New paper on the Schwabe organ in lepidopleuridan chitons

Last week a new study published by Sigwart et al. described a hitherto unreported structure in lepidopleuridan chitons called the Schwabe organ. Unlike chitonid chitons, the putatively plesiomorphic lepidopleuridan chitons lack an osphradium (sensory structure associated with the gills of Chitonids). However, in 2008 Enrico Schwabe observed "an elongated pair of patches of brown or purplish pigment stretching posterior from beneath the mouth lappets towards the start of the foot, and extending laterally on either side of the mouth" in Leptochiton algesirensis. The authors describe this structure in beautiful detail using light microscopy, 3D reconstruction, SEM, and TEM. Despite their thorough characterization, the exact function of this sensory structure remains unknown. They also review and compare the literature on chiton nervous systems and other known sensory structures.

Another interesting result of this study is the observation that at least the two species of Leptochiton examined both have a surprisingly large, well-developed brain. This is at odds with the traditional view that chitons have small, simple brains. More comparative studies are needed to determine if assumptions that chitons are "primitive" molluscs misled workers to presume they would have simple nervous systems and overlook this complexity or if there is a substantial difference between lepidopleuridan and chitonid chitons in terms of the size and complexity of the brain.

Leptochiton rugatus - my photo of a somewhat beat-up specimen
collected from Reid Rock near Friday Harbor Laboratories.