Structural analysis of the branchiae and dorsal cirri in Eurythoe complanata (Annelida, Amphinomida)

In many polychaete species filamentous, pectinate or arborescence branchiae may be present. In species with well-developed parapodia, such as Amphinomidae and Eunicidae, they are often associated with the notopodia. Mostly they arise close to the dorsal cirrus. Whereas the branchiae are regarded as the main site for respiration, the cirri function as segmentally repeated mechanosensory and chemosensory organs. To date, ultrastructural studies exist only for a limited number of polychaete species, especially with respect to the sensory appendages of parapodia. Therefore, the branchiae and dorsal cirri were investigated with CLSM, SEM and TEM in a species of Amphinomidae, Eurythoe complanata. These studies revealed that the branchiae are complex organs comprising blood vessels, highly developed circular and longitudinal musculature, neurite bundles, numerous sensory cells and a specialized epidermis and cuticle. The cuticle is thinner than on the trunk, and the blood spaces at the presumed respiratory sites are covered by cell processes, about 130-350 nm thin, thus providing a short diffusion distance. Ventilation is facilitated by a continuous, longitudinal ciliary band. In contrast to the uniform sensory equipment of the branchiae, several types of receptor cells are present on the cirri. Besides two different collar receptors as well as other uniciliated and multiciliated sensory cells, the basal joint of the cirri comprises a few phaosomous photoreceptor cells. These are of the rhabdomeric type, and although ectopic light sensitivity was known for other species, herein such cells are structurally characterized in polychaetes for the first time.