Poecilia vandepolli

Poecilia vandepolli, or Van de Poll's molly, is a poeciliid fish native to the ABC islands of the Lesser Antilles, namely Aruba, Bonaire, and Curaçao. It is a euryhaline species and 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 the freshwater habitats which disappear in each dry season and must be recolonized when the rains return.

Van de Poll's molly
A freshwater male
Conservation status
Least Concern  (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Cyprinodontiformes
Family:	Poeciliidae
Genus:	Poecilia
Species:	P. vandepolli
Binomial name
Poecilia vandepolli Van Lidth de Jeude, 1887

The molly feeds on algae, biofilm, and aquatic invertebrates, and is in turn prey to other fish and birds. Females give birth to live fry. Cannibalism of the fry is common when food is scarce. Males can develop an intense orange coloration, which may make the species attractive to aquarists.

Taxonomy

Poecilia vandepolli was originally described by Van Lidth de Jeude in 1887. He simultaneously described a 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. Following scrutinous morphological comparisons, Poeser resurrected P. vandepolli as a separate species in 1992. A genetic analysis in 2016 confirmed that P. vandepolli is a distinct species. Within the genus Poecilia, this species is placed among the mollies (subgenus Mollienesia).

There is no officially recorded common name for P. vandepolli. 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 the "molly" and machuri, respectively.

Mollienesia   P. mexicana P. gillii P. orri P. butleri P. vandepolli P. wandae P. koperi P. cf. gillii P. hondurensis P. catemaconis P. sphenops P. cf. sphenops P. kykesis P. latipinna

Phylogenetic relationships of P. vandepolli, based on the concatenated sequences of mitochondrial and nuclear DNA

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, showed that the sister species of P. vandepolli is P. wandae and that the two form a clade with P. koperi and an undescribed species provisionally called P. cf. gillii. P. vandepolli is the youngest of these. 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 enabled 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 P. mexicana cuneata. The Caribbean Sea ensures the Antillean molly's reproductive isolation, however, and no intergrades have been found between the two taxa.

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, helping them maneuver 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.

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  A saltwater male
• 
  A saltwater female
• 
  A female from the Spanish Lagoon
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  A freshwater male with a humeral blotch

Distribution and habitats

The ABC islands (green, from left to right: Aruba, Bonaire, and Curaçao), off the coast of Venezuela

P. vandepolli is native to three islands of the Lesser Antilles in the Caribbean Sea: Aruba, Bonaire, and Curaçao. These three islands, collectively known as the ABC islands, belong to the Leeward Antilles. 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. Occurrences have been reported from mainland Venezuela, including those in the Lake Maracaibo river estuary and the "thousands of individuals" in a drainage flowing through the city of Maracaibo, but the validity of these collection records remains unconfirmed.

A mangrove habitat of P. vandepolli

P. vandepolli is a euryhaline species: it inhabits freshwater, brackish, saltwater, and hypersaline coastal habitats. P. vandepolli is one of the most common fish species 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). On Aruba, most of the fresh and brackish ponds are muddy and cloudy. Salinity and temperature oscillate little across the various habitats, including pools and lagoons, where the temperature ranges from 25 to 26°C in the colder part of the year and from 27 to 31°C in the warmer months. The sole exception are the hypersaline environments, where salinity levels change through evaporation and rainfall, the latter also occasionally leading temperatures to rise far above 36°C, which is lethal to fish.

An inland stream inhabited in the wet season (left) and the same stream in the dry season (right), Aruba

The mollies are 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. Most of the freshwater pools and all the ephemeral streams inhabited by the mollies vanish during the dry season, causing mass mortalities and 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.

Ecology

Mollies browsing algae in an estuary

P. vandepolli prefers to feed on unicellular and other small algae, biofilm, and aquatic insects. When food becomes scarce, Van de Poll'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 as food dwindles. The pupfish is accordingly nearly absent on Aruba, where the molly is ubiquitous; but on Bonaire the pupfish is abundant while the molly is known from only one freshwater locality. In confined water bodies population density is limited by the availability of food: most offspring starve or fall prey to adults. In the sea, on the other hand, 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. Avian predators include egrets and pelicans.

Reproduction

An observation of hundreds of mollies in a stream on Aruba revealed large males chasing females "incessantly". These males were found to exhibit more orange coloration than the other males in the area. Studies of other Poecilia species, such as the guppy (P. reticulata), have shown that such increased coloration is attractive to females.

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 water and slowest in hypersaline environments. Adults readily predate on their own offspring. The females usually outnumber the males 2:1, sometimes less.

Fishkeeping

A saltwater male exhibiting numerous blotches of black and orange

The ability to intensify its orange coloration after a high-carotenoid diet and the existence of highly-blotched specimens might qualify P. vandepolli for ornamental fish trade. The species 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. A 20 US gallons (76 L; 17 imp gal) aquarium may house about 10 individuals.