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

Multisensory integration (MSI) combines information from more than one sensory modality to elicit behaviors distinct from unisensory behaviors. MSI is best understood in animals with complex brains and specialized centers for parsing sensory information, but the dispersive larvae of sessile marine invertebrates utilize multimodal environmental sensory stimuli to base irreversible settlement decisions on, and most lack complex brains. Here, we examined the sensory determinants of settlement in actinula larvae of the hydrozoan Ectopleura crocea (Cnidaria), which possess a diffuse nerve net. A factorial settlement study revealed that photo-, chemo-, and mechano-sensory cues each influence the settlement response, which was complex and dependent on specific combinations of cues, therefore indicating MSI. Mechanosensory cues either inhibited or enhanced settlement rates depending on the presence or absence of chemical and light cues in the environment. Sensory gene expression over development peaked with developmental competence to settle, which in actinulae, requires cnidocyte discharge. Transcriptome analyses also highlighted several deep homological links between cnidarian and bilaterian mechano- chemo- and photo-sensory pathways. Fluorescent in situ hybridization studies of candidate transcripts suggested cellular partitioning of sensory function among the few cell types that comprise the actinula nervous system, where ubiquitous polymodal sensory neurons with putative chemo- and photo-sensitivity interface with mechanoreceptive cnidocytes. We propose that a simple multisensory processing circuit, involving polymodal chemo/photosensory neurons and mechanoreceptive cnidocytes, is sufficient to explain MSI in actinulae settlement. Our study demonstrates that MSI is not exclusive to complex brains, but likely predated and contextualized their evolution.