Comparative sea spider neuroanatomy: exploring neural characters to illuminate phylogeny and evolution of an old arthropod lineage with limited morphological disparity

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This project aims to compare adult neuroanatomy across the major taxa (“families”) of the marine Pycnogonida (sea spiders), as well as to exemplarily study adult neurogenesis in this arthropod lineage. As likely sister group to all other chelicerates, pycnogonids are a pivotal taxon to consider, when trying to elucidate chelicerate nervous system evolution. Extant pycnogonids show a remarkably uniform body organization, featuring the “head” region with three specialized appendage pairs and the trunk with four walking leg pairs. However, while the walking leg structure is strictly conserved, the “head” appendages show drastic taxon-specific variations. Yet, neither external morphology, nor molecular sequence data have hitherto enabled robust reconstruction of pycnogonid phylogeny, rendering scrutiny of internal anatomy overdue. Owing to the documented usefulness of neural characters for phylogenetic inference at the level of major arthropod lineages, the pycnogonid central nervous system (CNS) is a promising internal organ system to begin with. Using modern-day techniques (incl. micro-CT) 3D-reconstruction of supracellular CNS structures will provide the backbone for studies at the cell level via immunolabeling of various neuroactive substances and subsequent high-resolution imaging. Additionally, in-vivo cell proliferation experiments coupled with gene expression studies and neuronal backfills will exemplarily explore adult neurogenic processes – an understudied phenomenon that may underlie plasticity of specific CNS areas in many arthropod taxa. The project will result in the first data matrix for an internal organ system of Pycnogonida, covering all extant families. Cladistic analyses will (1) test the utility of supracellular and cell-level neural characters in this old arthropod lineage with limited morphological disparity, (2) illuminate potential conflicts between neural and external morphological characters, especially in the structurally variable “head” versus the conserved trunk, and (3) allow critical evaluation of molecular pycnogonid phylogenies. Further, neuroanatomical features of the pycnogonid ground pattern will be reconstructed. Via comparison to other major arthropod lineages, this will inform on chelicerate and arthropod nervous system evolution. Among others, it is hoped to reveal additional apomorphies of the – even molecularly – still unsatisfactorily supported monophylum (Pycnogonida + Euchelicerata).