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

Abstract

Alcyonidium nodosum O'Donoghue and de Watteville occurs on whelks, Burnupena 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, each mammilla comprising a group of large male zooids bearing tall, funnel-shaped lophophores. Between mammillae are smaller female zooids bearing shorter, goblet-shaped lophophores. At the time of collection, October to November, colonies were actively reproducing, with the coelom of male zooids full of mature sperm. The lophophore of female zooids incorporated an intertentacular organ, while ovaries with developing oocytes and free ovulated eggs were both present. Eggs were twice observed being discharged through an intertentacular organ. Hippoporidra dictyota sp. n., from rocky reefs off Beaufort, North Carolina, is described. Its colonies encrust pagurid-inhabited shells and display the same colonial, and inferred functional, characteristics as those of A. nodosum. Male zooids on the mammillae bear reduced, non-feeding lophophores and are surrounded by autozooids, many bearing ovicells and presumed female, with normal lophophores. The feeding currents and flow patterns found in mammillate colonies, and their role in sperm discharge, are discussed. It is concluded that these two unrelated, shell-encrusting bryozoans have evolved identical hydrodynamic feeding systems which have been utilized to benefit reproduction. The mammillae provide foci for exhalant currents: the placement of male zooids therein concentrates all the sperm in a strong discharge, rising clear of the parent and enhancing its water-borne passage between colonies.