Poecilia vandepolli
Poecilia vandepolli, or Van de Poll's molly, is a poeciliid fish native to the Lesser Antilles. A euryhaline species, it is one of the most common fish in its range, inhabiting fresh, brackish, salt, and hypersaline waters. The fish vary significantly in size and color depending mostly on the salinity of their environment. The saltwater specimens grow faster and become more robust and more colorful, but the species is drawn to freshwater habitats, which disappear in each dry season and must be recolonized when the rains return. The species feeds on algae, biofilm, and aquatic invertebrates. Females give birth to live fry. Cannibalism of the fry is common when food is scarce.
Taxonomy
Poecilia vandepolli was originally described by Van Lidth de Jeude in 1887. He simultaneously described the subspecies P. vandepolli arubensis, but this taxon has not been recognized by later researchers. Hubbs considered P. vandepolli a subspecies of P. sphenops. In 1963, Rosen and Bailey challenged the validity of this and other short-fin molly species and made them synonyms for P. sphenops. Poeser resurrected P. vandepolli as a separate species in 1992. A genetic analysis in 2016 confirmed that P. vandepolli is a distinct species.

There is no officially recorded common name for the species. Proposed common names include Van de Poll's molly, orange-tail molly, and Dutch-Antillean molly. On the islands of Aruba and Curaçao, where it is native, it is known simply as "molly" and machuri, respectively.

The ancestral form of P. vandepolli diverged approximately 150,000 years ago when the Lesser Antilles, which the species inhabits today, were connected to mainland Venezuela. Based on the similarities between the two species noted by Hubbs, Poeser considered P. vandepolli to have common ancestry with P. vivipara from South America. The 2016 genetic study, however, has shown that the species most closely related to P. vanderpolli are the Central American P. mexicana, P. gillii, and P. sulphuraria.

Description
P. vandepolli is variable, or polymorphic, in many respects. Sexually mature females normally reach 30–45 mm in standard length, while males grow to 25–35 mm SL. Though these measurements place them among the smaller mollies, large specimens are sometimes encountered as well: a 72.7 mm SL female and a 51.9 SL male have been reported. These size variations pertain to distinct populations. The fish vary in color as well, even within the same population. Males are more intensely colored than females and have a larger dorsal fin.

The physical differences between populations are the result of varying ecological conditions, especially salinity. The specimens living in marine habitats have larger bodies, longer dorsal fins, and stronger coloration. Their caudal peduncle is stronger, allowing them a better maneuver against large waves and tidal fluxes. They also exhibit more orange coloration, especially in their dorsal and caudal fins, and more black spots in the caudal fin. The freshwater fish are cream to light yellow. A black band may be present on the dorsal fin in freshwater males, as may a humeral blotch. Individuals from brackish habitats resemble those from fresh waters but never have a humeral blotch.

Distribution and habitats
P. vandepolli is native to the Caribbean islands of Aruba, Bonaire, and Curaçao (ABC islands). It is a euryhaline species: it inhabits freshwater, brackish, saltwater, and hypersaline coastal habitats. It is also found in the freshwater habitats of the island of Saint Martin, but was probably introduced there by Dutch settlers. P. vandepolli is one of the most common fish species found in the coastal habitats across the Lesser Antilles and is found in nearly all freshwater localities. It is one of only five native species inhabiting the fresh waters of Aruba, the others including the American eel (Anguilla rostrata) and the mountain mullet (Dajaus monticola), and is by far the most abundant and dominant along with the invasive Mozambique tilapia (Oreochromis mossambicus). Occurrences have been reported from mainland Venezuela, including the Lake Maracaibo river estuary, but the validity of these collection records remains unconfirmed.

The molly is widespread in rivulets and freshwater ponds on the Lesser Antilles. Most of these vanish during the dry season, essentially purging the islands of freshwater animals. Once the torrential rain flooding commences, the chemical cue from the surface runoff laden with terrestrial organic compounds attracts the mollies from the sea to colonize the reemergent freshwater systems, where they reproduce quickly. The mollies are likewise common in sheltered lagoons and inner bays, particularly in localities with mangroves. Their population in these saltwater habitats is small, likely due to high predation, but crucial because of its stability.

Ecology
P. vandepolli prefers to feed on unicellular and other small algae, biofilm, and aquatic insects. When food becomes scarce, Vandepoll's mollies may take plankton and Artemia. Stomach content analyses show that cannibalistic adults eat fry when food is lacking.

The pupfish Cyprinodon dearborni, which can also live in all salinities, shares the molly's dietary habits and is a major competitor. They may only coexist if their habitat has a connection to the sea; if the habitat is cut off, one of the species vanishes. In confined water bodies population density is kept down by a lack of food: most offspring starve or fall prey to adults. In sea the molly is heavily predated on by other fish. Freshwater and supersaline habitats are generally free of predators, but on Curaçao birds and trematodes prey on the mollies. Bird predators include egrets and pelicans.

Reproduction
P. vandepolli is a ovoviparous livebearer. Small females produce about 10 live fry per litter; particularly large ones may produce over 100. Newborn fry measure 8-9 mm. Their growth is faster in the sea than in fresh wataer and slowest in hypersaline environments. Adults readily predate on their own offspring. The females usually outnumber the males 2:1, sometimes less.

Fishkeeping
thumb|A saltwater male exhibiting numerous blotches of black and orange
P. vandepolli is easy to care for in a home aquarium and will breed readily. It demands plenty of space on account of its activity as well as warm water. Its ability to intensify its orange coloration after a high-carotenoid diet, and the existence of highly-blotched specimens, might quality the species for ornamental fish trade.

References

vandepolli
Category:Fish of Aruba
Category:Fish of Bonaire
Category:Fish described in 1887
Category:Taxa named by Theodorus Willem van Lidth de Jeude