PETROV, A.*; HOOGE, M.; TYLER, S.; Univ. of Maine: Bursal nozzles in Acoela (Acoelomorpha): comparative morphology and taxonomic implications

Systematics of the Acoela is particularly difficult because of the paucity of readily discernable morphological features. In other soft-bodied worms, sclerotized structures, such as copulatory stylets, provide important characters that can be seen in whole mounts, but acoels generally lack such features. Among the few sclerotized structures in acoels are bursal nozzles – tube-shaped refractile appendages of a bursa copulatrix. Early classifications of the Acoela used the presence/absence of a nozzle for distinguishing major groups, but the current system essentially ignores it as too plastic to provide higher-level distinctions. We have used confocal and electron microscopy to further characterize bursal nozzles, and have found all composed of conjunctions of actin-reinforced cells. Such cell conjunctions can be found in some genera not previously known to have them, appearing in Aphanostoma bruscai, for example, as a series of small disjunct nozzles by confocal microscopy. Extensions of these cells form the wall of the bursa itself. By contrast, nozzle cells are distinct from those of the bursa in Wulguru cuspidata, for example. Multiple nozzles may appear as a series connecting vacuoles within a bursa, as in Daku woorimensis. Whereas most nozzles sit at the end of the bursa facing the ovary, presumably serving to deliver sperm (which we did found in the lumen of some nozzles) to mature oocytes, in species of Pseudmecynostomum they sit between the female pore and the bursa, at the functionally opposite end. Further characterization of bursae and nozzles in more acoel species may yet reveal characters for systematics.