Tunicata

Tunicata comprise approximately 3000 species of marine animals. They have evolved different reproductive strategies and life cycles, also comprising asexual reproduction. Their larva has a typical chordate body plan, with a dorsal hollow neural tube. At metamorphosis, the larval nervous system undergoes deep rearrangements and in the adult the central nervous system consists of a brain located between the two siphons. Studies on the tunicate nervous system began at the end of the nineteenth century and saw a renewal of interest from the early 1980s with the advent of molecular biology technologies. The central nervous system of the ascidian larva is considered a miniaturized model of the vertebrate nervous system. For this reason, it is the focus of research aimed at investigating the organization and evolution of the vertebrate nervous system. Alternatively, the ascidian adult nervous system has been generally considered to be uninteresting, even if it shows peculiarities, such as the ability to regenerate, the interconnection with excitable epithelia, and alternative developmental pathways in colonial species. This chapter integrates knowledge from classical morphological studies with recent findings from molecular investigations, mainly focusing on ascidians, the most studied tunicate taxon. An overview of the neuro-anatomy of adults and larvae is reported, with particular emphasis on the sensory organs and neurotransmitter localization.