A microfluidic device for controlled exposure of transgenic Ciona intestinalis larvae to chemical stimuli demonstrates they can respond to carbon dioxide.

The larva of the ascidian Ciona intestinalis controls a small repertoire of behaviours with a simple nervous system in which each cell is identifiable. As such it offers the prospect of building a cohesive cell-level picture of how a nervous system integrates sensory inputs to produce specific behavioural outcomes. Here, we report the development of a microfluidic chip in which larvae can be immobilised and exposed to chemical stimuli. We generate transgenic larvae in which the calcium ion reporter GCaMP6m is expressed in a defined population of cells, allowing us to record real-time neural activity following stimulation. We then use this to establish that some cell populations can sense dissolved carbon dioxide. We also leverage genome and transcriptome data coupled with molecular evolutionary analysis to identify putative chemoreceptors of the MS4A family in Ciona. Our study demonstrates that Ciona larvae can respond to dissolved carbon dioxide, identifies the cells that are likely responsible for chemosensation, and establishes a chip based imaging platform coupled with transgenic technology that could be adapted to establish where other stimuli are sensed and how such incoming signals are processed in the brain to yield behavioural output.