Supplementary information for: Multisensory integration by polymodal sensory neurons dictates settlement in actinulae larvae

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.