Multisensory integration (MSI) combines information from more than one sensory modality to elicit behaviours distinct from unisensory behaviours. MSI is best understood in animals with complex brains and specialized centres for parsing different modes of sensory information, but 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 mechanosensory cues each influenced the settlement response in a complex and hierarchical manner that was dependent on specific combinations of cues, an indication of MSI. Additionally, 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 photosensory 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 expressing putative chemo- and photosensitivity interface with mechanoreceptive cnidocytes. We propose a simple multisensory processing circuit, involving polymodal chemo/photosensory neurons and mechanoreceptive cnidocytes, that 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.
