Yucatan Molly

The Yucatan Molly is closely related to the Poecilia latipinna, another popular species among aquarists. Young fish of both species are almost indistinguishable; identification is possible only by counting the rays in the dorsal fin: Poecilia velifera has 18–19, while Poecilia latipinna has only 14. In adults, the differences are much more obvious. The Yucatan Molly is significantly larger: females can reach 17 cm in length. Males are smaller (up to 15 cm) and have a more massive dorsal fin, which is the basis for the name "sailfin". In nature the fish are grey with a pattern of dotted horizontal lines. Over the years, however, many hybrid varieties have been developed, displaying a rich diversity of colours and shades. The most popular are solid yellow, orange, black, white (albino), and several spotted forms.

This species is endemic to south-east Mexico on the Yucatan Peninsula; humans have introduced it to several other countries, including Colombia, Israel, Singapore, and Taiwan. It inhabits numerous river systems that flow into the Caribbean Sea, including brackish river estuaries. Outside its native range, the species has established itself in the Middle East and South-East Asia, where it probably came from home aquariums and became invasive.

The Yucatan Molly is peaceful and calm. Although these fish could be good neighbours to other freshwater species, their need for high pH and dH values limits the number of compatible tank mates.

For keeping one or two fish, a minimum aquarium volume of 170 litres is recommended. The tank should have plenty of rooted and floating aquatic plants, with enough open space for swimming. Avoid excessive plant growth, though, as males with their impressive sail-like dorsal fins need room to manoeuvre. The substrate is not critical for this species. As livebearers, they are generally undemanding, but the Yucatan Molly has its peculiarities. It needs fairly alkaline water with high carbonate hardness. It can live in brackish conditions at around 5 g/L salt. Soft, acidic water harms them. Maintaining the right water chemistry is the main challenge of keeping this fish. Otherwise, care is routine: weekly partial water changes with removal of organic waste such as uneaten food and faeces, and regular equipment maintenance.

These fish readily take most common aquarium foods: dry, frozen, and live. Include some plant matter in their diet. If it is not in the flakes or pellets, supplement live food such as bloodworms or brine shrimp with spirulina flakes or similar products.

During breeding season males become aggressive; in limited space, it is best to keep just one male for every 2–3 females. Like all livebearers, the eggs incubate inside the female without being laid. Pregnancy lasts 4 to 8 weeks on average. A single brood can number up to two hundred fry, but usually 40–60. The young should be moved to a separate tank to prevent predation by parents and tank mates. Feed them with specialized powdered foods, suspensions, or brine shrimp nauplii. Note that the Yucatan Molly can produce hybrid offspring with Poecilia latipinna.

When conditions are good and the fish are not stressed and well-fed, disease risk is minimal. However, as noted, this fish is sensitive to water chemistry; low pH and dH values stress it and can lead to fungal or bacterial infections. Correcting the water parameters often allows the immune system to cope, but if disease progresses, medication is needed.

When choosing healthy fish, check that the scales and fins are clean: there should be no white coating, sores, or visible parasites. Fins should be fully spread, with no signs of rot or clumping. Avoid fish with a sunken belly or an unnaturally curved spine, which may indicate starvation or genetic defects. Active, curious fish showing interest in food are signs of good health. Inspect the shop tank: there should be no dead fish or any showing obvious signs of disease, as this indicates poor keeping conditions.