Pedal serotonergic neuron clusters of the pteropod mollusc, Clione limacina, contain two morphological subtypes with different innervation targets

Each pedal ganglion of the pteropod mollusc Clione limacina contains a cluster of serotonin-immunoreactive neurons that have been shown to modulate contractions of the slow-twitch musculature of the wing-like parapodia, and contribute to swim accelerations. Each cluster has a variable number of neurons, between 5 and 9, but there is no significant difference between right and left ganglia. In experiments with electrophysiological recordings followed by dye-injection (carboxyfluorescein), the clusters were found to contain two subsets of neurons. The majority innervate the ipsilateral wing via nerve n4. Two of the neurons in each cluster send processes out of the pedal ganglion in nerves n3 and n8. The processes in nerve n3 innervate the body wall of the neck region, while those in nerve n8 innervate the body wall of the tail. The baseline electrophysiological activity of the two subsets of neurons was different as "wing" neurons had constant barrages of small synaptic activity, while the "body wall" neurons had few synaptic inputs. The potential roles of the Pd-SW cluster in swim acceleration (wing neurons) and control of fluid pressure in the body and wing hemocoelic compartments (body wall neurons) are discussed.