Some animals form characteristic infertile spermatozoa called parasperm, which differ in size and shape compared to fertile sperm produced by single males. Species that have been reported to produce parasperm include snails, cottoid fish, moths and butterflies. Moths and butterflies produce fertile eupyrene sperm and anucleate non-fertile parasperm, which are known as apyrene sperm. A research team at the National Institute for Basic Biology in Japan has identified the gene involved in the formation of the apyrene sperm and has revealed the important function of the apyrene sperm in fertilization using the silk moth, Bombyx mori.