Meeting Abstract

95.2  Thursday, Jan. 7  How do chitons see their world? A new sensory organ in basal molluscs (Polyplacophora: Lepidopleurida) SIGWART, JD; Queen’s University Belfast j.sigwart@qub.ac.uk

Polyplacophoran molluscs (chitons) are “primitive” animals that have a simple nerve system of non-ganglionised longitudinal nerve cords and no cephalised sense organs. Two sensory structures in the pallial cavity have previously been described in the early-diverging chiton order Lepidopleurida. The examination of these structures seems historically to have been motivated primarily by a quest for an osphradium homolog, which is not convincingly present in lepidopleuran chitons. Recent work has described the three-dimensional anatomy of a new additional sensory organ in basal living chitons, referred to as the “Schwabe organ” after the discoverer. Anatomical examination has included semi-thin sectioning and three dimensional computer reconstruction of the anterior nervous system as well as TEM imaging of the fine structure. This sense organ is macroscopic and clearly visible on the external epithelium in living animals, yet has never been reported in 150 years of anatomical literature. As this structure has only been found in the basal clade of chitons, future work will focus on developmental work and identifying potential homologous structures in the putative sister taxa to this clade. More detailed assessment of available morphological characters is needed to uncover usable morphological synapomorpies to define clades. Sensory structures, which clearly have a significant relationship on how an organism interacts with its environment, may be especially useful in this context. The datasets currently available already provide interesting insights into the analytical power of traditional morphology, as well as some knowledge about the early evolution and radiation of this group.