Who is Molly?

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Sailfin mollies, as well as guppies, platys and swordtails, belong to the family of livebearing fishes (Poeciliidae) and live in smaller lakes and rivers. Sailfin mollies are euryhaline, that is, they have a high tolerance to the salt concentration of a water and are therefore found in both freshwater, as well as in brackish water. Livebearers are characterized by internal fertilization and, with one exception (Tomeurus gracilis), by giving birth to fully developed and live offspring. The gestation period of the eggs amounts to approximately 30 days. To ensure internal fertilization, males develop a tube-like anal fin, the gonopodium, which is used to transfer the sperm to the female internally.

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The gonopodium is a key differentiator between males and females. Moreover, sailfin mollies show a distinct sexual dimorphism, with males mostly having large, sail-shaped and highly colored dorsal fin, as well as an enlarged and colored caudal fin (Figure 1). Frequently, males additionally express dark bars on the lateral sides of their body. Coloration and fin size of a male depend on its dominance, and its reproductive strategy. Males either actively court females, while presenting their erected dorsal fin, or they rely on forced copulation without any courtship. Courting males are generally more conspicuous.