Due to insemination, the sperm of glandulocaudines has adapted. In many species, an elongate cytoplasmic collar binds the flagellum to the elongate nucleus at some stage of spermiogenesis. In almost all species, the sperm cell bodies are elongate. In the tribe Diapomini, the genus "Planaltina" expresses only round sperm (like that of externally fertilizing characins) and the genera "Diapoma" and "Acrobrycon" only express slightly elongated sperm; this may indicate a possible plesiomorphy. Some sperm have enlarged regions containing mitochondria, which may help in prolonging the life of the sperm while stored in the ovary. In some genera, sperm clumping and patterns of arrangement are observed in the sperm ducts and storage regions. In the tribes Xenobryconini (in the genera "Tyttocharax", "Scopaeocharax", and "Xenurobrycon") and Glandulocaudini, there is a form of sperm packaging which would allow for a higher sperm density during transfer from the male to the female. These packets are called "spermatozeugmata", and the sperm are packaged to each other; this packaging is further increased by the elongation of the sperm cells. In Xenobryconini, each spermatozeugma is produced and is released fully formed in the spermatocysts, but in Glandulocaudini, the sperm is released from the spermatocysts and packaged elsewhere. The spermatozeugmata are situated in the posterior end of the testes, which serves as a storage area for sperm.