proceed in the small, shifting water bodies the species occupies without any need for a fixed breeding substrate.
As is general for the genus, females are able to store viable sperm internally and can produce several successive broods from a single mating. This capacity is particularly valuable for an island fish whose habitats dry out and refill seasonally: a female arriving in or surviving in an isolated pool can found new generations without needing a male present at each cycle, which suits the recolonisation dynamics seen across the ABC islands.
Species-specific data on courtship behaviour and mate choice in P. vandepolli are sparse, so the finer details are best inferred from the well-studied biology of its close relatives rather than asserted directly. What is secure is the underlying poeciliid pattern: internal fertilisation via the gonopodium, sperm storage, and live birth.
Breeding
Poecilia vandepolli is viviparous — a livebearer, as confirmed for the species and as universal in the genus Poecilia. After internal fertilisation the female carries the developing embryos and gives birth to free-swimming, fully formed fry rather than laying eggs. The young are independent miniatures from birth, able to swim and feed at once, which lets them exploit ephemeral island pools quickly.
The species shows high biological resilience, with a minimum population doubling time estimated at under 15 months (FishBase) — a reproductive output that, combined with sperm storage and live birth, lets populations rebound rapidly after the seasonal drying that periodically wipes out freshwater habitats. This fecundity is the engine behind the species' abundance and its ability to recolonise inland waters from brackish and marine refuges.
Precise brood sizes, gestation length and any tendency toward superfetation (the carrying of broods at more than one stage of development, known in some poeciliids) are not well documented for P. vandepolli specifically, and are noted here as data-sparse rather than guessed. As with other mollies, gestation in livebearers generally runs on the order of a few weeks and varies with temperature; the species' warm habitat would favour the shorter end of that range.
In the aquarium
Poecilia vandepolli is rarely encountered in the aquarium trade and is more a fish of biological and regional interest than a hobby staple; dedicated husbandry literature is correspondingly thin. Where it is kept, it is best approached as a hardy, salinity-tolerant molly, and its natural environment gives clear guidance: warm water around 79–84 °F, and hard, alkaline conditions with pH roughly 7.5–8.5 and a moderate-to-high hardness floor near 10 dH.
The species' defining feature for a keeper is its euryhaline nature. It thrives across fresh, brackish and salt water, and like many mollies is likely to do best — and to show its faster growth, greater robustness and stronger colour — in hard, mineral-rich or lightly brackish water rather than soft, acidic conditions. A stable, well-mineralised setup that mirrors its limestone-island origins is the safest bet.
As a small, peaceful, prolific livebearer it would suit a single-species or hardy-livebearer setup, with the usual caveat that livebearers reproduce readily and population control becomes the main management task. Given how little species-specific care information exists, a keeper should generalise cautiously from molly husbandry and prioritise water hardness, alkalinity and warmth over precise numbers.
Conservation
Poecilia vandepolli is assessed as Least Concern on the IUCN Red List (species ID 125989461), in a global assessment last carried out in 2020. CITES and CMS both list it as Not Evaluated. The Least Concern status reflects a fish that is abundant across its range and remarkably tolerant of harsh and variable conditions.
The basis for that secure status is the species' own biology. It is one of the most common fishes on the ABC islands, occupies an exceptionally broad range of salinities from fresh through hypersaline, recolonises seasonally drying freshwater habitats from marine and brackish refuges, and reproduces rapidly with high resilience. A fish this adaptable, this fecund and this widespread within its native islands faces no significant species-level threat under current conditions.
As an island endemic of the southern Caribbean, however, its long-term security still rests on the health of the ABC islands' coastal and inland waters. Localised threats common to small arid islands — water abstraction, pollution of the limited freshwater pools, and the introduction of non-native fishes — are the kinds of pressures worth watching, even though the species is presently abundant and of Least Concern.