Vestibular Glands in Cheilostome Bryozoans: Structure, Distribution and Probable Function

In the evolutionary history of non-motile benthos, the production of bioactive compounds is an important factor for survival in surroundings with aggressive grazing and predation pressures. We hypothesize that so-called vestibular glands, long known in the ascophoran cheilostome bryozoans, produce toxic compounds, thus providing a chemical defense for these vulnerable sessile organisms. Our own microanatomical and ultrastructural research along with a comprehensive analysis of literature revealed the wide distribution of various glandular organs among bryozoans in general. Their positional and structural differences suggest that they evolved independently in two classes, Phylactolaemata and Gymnolaemata. They were also independently acquired in the gymnolaemate orders Ctenostomatida and Cheilostomatida, and several times in the latter. Despite the aforementioned differences, all these organs are situated within the apertural area, indicating their defensive function in protecting the zooidal orifice. We also briefly discuss the distribution and function of so-called funicular bodies containing symbiotic bacteria that are now known to produce bioactive compounds, and speculate on the importance of the chemical defenses for the evolutionary success of bryozoans.