Structure and function of the nervous system in nectophores of the siphonophore Nanomia bijuga

Abstract

SummaryAlthough Nanomia nectophores are specialized for locomotion, their cellular elements and complex nerve structures suggest they have multiple subsidiary functions.The main nerve complex is a nerve ring, an adjacent columnar-shaped matrix plus two associated nerve projections. An upper nerve tract appears to provide a sensory input while a lower nerve tract connects with the rest of the colony.The nerve cell cluster that gives rise to the lower nerve tract may relay information from the colony stem.The structure of the extensively innervated “flask cells” located around the bell margin suggests a secretory function. They are ideally placed to release chemical messengers or toxins into the jet of water that leaves the nectophore during each swim.The numerous nematocytes present on exposed nectophore ridges appear to have an entangling rather than a penetrating role.Movements of the velum, produced by contraction of the Claus’ muscle system during backwards swimming, can be elicited by electrical stimulation of the surface epithelium even when the major nerve tracts serving the nerve ring have been destroyed (confirming Mackie, 1964).Epithelial impulses generated by electrical stimulation elicit synaptic potentials in Claus’ muscle fibres. Their amplitude suggests a neural input in the vicinity of the Claus’ muscle system. The synaptic delay is <1.3 ms (Temperature 11.5 to 15° C).During backward swimming radial muscle fibres in the endoderm contract isometrically providing the Claus’ fibres with a firm foundation.Summary StatementNanomia colonies have specialized swimming bells capable of backwards swimming; thrust is redirected by an epithelial signal that leads to muscle contraction via a synaptic rather than an electrotonic event.