Dwarf killifish (Heterandria formosa) are among the most fascinating aquarium inhabitants, despite their rather plain coloration and small size. Their reproductive biology is particularly interesting. The males, measuring only 2 cm, defend small territories and impress each other with their fins stretched to the point of breaking. They mate with the females, which are about one third larger, using a special, highly mobile mating organ known as the gonopodium. This is followed by an extraordinary gestation period during which the females provide nutrients to their differently developed embryos. After a gestation period of several weeks, they give birth to only a few fully independent fry. A few siblings follow every few days. Michael Kempkes describes the dwarf killifish in all its facets in his monograph. They are charming creatures.