The endemic mollies of the ABC islands
Lush tropical rainforests and pristine white beaches lapped by turquoise waters characterize the quintessential Caribbean shorescape. Travel to the southern belt of the Caribbean and you’ll encounter the ABC islands of the Leeward Antilles-Aruba, Bonaire, and Curacao. Aruba and Curacao are autonomous partners (countries) in the Kingdom of the Netherlands, while Bonaire is a special municipality of the Netherlands. On this small cluster of islands, beautiful beaches give way to stunning fringing reefs full of tantalizing marine fishes. But head inland, and instead of lush tropical forests you will find dusty red earth dotted with cacti and Acacia trees. The ABC islands are highly influenced by the Intertropical Convergence Zone (ITCZ), a climatological system that deprives these islands of their greater share of rainfall. Oscillation in the ITCZ leads to distinct wet and dry periods, with peak rain in December and peak drought in June. Growing up in Aruba taught me that one of the greater joys in life is to feel the refreshing touch of the first rain shower at the end of the dry season to break the searing heat and soothe the dry, cracked earth. This relief must be felt intensely by any freshwater fish lucky enough to hold over in the thick, muddy ooze of any lingering refugial puddles.
The large fluctuation in precipitation results in ephemeral stream and pond systems across these islands, and the transitory nature of these bodies of (mostly) fresh, standing water poses unique challenges for the freshwater that attempt to live in them.
The geology and hydrology of Aruba leave few, if any, ponds during the peak of the dry season. For this reason Aruba only has secondary freshwater fishes (secondary division). Secondary freshwater fishes are species that primarily reside in fresh water, but have some degree of halo tolerance that allows them to recolonize freshwater habitats from salty ones.
Aruba has five recorded native species that are known to inhabit fresh water and four exotic ones that were either ornamental releases or were introduced for mosquito control. However, in my recent surveys in Aruba no specimens of either Poecilia reticulata or Xiphophorus hel-leri have been observed; they have likely been extirpated through various dry cycles, while Gambusia sp. is still present in two areas on the island. Of the native species, most occur in very low numbers or appear sporadically, depending on recruitment, and include the American Eel (Anguilla rostrata) and Mountain Mullet (Agonostomus monticola).
By far the most plentiful and dominant species are the invasive Mozambique Tilapia (Oreochromis mossambicus) and the molly Poecilia vandepolli, which is endemic to the three ABC islands and has been introduced into Sint Maarten (St. Martin). On Aruba P. vandepolli is known as just “molly,” and on Curacao as machuri. Although no accepted common name is registered for the species in scientific records, following the species epithet it stands to reason that the most appropriate common name would be van de Poll’s Molly. Other common names for the species include Orange-Tail Molly and Dutch-Antillean Molly.
There has been much confusion and disagreement on the validity of many species and subspecies of Poecilia.
Bailey doubted its validity and synonymized P. vandepolli with P. sphenops. However, after more scruti-nous morphological comparisons, Poeser resurrected P. vandepolli from its synonymy with P. sphenops, and our recent genetic analysis supports the validity of P. vandepolli as a distinct species. Although P. vandepolli is located off South America, our genetic data shows that it is most closely related to P. sulphuraria, P. gillii, and P. mexicana, species that are located in geographically distant Central America, not South America. This is certainly interesting from an evolutionary standpoint, and I look forward to tackling this question through sampling in Venezuela.
Poecilia vandepolli is euryhaline and inhabits salinities rangin to 35% (sea water), but during the dry months, when salt ponds become hypersaline, P. vandepolli have been observed to tolerate salinities of 80% and up to 135% in exceptional cases. At the peak of the dry season, only a handful of ponds retain enough water to sustain fish life. The freshwater systems are essentially purged of freshwater fauna.
The discord arises from the overlapping distribution ranges of many species and is confounded by phenotypic plasticity — the ability of individuals to tailor their morphological development to their environment. For example, Guppies that feed on hard-to-digest foods develop longer guts compared to their siblings that feed on easily digested food. This confusion is so rampant that in some cases, 50 percent of specimens identified based on morphology are shown by genetic analysis to be a different species. It is no surprise, then, that after the initial description of P. vandepolli by van Lidth de Jeude Rosen and that live in areas that are connected to the sea can seek refuge in salt water. Once the rain returns, the ephemeral streams (locally called roois) start to run again. The roois only run during strong bouts of rain. The initial rain runoff is laden with terrestrial organic compounds. These chemical cues are irresistible to P. vandepolli and lure them back inland from the coast to recolonize the virgin freshwater systems. Once they make it inland, they reproduce prolifically and quickly populate the recently formed ponds and puddles. The longevity of these ponds depends on subsequent rain and evaporation. During the wet season, it rains frequently enough to maintain these puddles and ponds for months. They undergo a yearly cycle of boom and bust, starting with algae and biofilms that feed a myriad of primary aquatic insect colonizers and tadpoles. The algae, biofilm, and insects provide ample food sources for the mollies. The mollies are important prey items for the local avifauna, which range from egrets to pelicans, making them a crucial connection in energy transfer between the aquatic and terrestrial ecosystems.
Poecilia vandepolli exhibits various phenotypes, depending on habitat. Growth rate is influenced by salinity, with populations in sea water growing the fastest, followed by those in fresh water; populations in hypersaline waters grow most slowly. In hypersaline habitats (70-80%), the energetic cost of osmoregulation is so high that no acquired energy can be allotted to either growth or reproduction. The fish essentially scrape by, awaiting the rains to release them from the grip of high salinity. Although size is the most obvious difference, their shape and coloration also varies markedly between habitats.
The species is both sexually dimorphic and sexually dichromatic. Males have larger dorsal fins, tend to be more colorful, and are usually smaller than the females. Although they are attracted by terrestrial chemical cues, permanent saltwater populations exist. Individuals in sea water have robust bodies and a thick caudal peduncle (tail base) that can aid in maneuvering a more energetic environment with large waves and tidal fluxes. In sea water their bodies are generally light orange in color, and their tails and dorsal fins are orange. The fins are riddled with black spots, particularly in the males. The freshwater specimens — tend to be much smaller and have a thinner caudal peduncle. They are cream to light yellow in color, and usually have fewer and lighter black spots on their tails and dorsal fins. Some of the males have a thick black band at the base of the dorsal fin, while other males have a humeral blotch in varying shades of gray and black. Brackish water fish specimens are more similar in size and shape to freshwater fish than to saltwater fish. However, brackish water males never present with humeral blotches.
Most fresh and brackish water ponds in Aruba are muddy and cloudy. However, I found one brackish rooi that holds crystal-clear water year-round. Sitting on the bank, I could easily observe hundreds of mollies going about their daily business. Although I didn’t see much aggression between individuals, there appeared to be small territories in which large males incessantly chased the females back and forth. When I caught these males (see xanthic male image, previous page), it was evident that they were extremely xanthic compared to the other males in the same area. In these males, the tails and the area under the mouth and gills were bright orange. Furthermore, the scales on the flanks were an iridescent light blue, with five prominent rows of orange spots.
The yellow color likely comes from carotenoids, which are powerful antioxidants and provide numerous immune system and health benefits. In various species, including Guppies (P. reticulata), females tend to select males that are brighter yellow or red. Biologists call this an “honest signal” of health. The males are signaling their genetic prowess by showing not only that they are capable of gathering sufficient resources to survive, but also that they have enough energy left over to improve their coloration with costly carotenoids. This ability to absorb and display carotenoids appears to be innately encoded in P. vandepolli. When some aquarium specimens were exposed to high doses of astaxanthin from captive feed, their bellies turned highly reflective yellow, while their tails and bodies turned blood red. The red skin intensified the coloration of the lateral scale iridescence, changing it from a light blue to a brighter teal-green color. Given the exemplary history of the captive rearing of poeciliids and this large discrepancy between wild and captive phenotypes, there is a great opportunity for further investigation and incorporation of this species into the ornamental trade.
This is further exemplified by an aberrant specimen I collected from the seawater habitat (see image on facing page). This phenotype is characterized by random blotches of black pigment across the entire body and fins. The blotches also extend onto the gonopodium and iris, structures that usually do not have black pigments in P. vandepolli. Furthermore, the ordered rows of orange spots and iridescent scales on the laterals are broken down and appear at random across the body as well. Although I have only encountered one specimen of this phenotype in Aruba over the years, the gene(s) that control coloration appear to mutate easily. The sailfin clade of mollies is genetically distinct from other mollies (shortfins) and includes P. latipinna, P. velifera, and P. petenensis.
All members of the sailfin clade have been documented to exhibit a wild phenotype similar to the one described here. These phenotypes, in particular the spotted phenotype of P. latipinna, are often cited as the source of the black spotted pattern of the Dalmatian Molly, a captive hybrid strain. However, this is the first time that I have come across a non-sailfin molly exhibiting this patterning. Given the numerous specimens of sailfins that I have observed, I would not be surprised if this phenotype occurs much more frequently in the wild but is simply easily weeded out. These aberrant fish would essentially act as beacons calling wading birds in to lunch.
During the wet season P. vandepolli is almost ubiquitous across Aruba. On Curacao they are more restricted to mangrove areas, and on Bonaire I encountered only one small population. Interestingly, the opposite trend was observed for Cyprinodon dearborni, which are almost absent in Aruba and abundant in Bonaire. This trend has also been noted by others. It is possible that P. vandepolli and C. dearborni fill very similar niches. However, there may be environmental gradients across these islands that we are unaware of that dictate the current distribution. Genetic analysis across the ABC islands has revealed that there is a minor genetic division between Aruba and Curacao, and analysis for Bonaire is pending. But different drainages in Aruba and Curacao show no genetic subdivision whatsoever within each island, which supports the model of the sea serving as a refuge and fish recolonizing the inland ponds. This annual cycle leads to complete homogenization of the populations within each island.
There is one notable exception. There is a freshwater spring in Aruba called Fontijn, a Dutch word for fountain. This spring trickles constantly year-round and is located on the central north coast of Aruba. The north coast is highly energetic: huge waves created by northeasterly trade winds smash into the rocky cliffs. There are only a few areas where the cliffs are interrupted and the roois make contact with the ocean. Specimens from Fontijn are genetically as dissimilar to the rest of Aruba as those from Aruba are to those from Curacao. It appears that the high-energy systems on the north coast form a genetic barrier for P. vandepolli inhabiting the Fontijn drainage, effectively isolating this population from the rest of the island and increasing the overall genetic diversity of the species.
The ABC islands, especially Aruba, have seen great economic growth over the past few decades, which has led to urban sprawl and loss of habitat. Although many molly habitats on Aruba have been lost and the population has been extirpated due to continual development, the sheer number of mollies still present, the observed genetic diversity, and their ability to recolonize fresh water from salty areas give this species a good fighting chance to survive on the island. It seems that the millennia of evolution to suit the hostile, dry environment of these islands have led to the opportunistic nature of this species and has primed them well to adapt in the face of anthropogenic pressures. If a solid management plan is drafted to safeguard the current diversity of this species, I have no doubt that P. vandepolli will persist in the ABC islands and continue to perform crucial ecosystem services for these countries.