Multisensory integration (MSI) by polymodal sensory neurons dictates larval settlement in a brainless cnidarian larva

Multisensory integration (MSI) combines information from two or more sensory modalities and results in a different response from the individual sensory stimuli. Larvae of sessile marine invertebrates use environmental sensory stimuli to irreversibly settle and metamorphose. It can be expected that with such an important survival decision larvae leverage information from multiple sensory modalities. However, MSI has yet to be deeply investigated in marine larvae that lack complex nervous systems and brains. Here, we examined the settlement response of the actinula larva of the hydrozoan Ectopleura crocea, which possess a diffuse nerve net, and found that actinulae utilize MSI during larval settlement. We performed a factorial larval settlement study that revealed larvae are sensitive to photo-chemo- and mechanosensory cues and settle in complex ways to different combinations of cues, indicating MSI. In addition, a developmental transcriptome study revealed peak expression of a comprehensive set of candidate sensory genes at the developmental stage when larvae are competent to settle. Finally, RNA fluorescent in situ hybridization studies of candidate sensory genes showed that many of the sensory genes of interest are expressed in the same sensory neuron, suggesting that the process of MSI in E. crocea takes place within a sensory neuron and not between specialized neurons or neural regions as in other animals. Our results indicate that MSI plays an important role in mediating the larval settlement decision in E. crocea and suggests that this process may be accomplished by integrating different lines of sensory information within a polymodal sensory neuron.