Poecilia vandepolli, or Van de Poll's molly, is a poeciliid fish native to the Caribbean islands of Aruba, Bonaire, and Curaçao. It is one of the most common fish in its range, inhabiting fresh, brackish, salt, and hypersaline waters such as streams, ponds, bays, salt pans, and lagoons. For much of the twentieth century, it was regarded as a subspecies or synonym of P. sphenops, but morphological and genetic studies have confirmed its status as a distinct species. The molly is highly variable in size and color, with males ranging from 2.6 to 5.3 cm and females from 3.2 to 9.6 cm, and coloration varying from grayish-brown to yellow or orange in any given population. Marine fish are generally larger, stronger, and more colorful than freshwater specimens. Males' ability to develop an intense orange coloration may make the species attractive to aquarists.
Forming shoals near the surface, mollies feed mainly on algae, detritus, and aquatic invertebrates, and are preyed upon by other fish and birds. Females produce live fry, giving the species an edge over killifish, with which it competes across its range. Successful reproduction depends on the availability of shelter for fry, most of which fall prey to adults. The species is drawn to freshwater habitats, which disappear in each dry season, killing freshwater populations; these habitats are recolonized from the sea when the rains return.
Taxonomy
In 1887, Van Lidth de Jeude described a new Poecilia species, P. vandepolli, from specimens collected in a fresh water rivulet on Curaçao. He simultaneously described a subspecies from Aruba, P. vandepolli arubensis, on the basis of a slight difference in the proportional relationship between overall body length and the depth of the caudal peduncle (tail stalk). Metzelaar argued that the observed distinction did not warrant taxonomic separation, as it could be explained by normal variation within the species, compounded by the limited number of specimens available for study, and later authors have not recognized the subspecies.
In 1926, Hubbs reduced P. vandepolli to a subspecies of P. sphenops. Rosen and Bailey went further in 1963, questioning the validity of this and other short-fin molly species and placing them in synonymy with P. sphenops. In 1971, Schultz and Miller restored P. butleri to species rank on the grounds of partial reproductive and geographic isolation. Two years later, Menzel and Darnell resurrected P. mexicana from synonymy of P. sphenops, noting that it differs from P. sphenops in ways similar to P. butleri. The key difference is dental: P. butleri and P. mexicana have inner jaw teeth with a single point (unicuspid), whereas P. sphenops has inner jaw teeth with three points (tricuspid). P. vandepolli also has single-pointed teeth, but it diverges from all other Poecilia in additional traits, including the morphology of the gonopodium (male reproductive appendage), coloration patterns, body measurements, and scale counts. On this basis, and in the absence of intergrades (intermediate forms), Poeser reinstated P. vandepolli as a distinct species in 1992.
Although he treated it as a subspecies of the Central American P. sphenops, Hubbs observed that P. vandepolli resembles the South American P. vivipara in gonopodial structure, body size, fin placement, general form, and coloration. Poeser found this indicative of a common ancestry between P. vandepolli and P. vivipara; he objected to Miller's 1975 resurrection of Mollienesia as a subgenus of Poecilia because it separated P. vandepolli from P. vivipara.
A genetic analysis in 2016 confirmed that P. vandepolli is a distinct species and supported its placement among the mollies (Mollienesia). It identified P. wandae as the sister species of P. vandepolli; the two form a clade with P. koperi and an undescribed species similar to P. gillii. P. vandepolli is the youngest member of this group. Its ancestral form diverged approximately 150,000 years ago when the sea levels were lower and the Lesser Antilles islands, which the species inhabits today, were connected to mainland Venezuela. The connection likely allowed the most recent common ancestor of P. vandepolli and P. wandae to populate the area, while the subsequent sea level rise led to the emergence of two distinct species. Geographically, the closest relative of P. vandepolli is the Panamian P. gillii cuneata, but the Caribbean Sea ensures reproductive isolation and no intergrades have been reported between them.
The specific epithet commemorates the Dutch entomologist Jacob R. H. Neervoort van de Poll. The species has no established common name. Proposed common names include Van de Poll's molly, orange-tail molly, and Dutch-Antillean molly. On Curaçao, it is known by the Papiamento name machuri; on Aruba, where it is also native, it is called "molly".
Description
In Poecilia vandepolli, the gonopodium bears no external spines or hooks, and its fourth ray usually carries 10–11 fine serrations (tooth-like bumps) that begin on the sixth or seventh segment from the tip; in fully developed males, a small membranous swelling on the third ray partly veils these projecting serrations. The fins follow a consistent pattern: the caudal fin typically has about 17 rays, the dorsal and anal fins each about 8, the ventral fin 6, and the pectoral fin about 12. Along the body, the fish usually shows 27 scales in a row running from head to tail, around 13 before the dorsal fin, about 23 encircling the mid-body, and around 16 surrounding the caudal peduncle. The species can be distinguished from P. sphenops and P. mexicana by its rearward dorsal and anal fins with fewer rays, a smooth gonopodium, and the humeral spot—a mark on the side of the body just behind the head—which appears in some specimens, more commonly in those from fresh water.
Poecilia vandepolli is highly polymorphic. Size varies considerably among populations, with males ranging in total length from 2.6 to 5.3 cm and females from 3.2 to 9.6 cm. Regan, who treated P. vandepolli as synonymous with P. sphenops, examined specimens from the mainland and from the Leeward Antilles, finding "numerous examples to [12.0 cm] in total length from all parts of the range of [P. sphenops]".
Pigmentation is equally variable. Males and females have the same color patterns, but males are more vivid and have a larger dorsal fin. While females are generally grayish-brown, their backs may carry a faint wash of green or blue. Within a single population, males may match that subdued coloration, display yellow flanks tinged with blue, or have an orange-red dorsal fin bordered in black and an orange throat. Often, 10–12 grey vertical stripes can be seen on the trunk and tail. Speckled specimens also occur.
The physical differences between populations reflect local ecological conditions, above all salinity. Fish from fresh or hypersaline habitats are smaller, show weaker coloration, and have shorter dorsal fins than individuals living in marine conditions. A more robust caudal peduncle allows saltwater specimens to withstand the challenges of strong waves and tidal currents. In contrast to their pale freshwater conspecifics, marine fish exhibit more orange pigmentation across their bodies, especially in their dorsal and caudal fins, as well as darker and more numerous spots in the caudal fin. Brackish water specimens resemble freshwater fish in size and body shape, but differ in that males never have a humeral spot. Aside from those with humeral spots, freshwater populations include males with a broad black band at the base of the dorsal fin. Aruban specimens more commonly exhibit a minor gonopodial variant: in some fully pigmented adult males of all color types, one gonopodial ray bears a small hook, a feature that likely represents a late maturation stage.
Distribution and habitats Range
Poecilia vandepolli is native to three islands of the Leeward Antilles: Aruba, Bonaire, and Curaçao. The species has not been recorded on other Leeward Antilles islands, but it is found in the freshwater habitats of Saint Martin in the Antillean Windward Islands, where it was probably introduced by Dutch settlers. The ecological consequences of its establishment on Saint Martin have not been determined.
Populations from mainland Venezuela previously attributed to Poecilia vandepolli, including those in the Lake Maracaibo river estuary and "thousands of individuals" in a drainage flowing through the city of Maracaibo, are now identified as P. koperi.
Habitat and environment
Poecilia vandepolli is a euryhaline species: it inhabits freshwater, brackish, saltwater ( 35‰), and hypersaline coastal habitats. It is one of the most common fish in the coastal habitats across the Lesser Antilles and is found in nearly all freshwater localities. On Aruba, it is especially abundant in rivulets and ponds, the Spanish Lagoon, and the salt evaporation ponds (salinas) in the northwestern part of the island. It is one of only five native species inhabiting the fresh waters of Aruba, the others including the American eel and the mountain mullet, and is by far the most abundant and dominant along with the invasive Mozambique tilapia. Most of the fresh and brackish ponds on the island are muddy and cloudy. On Curaçao, mollies abound in temporary freshwater pools. They frequently occur in shallow areas of inner bays and lagoons and are also present in all salinas connected to the sea, as well as in the single landlocked salina at Cas Abou. Along the shoreline of Bonaire, mollies are common in Lac and Lagun bays. In the bay of Slagbaai, they are numerous wherever salinity levels are below 90‰. They are nearly absent on the island itself, where freshwater habitats tend to stay isolated from the sea.
Calm lagoons and secluded parts of bays harbor abundant populations of mollies; their numbers are particularly high in areas where mangroves grow. Habitats are between 10 and 200 cm deep; the species cannot survive if water dries up. Waters containing green filamentous algae are preferred regardless of depth. In deeper water, the fish remain near the surface, forming relatively small shoals, typically of a few dozen individuals. Despite habitat pressures across the islands—including invasive species, altered water flow, and tourism-related shoreline development—the species has not experienced major overall habitat degradation, and even on heavily developed Aruba, where many sites have been lost, the survival of mollies does not appear to be under threat.
P. vandepolli inhabits waters with salinities up to about 80‰, and occasionally as high as 135‰, but it requires access to lower-salinity areas for refuge. It withstands a thermal range of 16–36 °C, although neither temperature nor salinity varies significantly in most habitats. Some lagoons are brackish in the rainy season and become supersaline in the dry season; others remain supersaline for most of the year. In pools and lagoons, the temperature is 25–26 °C in the colder part of the year and 27–31 °C in the warmer months. Hypersaline environments are an exception, as evaporation increases salinity and rainfall decreases it. Rainwater can also accumulate as a surface layer that traps heat in the underlying hypersaline water, pushing temperatures to lethal heights.
Population patterns