Immunohistochemical analysis of the adhesive papillae of Botrylloides leachi (Chordata, Tunicata, Ascidiacea): Implications for their sensory function

Most ascidian larvae settle and begin adhesion by means of three mucus secreting and sensory organs, the adhesive papillae or palps. However, the adhesive papillae of Botrylloides genus larvae, despite their name, have only a sensory function. By immunohistochemical localization of serotonin and beta-tubulin, we demonstrated that the adhesive papillae of Botrylloides leachi contain two distinct types of neurons with different localization and possibly a different function. The central neurons emerge from the tunic at the apex of papillae and probably have a mechanosensory function. The lateral neurons contain serotonin and may play a role in the mechanism of metamorphosis triggering. Moreover, by histological analysis we found numerous secreting cells, clustered in the centre of the area among the three papillae, forming a glandular organ. This organ could perform the attachment of the larva to the substrate, which is traditionally considered to be operated by the anterior epidermis of the larva, with a sucker-like mechanism.