Anna Seybold ^1,2* Suman Kumar ^1,3* Sharat Chandra Tumu ¹ Harald Hausen ^1*

• ¹ University of Bergen, Bergen, Norway
• ² University of Innsbruck, Innsbruck, Tyrol, Austria
• ³ University of Oslo, Oslo, Oslo, Norway

Comparative studies on the development of nervous systems have a significant impact on the understanding of animal nervous system evolution. Yet, an important question is to what degree neuronal structures, which play an important role in early stages, become part of the adult nervous system or are relevant for its formation. This is likely in many direct developers, but it is not the case in forms with catastrophic metamorphosis, and it is not clear in many forms with gradual metamorphosis. This introduces uncertainty in tracing the evolution of nervous systems and of larval forms. One of the most obvious larval features of many planktonic marine organisms is the epidermal ciliation used for swimming and steering, which disappears during metamorphosis. Hence, also the neuronal elements controlling the ciliary beating are often assumed to vanish with the cilia and regarded as purely larval specializations. With volume EM we followed the neuronal targets of the very first pioneer neurons at the apical and posterior ends in the larva of the annelid Malacoceros fuliginosus. We observed that all these pioneers seem to have a dual function. While they are laying the paths for the later adult nervous system, they also make synaptic contact to the main ciliated ring of the larva. We propose that the formation of the later adult nervous system and the innervation of the larval locomotory organ are indeed closely linked to each other. This has implications on the understanding of early nervous system development in marine larvae and on existing hypothesis on nervous system evolution.