Comparative neuroanatomy of Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda (Mollusca) and its phylogenetic implications

Abstract

The nervous system of invertebrates is considered to be a very conservative organ system and thus can be helpful to elucidate questions of phylogenetic relationships. Up to now, comparative neuroanatomical studies have been mainly focused on arthropods, where in-depth studies on major brain structures are abundant. In contrast, except for Gastropoda and Cephalopoda, the nervous system of representatives of the second largest phylum of invertebrates, the Mollusca, is as yet hardly investigated. We therefore initiated an immunohistochemical survey to contribute new neuroanatomical data for several molluscan taxa, especially the lesser known Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda, focusing on the cellular architecture and distribution of neurotransmitters in the brain. Antisera against the widespread neuroactive substances FMRFamide and serotonin were used to label subsets of neurons. Both antisera were additionally used in combination with acetylated α-tubulin and the nuclear marker DAPI. This enables us to describe the morphology of the nervous system at a fine resolution and to compare its cellular architecture between different species of one taxon, as well as between different taxa of mollusks. On the basis of these results, the nervous system of caudofoveates seems to be most highly derived within the so-called basal (non-conchiferan) mollusks, and a monophyly of a clade Aplacophora could not be confirmed. In general, the brain as well as the remaining nervous system of the molluscan taxa investigated shows a great variability, suggesting a deep time origin of the diversification of this prominent protostome clade.