Tower cells of the marine bryozoan Membranipora membranacea

We report on the occurrence, distribution, and anatomy of tower cells in colonies of the marine gymnolaemate bryozoan Membranipora membranacea that occurs epiphytically on the brown alga Fucus vesiculosus in the intertidal at Nahant, Massachusetts. Tower cells are a category of heterozooid that lack a polypide and possess an elongated finger-shaped extension of the frontal surface. Two stages of tower-cell development are examined. Developing tower cells possess an endocyst composed of epidermal and mesodermal cells. In fully developed tower cells, the endocyst is incomplete and consists in part of a mass of intermixed epi- and mesodermal cells suspended in the coelomic cavity. The coelomic fluid contains two types of coelomocytes: amoebocytes and granulocytes. The position and orientation of 1,592 tower cells are recorded. Fifty-four percent of these tower cells occur at the contact zone between two different colonies, which suggests that tower cells might be a defense-related structure preventing overgrowth by neighboring colonies. Additionally, 89% of the tower cells occurring at the lateral margin of the frond are parallel to the surface of the algal frond, which suggests that they may increase the rigidity of the frond-colony complex, thereby protecting the colonies from abrasion. J. Morphol. 239:121-130, 1999. © 1999 Wiley-Liss, Inc.