One or more males attach themselves permanently to a female, eventually merging circulatory systems. As this genetic chimera matures, the male grows large testicles, while the rest of its body atrophies. Ceratiidae are the only creatures known to become chimeras as a normal part of their lifecycle.
In some species of Ceratiidae, males permanently bond with females, merging their circulatory systems. As this chimeric creature matures, the male develops large testicles while the rest of its body shrinks.