New data on nudibranchs rhinophore morphology and their spicule complex in Onchidoris muricata (Doridina, Gastropoda)

Abstract

Nudibranchia is a group of shell-less heterobranch gastropods characterized by highly specialized chemosensory cephalic organs known as rhinophores. Whereas the morphology of the rhinophore is an important taxonomic character, as the external structures are used when describing species. However, the internal morphology of the rhinophore has only been studied in terms of neurophysiology. In this study, for the first time, the external and internal structure of the rhinophore in Onchidoris muricata (O.F. Müller, 1776) is described in detail using methods of light, scanning electron-, transmission electron- and confocal laser scanning microscopy, as well as computer microtomography (micro-CT).Data obtained showed that spicules of the rhinophore significantly differ from the body spicules in both general morphology and localization. The thinner and longer spicules of the rhinophore form the spiral square mesh network, allowing mobility which permits contraction of the rhinophore while maintaining a strong framework. Interestingly, the ultrathin structure of the spicules and scleroblasts resemble those of the body spicules. The lymphatic canal and the large nerve and associated longitudinal muscles are situated in the central part of the rhinophore. Stemming from the large nerve are thin transverse muscles and nerve branches associated with the rhinophore lamellae. Morphological analyses suggest a lymphatically driven mechanism in O. muricata, which is capable of quick muscular contraction and slow retraction of the rhinophore.