Physalia reproduction takes place mostly in the fall. Physalia are dioecious, meaning that each “individual” is either male or female. The polyps responsible for reproduction are the gonozooids, which are comprised of gonophores; sacs containing either ovaries or testes. Fertilization occurs externally when the men of war shed their gametes into the open ocean; sperm from one colony fuses with the eggs of another colony. It is unknown what causes this spawning cycle to begin. However, the release of gametes may be triggered by a chemical response due to the presence of men of war in large quantities in a single locality. This critical density is probably needed for successful fertilization. Physalia physalis also reproduces a sexually by means of budding or mitotic division.