A diverse array of sessile marine invertebrates mate by passive dispersal of sperm which fertilize the brooded eggs of neighbours. In two such species, a sea–mat (phylum Bryozoa) and an ascidian (phylum Chordata), vitellogenic egg growth is absent in reproductively isolated specimens, but is triggered by a water–borne factor released by conspecifics. In both of these colonial, hermaphroditic species, the active factor can be removed from water by filtration. The effect involves self–/non–self–recognition: water conditioned by a separate subcolony of the same genetic individual does not prompt oocyte growth. In each species, allosperm move from the surrounding water to the ovary and are then stored in close association with the growing oocytes. We concluded that sperm themselves are the water–borne factor that triggers the major phase of female reproductive investment. This mechanism is, to our knowledge, previously undescribed in animals, but has parallels with the initiation of maternal investment in flowering plants following the receipt of compatible pollen. The species studied may be representative of many other aquatic invertebrates which mate in a similar way. The stimulation of egg growth by allosperm could lead to intersexual conflict during oogenesis.