Meeting Abstract
Sensory systems in animals with hard parts often require penetration of nerves through the skeleton: nerves penetrate bone in vertebrates, brachiopods have shell pores called punctae, and arthropod exoskeletons bear innervated sensilla. Polyplacophoran molluscs, or chitons, have “aesthetes”, photosensory pores in their shells. Chitons have a distinctive armature of eight articulating calcareous shells that cover their dorsal body surface. In all living species, the eight shell valves are covered by a dense array of aesthetes, but in some taxa, a subset of these pores is elaborated into lensed eyes, which have been shown to be capable of simple vision. The aesthete pores and “eyes” are connected to the main body beneath the shell by a network of tissue-filled channels, meaning the shell comprises a substantial proportion of volume that is not solid. In more derived species, the channels coalesce in distinct arrangements and their exit points form shell features that are routinely used in taxonomic identification. The internal arrangement of the aesthete canals is challenging to visualise within the solid opaque shell. We used synchrotron radiation X-ray tomographic microscopy, similar to micro-CT scanning, to create three-dimensional models of chiton valves from more than 20 species across the diversity of Polyplacophora and trace these canals within them. Aesthete features are known to vary among major clades, and these represent neurological features with important phylogenetic signal, and our new data could also act as important taxonomic characters. Simultaneously, this dataset allows us to investigate the potential constraints on the arrangements of these sensory structures, as in all cases in Metazoa the penetration of the skeletal elements by nerve tissue should not overly compromise its structural integrity.
