Novel Tools to Understand the Behaviour of the Ctenophore Pleurobrachia pileus

Neuroscience explores how nervous systems produce behaviour. One approach is to study simple animals with basic nervous systems and clear, easily defined behaviours to understand the structure-function relationship. Researchers typically choose either inherently simple organisms or animals at early developmental stages. We propose that Pleurobrachia pileus, a ctenophore, is well suited for this approach due to its simple nervous systems, simple behavioural repertoire, stereotyped developmental and reliable lab culturing. While tools exist to study ctenophore nervous systems, behaivoural recording and description methods are less developed. To address this we developed a flexible dual-view imaging platform (the MoTraScope) to can record Pleurobrachia behaviour across developmental stages and contexts. This system integrates automated tracking methods to facilitate large-scale behavioural data collection and can be applied to various micro- and mesoscale organisms. We developed an unsupervised behavioural analysis pipeline to compare methods for characterising Pleurobrachia behaviour, including evaluation metrics to select the best approach for the dataset. Using this system, we found behavioural differences between Pleurobrachia larvae that were one-day-old and two-to-five-day-old. We also analysed Beroe cucumis larvae but found no significant behavioural changes across larval stages. We also applied this pipeline to footage of adult Pleurobrachia in a Kriesel tank. However, in this case the circulating flow in the tank made it difficult to disentangle active behaviours from passive movement due to flow. Using Pleurobrachia to study structure-function relationships in nervous system, however, relies on the assumption that this relationship is relatively straightforward. To test this, we investigated whether the juxtatentacular nerve, a component of the subepithelial nerve net thought to play a role in transmitting signals between the body wall and the tentacle, is involved in either feeding behaviour or the tentacle retraction reflex. Using a lesion study and tailored behavioural paradigms, we found that lesioning the juxtatentacular nerve did not inhibit either behaviour. This suggests that the underlying neural control of these behaviours is more complex that initially assumed and the structure-function relationship in Pleurobrachia is not as simple as it appears. Together, these tools lay the foundation for establishing Pleurobrachia pileus as a model organism for investigating how nervous system structure gives rise to behaviour, while revealing the underlying complexity even in seemingly simple animals.