Morphology and evolution of ovicells in Scrupocellaria ferox (Bryozoa: Cheilostomata)

Scrupocellaria ferox forms erect colonies with a budding pattern that can be derived from the quincunxal budding pattern of primitive encrusting cheilostomes, including Membranipora and Wilbertopora by suppression of peripheral lateral buds into heterozoids and development of cuticular joints and stolons. The reduced size of the vestibule relative to the tentacle sheath suggests that the distal wall of the vestibule has been everted to form a muscular "ovicell plug" that closes the ovicell aperture and provides nutrients to the embryo. Traces of cuticle and a cell mass located at the base of the vestibule we infer to be a vestigial setigerous collar similar to that found in ctenostomes and some cheilostomes, including Membranipora chesapeakensis. The earliest cheilostomes produced plaktotrophic larvae. We propose that in an early cheilostome ancestor of S. ferox the embryos adhered to the lower neck of an everted polypide and was retained in retracted zoids in the introverted vestibule. Brooded embryos became so large that they no longer could be retracted into the vestibule, but remained outside adherent to a portion of uninvaginated vestibular wall. The developing embryo was protected by modifications on the next distal zoid probably formed by a fusion of a pair of proximal spines. This structure became the ovicell. The ovicell plug, representing evaginated vestibule, was modified into a muscular seal, protecting the embryo from predation. These hypotheses imply the following: (1) The cheilostome grade of evolution was attained before development of the ovicell. (2) Vestibules of ovicelled cheilostomes do not contain setigerous collars, at least not as large as those found in the ctenostomes. (3) Since a wide variety of zoecial components were available to take part in protective innovations, they may have evolved separately from the vestibular component of brooding. (Abstract shortened by UMI.).