Animal behavior depends both on the intrinsic properties of individual neurons and how these neurons connect to and modulate one another. A major focus of modern neuroscience is to dissect behavior at the level of individual genes, neurons, and specific synaptic connections, but we are far from fully understanding how the composition and connectivity of even the smallest nervous systems can determine the wide range of behaviors observed in a free-living animal. Our lab is investigating the development of the simple larval nervous systems of tunicates like Ciona, marine invertebrates closely related to vertebrates. Although tunicates are chordates like us, Ciona larvae possess the smallest nervous system ever described at only 231 total neurons (177 central nervous system neurons and 54 peripheral sensory cells), comprising only the second complete “connectome” ever mapped. Using experimental tools such as CRISPR/Cas9-mediated mutagenesis and single-cell RNAseq, we have uncovered neurodevelopmental processes that shape this minimal nervous system, some of which are conserved even in mammals. We are also interested in studying an even more extreme example of the “minimization” of the tunicate nervous system, focusing on certain species that bypass the swimming larval phase and are therefore undergoing evolutionary loss of the larval nervous system altogether.