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Abstract:
Chemosensation drives animal behavior. Adapting to different habi-
tats, animals possess chemosensory systems of diverse morphology
and cellular architecture, and using distinct types of molecular recep-
tors. To elucidate the early evolution of chemosensation, it is essen-
tial to study chemosensory systems in their original environment, the
sea. Yet, most animals investigated so far are terrestrial or freshwater
species. Here, we test the role of candidate chemosensory head organs
in Platynereis dumerilii, a marine annelid worm. Using a customized
microfluidic setup for precise delivery of chemical compounds, we per-
form calcium imaging of whole heads to visualize neuronal activity in
6-days-old larvae. We find evidence of chemosensitivity in all types of
adult organs, but not in the larval apical organ. Antennae appear to be
the main chemosensory organs, whereas nuchal organs and palps ap-
pear more specialized, and tentacular cirri show low chemosensitivity.
The apical organ, a brain region thought to be responsible for larval
settlement and metamorphosis, shows prominent oscillatory activity.
Interestingly, we find chemically-induced activity in the Mushroom Bodies, a brain structure potentially involved in learning. Other previ-
ously unknown regions or cells are described which were activated by
chemical stimulation, and sensory integration between organs is ana-
lyzed. We provide the first comprehensive study of head chemosensory
organs in an annelid, establishing the 6-days-old Platynereis larvae as
a model for the study of annelid chemosensory systems.
