Poecilia velifera

Range

Naturally occurring populations of Poecilia velifera are confined to the Yucatán Peninsula in Mexico, where they occur at elevations below roughly 20 m (66 ft) above sea level.^[17][19] The species' native range extends along the peninsula's northern coast from Tulum to a tidal stream located about 50 km (31 mi) southwest of San Francisco de Campeche (just north of latitude 19°30′ N);^[12][17] it encompasses the states of Campeche, Yucatán, and Quintana Roo, including the islands of Mujeres, Cozumel,^[12] and Contoy Island.^[20] P. velifera reaches higher population densities in and around two major protected areas in Yucatán: the Ría Lagartos Biosphere Reserve and the state reserve Ciénegas y Manglares de la Costa Norte de Yucatán.^[21]

In addition to its coastal range, P. velifera is found in inland cenotes (sinkholes in the karst) in the vicinity of Mérida, the state capital of Yucatán.^[17] Most of these cenotes are artificial, and P. velifera was probably planted there by the International Health Board to control mosquitoes. Another inland locality is the Xlaká cenote, a natural formation associated with Maya ruins; the species' occurrence there is likely natural, although introduction by the pre-Columbian Maya remains a possibility.^[5] Some inland records may represent populations that recently colonized new areas during hurricane-driven flooding.^[14]

Use in mosquito control and the ornamental fish trade has spread P. velifera far beyond Yucatán.^[22] It was introduced to Thailand from Taiwan in the 1980s for use in shrimp farms, and formed established populations in a brackish section of the Chao Phraya River and in the Songkhla Lake basin.^[22] In the 1980s, P. velifera was introduced to Peru and has established itself in coastal environments of the Department of Lima, including the Pantanos de Villa Wildlife Refuge.^[23] In Israel, two established populations occur: one recorded in June 1993 in warm springs in the Beit She'an Valley and another in springs south of the Dead Sea, the latter traced to escapees from a nearby fish farm.^[24] In 1999, P. velifera was recorded in mangrove creeks in southern Taiwan.^[25] First records from the Ain Zayanah lagoon in Libya date from 2019, the fish likely originating from deliberate aquarium releases; P. velifera subsequently became well established and abundant.^[26] The species is further established in Singapore,^[27] Colombia,^[28] and in the Paraíba do Sul basin in Brazil.^[29]

Habitat preferences

Poecilia velifera can live in both fresh and salt water, tolerating a broad range of salinities, but is most successful in fresh water.^[30] It occupies a variety of near-coastal and coastal habitats, primarily in marine and brackish environments.^[12] These include lagoons such as Laguna de Términos,^[17] tidal streams, estuaries such as Ria Lagartos, salt marshes, mangrove channels, and coastal cenotes and freshwater pools.^[1][12] The contiguity of the Yucatán molly's habitats leaves movement largely unimpeded, resulting in potentially substantial gene flow among adjacent populations.^[19] Even so, populations from different parts of the peninsula are often genetically distinct from one another, possibly shaped by patterns of nearshore ocean currents between the Bay of Campeche and the Atlantic coast.^[31]

Habitat preference separates P. velifera from Yucatán's other sailfin molly, P. kykesis, which favors freshwater rivers and reservoirs in the interior of the peninsula.^[19] P. velifera occurs together with one of two shortfin mollies—P. mexicana or P. orri—at several sites.^[32] In shallow swamp waters near Sisal, Yucatán, dominated by red mangrove and with white and black mangroves also present, P. velifera occurs together with P. mexicana; other fish that sporadically occur there with mollies include Yucatán tetra, pike topminnow, twospot livebearer, giant killifish, Yucatán flagfish, and Jack Dempsey and firemouth cichlids.^[15] In Quintana Roo, both P. velifera and a smaller population of P. orri have been recorded in a small cenote. At a site in Campeche, P. velifera inhabits a small natural impoundment enlarged by dredging together with P. kykesis and P. mexicana, representing the only reported locality where all three species co-occur; males of P. velifera there are outnumbered almost 3:1 by those of the accompanying molly species.^[32]

P. velifera occurs over diverse substrates, including sand, mud, fine loose silt, shell debris, accumulations of decaying plant material, gravel, and rock. Plant growth is common across habitats but not ubiquitous. Where vegetation occurs, it typically consists of mangroves, cattails, and pondweed; algae such as Chara are also common. In freshwater cenotes, plant communities may also include water lilies, water milfoil, and bladderworts. Individuals are usually found in shallow water, typically less than 1 m (3.3 ft) deep; in some inland cenotes, however, adults have been recorded at greater depths, reaching about 3 m (9.8 ft). Water clarity ranges from clear to turbid, including green and dark, tea-stained waters colored by decaying vegetation. Currents are typically absent or weak,^[12] and temperatures range from 25 to 30°C.^[17]

Habitat loss and degradation pose a potential threat to Poecilia velifera. These impacts include changes to water flow, channel modification, pollution, and sediment accumulation within its range. Its population is declining; over roughly 20 years, urbanization and highway construction have wiped out two of the approximately 28 known sub-populations.^[17][33] Consequently, the International Union for Conservation of Nature lists P. velifera as a vulnerable species,^[1] and it is protected under Mexican law.^[34] Because it is widely recorded throughout its range, P. velifera has been proposed as a flagship species for wildlife conservation in coastal Yucatán habitats.^[21]

Routine activities

Yucatán mollies spend considerable time engaged in routine maintenance activities. Fish often seek shelter among mangrove roots, beneath submerged logs, or in other protected locations, where they remain still for extended periods. Body maintenance includes rubbing the sides of the body against hard surfaces such as rocks or wood.^[15]

When engaged in surface respiration, individuals congregate near the water surface, swimming slowly or remaining nearly stationary while rhythmically opening and closing the mouth and gill covers. Mollies may also briefly lunge upward to the surface, forcefully raising the dorsal fin to produce a splash—sometimes repeating this several times—and may also stop swimming to adopt a vertical, head-up posture; the purpose of these behaviors remains unknown.^[15]

Social interactions

Male interactions often begin with harassment and may escalate into territorial and aggressive displays. A male may then confront another by swimming directly toward it while fully extending the dorsal fin and opening the mouth. If the interaction continues to escalate, both individuals flare their dorsal fins, bend into distinct S-shaped postures, and strike with flanks, tails, or bites. Subordinate or threatened males typically end such encounters by breaking off abruptly and fleeing with rapid changes in direction. Where P. mexicana occurs alongside P. velifera, P. velifera males may be harassed—and females sexually pursued—by large-class P. mexicana males.^[15]

Space use varies with sex and size class. Large males move alone across broader areas, whereas smaller ornamented males are more restricted to particular sections of the habitat. Females range freely between open water and mangrove margins. Cryptic males, in contrast to others, remain within groups of females and avoid fighting with other males, which leaves them more time for body cleaning and mating. Some smaller or subordinate individuals also track larger males at a close but consistent distance, following them at roughly 10–30 cm (3.9–12 in) behind while matching their swimming speed.^[15]

In an experimental group setting, alpha males have been shown to exert dominance over all other group members. Males direct butting behavior—gentle nipping directed toward the genital region—primarily toward females, and this may at times be followed by thrusting the gonopodium toward the gonopore (genital opening). The same form of butting also occurs between males; it may be accompanied by gonopodial thrusting when interactions are not overtly aggressive. Females form their own hierarchies but engage in substantially fewer confrontations, and they too assert dominance over subordinate males.^[37]

Mating strategies

Reproductive behavior in Poecilia velifera includes courtship displays and rapid mating attempts, with these behaviors varying among males of different sizes and appearances. Males often converge on individuals already attended by rivals. They may briefly nibble the female's gonopore while moving behind or slightly beneath her; this is followed by swinging the gonopodium forward in an effort to insert its tip into the gonopore^[15] and transfer sperm to the female's oviduct.^[17] Females choose mates based on body size, with preference consistently given to males bearing larger dorsal fins, even when those males belong to a different sailfin species.^[31][14]

Larger, ornamented males rely on conspicuous courtship combined with thrusting attempts.^[38] They approach females with elaborate visual displays, fully extending the dorsal fin and curving the body into an S-shape, and may engage in vigorous chases.^[15][14] Even so, their mating attempts are frequently disrupted, as rival males seize the opportunity to rush in and attempt rapid inseminations while the displaying male is occupied. Courtship often does not lead to mating attempts and may function not only as a signal to females but also as a way of blocking rival males from accessing the female.^[15]

Small males adopt a markedly different strategy, dispensing with courtship and instead attempting quick, unannounced inseminations.^[38] Intermediate ornamented males show a more flexible approach, displaying primarily when larger competitors are absent but otherwise relying on similar rapid thrusting attempts. Intermediate cryptic males avoid direct competition altogether, remaining within groups of females rather than engaging rivals. They also perform gonoporal nibbling more frequently, which may help them identify females that are ready to mate. By staying close to females and making repeated mating attempts with little interference, they achieve more frequent copulations than larger males.^[15]

Fecundity and development

Poecilia velifera is a livebearing species, with females producing fully formed young. In its native range, breeding appears to extend from late February to late July, and in some cenotes, reproductive activity may occur almost throughout the year.^[12] Females have been recorded reproducing at lengths as small as about 4 cm TL,^[13] and the reported sex ratio is approximately one male to every two or three females.^[17] Brood size increases with body size:^[13] at around 6 cm TL, females give birth to approximately 47 fry,^[39] while the average brood is about 77.^[13] The smallest broods may contain as few as 10 fry, whereas larger females can produce up to 120.^[17]

Reproductive performance varies with environmental conditions. It is highest in fresh water, where individuals grow faster, reach larger sizes, and mature earlier, whereas increasing salinity is associated with reduced feeding, slower growth, and smaller adult size.^[30][40] Gestation lasts approximately 28 to 40 days at temperatures of 25–30 °C.^[13] The very young are robust and conspicuously marked by broad vertical bars and a white border along the edge of the anal fin, distinguishing them from P. latipinna fry.^[5] Newborns measure about 9–11 mm TL, regardless of the size of the mother.^[13]

Hybridization

Poecilia velifera has an XY sex-determination system, with females carrying two X chromosomes and males one X and one Y chromosome. In crosses with closely related species such as P. latipinna and certain domesticated stocks of P. sphenops, offspring showed normal male-to-female ratios, indicating that the sex chromosomes function properly even in hybrids. In contrast, crosses with the more distantly related P. reticulata produced only male offspring, suggesting disrupted female development.^[41]

P. velifera and the shortfin molly P. mexicana—which occur together at one site in Campeche—have been known to produce viable and fertile hybrid offspring both in nature and in laboratory settings since 1933.^[5] Such crosses are rare in the wild: despite decades of intensive searching, the next known specimens were not found until 2004. An early-generation hybrid was determined to be the offspring of a male P. velifera and a female P. mexicana. Laboratory tests have shown that female shortfin mollies prefer sailfin males to shortfin males, the former being larger than the latter, which may lead to the occasional breakdown of behavioral barriers that normally prevent crossbreeding.^[32]