An "individual" is actually a colony of unisexual organisms. Every individual has specific gonozooids (sex organs or reproductive parts of the animals, either male or female). Each gonozooid is comprised of gonophores, which are little more than sacs containing either ovaries or testes.
Physalia are dioecious. Their larvae probably develop very rapidly to small floating forms.
Fertilization of P. physalis is assumed to occur in the open water, because gametes from the gonozooids are shed into the water. This may happen as gonozooids themselves are broken off and released from the colony. The release of gonozooids may be a chemical response occurring when groups of individuals are present in one locality. Critical density is probably required for successful fertilization. Fertilization may take place close to the surface. Most reproduction takes place in the fall, producing the great abundance of young seen during the winter and spring. It is not known what triggers this spawning cycle but it probably begins in the Atlantic Ocean.
Germ Cell Development
Each gonophore has a central spadix of multinucleate endodermal cells separating the coelenteron from a layer of germ cells. Covering each germ cell is a layer of ectodermal tissue. When gonophores first bud, the germ layer is a cap of cells on top of the endodermal spadix. As gonophores mature, the germ cells develop into a layer covering the spadix. Spermatogonia form a thick layer, while oogonia form a convoluted band several cells wide, but only one cell layer thick. There is very little cytoplasmic material within these cells, except during rare instances when cell division is occurring. Oogonia begin development at approximately the same size as spermatogonia, but become considerably larger. All oogonia are apparently formed at an early stage of gonophore development prior to the occurrence of enlargement. Interestingly, there appears to be yolk globules within the cytoplasm of most oogonia.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); oviparous