Guppy couple (male bellow), Poecilia reticulata in aquarium. Is one of the world's most widely distributed tropical fish, and one of the most popular freshwater aquarium fish species. Male guppies, like other members of the Poeciliidae family, possess a modified tubular anal fin called the gonopodium, located directly behind the ventral fin. The gonopodium has a channel-like structure through which bundles of spermatozoa, called spermatozeugmata, are transferred to females.