Convergent colonial organization and reproductive function in two bryozoanspecies epizoic on gastropod shells

Alcyonidium nodosum O'Donoghue and de Watteville occurs on whelks, Burnupen a papyracea, off the south and western Cape, South Africa. A description is given of colony structure and organization in relation to reproductive and inferred feeding functions. The thick, encrusting colony is mammillate, ea ch mammilla comprising a group of large male zooids bearing tall, funnel-sh aped lophophores. Between mammillae are smaller female zooids bearing short er, goblet-shaped lophophores. At the time of collection, October to Novemb er, colonies were actively reproducing, with the coelom of male zooids full of mature sperm. The lophophore of female zooids incorporated an intertent acular organ, while ovaries with developing oocytes and free ovulated eggs were both present. Eggs were twice observed being discharged through an int ertentacular organ. Hippoporidra a dictyota sp. n., from rocky reefs off Be aufort, North Carolina, is described. Its colonies encrust pagurid-inhabite d shells and display the same colonial, and inferred functional, characteri stics as those of A. nodosum. Male zooids on the mammillae bear reduced, no n-feeding lophophores and are surrounded by autozooids, many bearing ovicel ls and presumed female, with normal lophophores. The feeding currents and f low patterns found in mammillate colonies, and their role in sperm discharg e, are discussed. It is concluded that these two unrelated, shell-encrustin g bryozoans have evolved identical hydrodynamic feeding systems which have been utilized to benefit reproduction. The mammillae provide foci for exhal ant currents: the placement of male zooids therein concentrates all the spe rm in a strong discharge, rising clear of the parent and enhancing its wate r-borne passage between colonies.