Bryozoans form colonies of iterated modules, termed zooids, and display varying degrees of polymorphism. Polymorphic colonies comprise autozooids (or feeding zooids) and heteromorphic zooids, among which the most common types are avicularia and kenozooids. Kenozooids differ in shape, size, and presumed function. Among this diversity, there are rhizoids, which serve to attach colonies to the substrate or to lift them above it. To date, only general data on anatomy of kenozooids at light microscopy level are available. Here, we present the first description of the ultrastructure of the holdfast-like rhizoids of the cheilostome bryozoan Dendrobeania fruticosa. The rhizoid wall is composed of a single-layered epidermis, which produces the ectocyst. The voluminous cavity is acoelomate: it has no special cellular lining, nor any signs of an extracellular matrix toward the epidermis. It is traversed by delicate branching funicular strands that originate from the pore plate. The only cells in contact with the epidermis are the cells of the funicular system and the storage cells. The pore plate between the rhizoid and autozooid includes a variable number of communication pores. Each pore is plugged with a rosette complex, which includes a cincture cell and four special cells extending through the pore. The limiting cells are absent, and the special cells are in direct contact with the funicular strands. Cell contacts between special cells are absent; moreover, there are spaces between their proximal lobes filled with a heterogeneous matrix similar to that in the lumen of the funicular strands. Such matrix is also found outside of the extracellular matrix surrounding the special cells. These findings allow us to suggest that nutrient transport most likely occurs between, rather than through, the special cells. However, further studies are needed to understand how the rosette complex functions.
Keywords: Gymnolaemata; kenozooid; marine bryozoans; polymorphism; rhizoid; ultrastructure.
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