Morphology and evolution of the central nervous system in adult tunicates

Tunicata, comprising approximately 3,000 marine invertebrate species, has recently been proposed to be the sister taxon to Craniota. Phylogenetic interrelationships of higher tunicate taxa are controversial, and it remains unclear whether traces of a fishlike ancestor with an active mode of life can be found in present‐day tunicates. To answer this question, we investigated the morphology of the central nervous systems of 18 tunicate species, belonging to the five major tunicate taxa. We reconstructed three‐dimensional anatomic models of the neural complexes and additionally conducted immunohistological staining experiments using antibodies against tyrosinated α‐tubulin. While the reconstructions of the neural complexes of selected species are consistent with previous studies, they also reveal an unappreciated variation of the tunicate nervous systems, especially on the interspecific level. The presence and number of brain nerves, that is, paired anterior and posterior nerves and lateral and ventral visceral nerves, as well as the presence of a dorsal strand, varies among representatives of a higher taxon, but can be very similar to species of another tunicate taxon. We ameliorated the availability of data on the neuroarchitecture of tunicate species providing detailed descriptions of the neural complexes of adult specimens. Moreover, we formulated a set of neuroanatomic characters and showed in a phylogenetic analysis that neuroanatomic characters are parsimony informative, but are insufficient to resolve phylogenetic relationships on their own. To understand neuroanatomic character evolution, we plotted our data on a recently published phylogenetic hypothesis showing that the evolution of the neural complex is clearly correlated with the evolution of life‐history strategies. Our study represents a first attempt to consistently reconstruct the evolution of the morphology of the adult tunicate central nervous system and allows evaluating and complementing molecular phylogenetic hypotheses of tunicate relationships. This study presents a large‐scale comparative analysis of the morphology of the adult CNS of 18 tunicate species. We reconstructed three‐dimensional anatomical models of the neural complexes and conducted immunohistological staining experiments. Now, more data on the neuroarchitecture are available. Our study reveals convergent evolution of characters, probably resulting from similar lifestyles, and considerable variation in the morphologies of