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Origin of Brooding and Ovicells in Cheilostome Bryozoans: Interpretive Morphology of Scrupocellaria ferox
Scott Santagata and William C. Banta
Vol. 115, No. 2 (Spring, 1996), pp. 170-180
Stable URL: http://www.jstor.org/stable/3227047
Page Count: 11
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Scrupocellaria ferox forms erect colonies whose budding pattern is similar to that of early fossil encrusting cheilostomes, except that peripheral lateral buds are suppressed into heterozoids, and stolons and flexible cuticular joints are present, presumably as a requirement of the erect colony form. The ovicell of S. ferox is a flattened chamber confluent with the zoid distal to the fertile zoid. The ovicell aperture is occluded by a muscular ovicell plug evaginated from the fertile zoid. The embryo more than doubles in size, probably nourished through the placental epithelium of the plug. The operculum and vestibule are strongly reduced. We propose a hypothesis to explain evolution of S. ferox and related species from encrusting ancestors with free gamete release and planktotrophic larvae. (1) Embryos of an ancestral cheilostome began to adhere to the base of the everted vestibule, which caused them to be withdrawn into the introverted vestibule when the lophophore was retracted. (2) Placental nutrition developed, accompanied by hypertrophy of the epithelium of the adherent vestibular wall. (3) Brooded embryos became so large that they no longer could be contained in the vestibule, but remained outside, adherent to evaginated vestibular wall. (4) The setigerous collar (and in Scrupocellaria the operculum) were lost because of this vestibular eversion. (5) The ovicell evolved from a pair of proximal spines on the next distal zoid, which were modified to enclose the vulnerable embryo. (6) The erect colony form of Scrupocellaria was attained, but only after ovicells evolved. This hypothesis implies that the vestibule is the essential part of cheilostome brooding and that external structures, including ovicells, developed subsequently as protection for the embryo. If correct, this hypothesis could account for much of the structural variation among cheilostome brood chambers.
Invertebrate Biology © 1996 American Microscopical Society