Overview: Understanding of the neural circuitry that underlies animal behavior is one of the greatest challenges facing biologists. Progress in this area requires both the assessment of quantifiable and reproducible behaviors, as well as detailed knowledge of neuronal connectivity and the dynamic properties of the neurons themselves. This later requirement - detailed knowledge of connectivity and neuron properties- is a daunting challenge even the simplest vertebrates, such as the zebrafish (~10,000,000 neurons). Our most complete and quantitative understanding of circuits driving behaviors come from the simplest nervous systems such as those of C. elegans and larval Drosophila. Here, an international collaborative team from three research institutions will focus their efforts on a uniquely positioned experimental animal for the quantitative investigation and modeling of circuits and behaviors - the tunicate Ciona intestinalis, one of the closest relatives of the vertebrates. The Ciona larval nervous system, is remarkable in two ways: it shows unmistakable anatomical, genetic, and functional homology to those of vertebrates, yet is very simple, with only 177 central nervous system neurons. As starting points for this proposal, the team has in hand a complete connectome (and a second on the way), a classification of the CNS neurons into 25 functional groups, quantitative assays for range of larval behaviors, and circuit models for those behaviors. In this project we will derive single-cell transcriptomic profiles for cells and cell-classes identified in the connectome, and integrate and experimentally validate this new knowledge in quantitative circuit models of Ciona behavior. Finally, we will create a web portal for Ciona larval connectomic data, something that is currently lacking and limiting the accessibility of this model system to the wider scientific community. Intellectual Merit: While the behavior of tunicate larvae has been studied for many decades, the recent publication of the Ciona larval connectome provides a unique opportunity for major advances. In terms of nervous system complexity, Ciona is most comparable to C. elegans, but fundamental differences in the underlying neurophysiology of these two very distantly related organisms highlights how investigation of both will be complementary. For example, Ciona neurons are known to have sodium-driven action potentials, unlike C. elegans, and Ciona has sensorimotor modalities not found in C. elegans, including photoreceptor-driven visuomotor responses, and a gravitaxis response driven by a well-described otolith organ. Broader Impacts:Impacts to Scientific Community: Despite the research opportunities made available by the Ciona connectome it remains relatively unexploited. We will increase the visibility and accessibility of the existing connectomics data, and new data collected as part of this project, by creating an internet-based resource within the highly successful ANISEED web portal. We expect this web portal to be of value beyond Ciona researchers to data miners, circuits modelers with diverse interests and researchers with particular interests in comparative neurobiology. Impacts to the society: The project will provide training and mentorship at the undergraduate, postdoctoral, and assistant professor levels. All three PIs are committed to increasing the participation of traditionally underrepresented groups in science careers. The inclusion of undergraduates in this project will help to ensure the continued pipeline of underrepresented students entering research careers. We will also undertake outreach efforts targeted to K-12 education. Finally, this project will further international scientific cooperation.