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

Abstract

O_LIAlthough Nanomia nectophores are specialized for locomotion, their cellular elements and complex nerve structures suggest they have multiple subsidiary functions. C_LIO_LIThe 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. C_LIO_LIThe nerve cell cluster that gives rise to the lower nerve tract may relay information from the colony stem. C_LIO_LIThe 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. C_LIO_LIThe numerous nematocytes present on exposed nectophore ridges appear to have an entangling rather than a penetrating role. C_LIO_LIMovements 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). C_LIO_LIEpithelial 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{degrees} C). C_LIO_LIDuring backward swimming radial muscle fibres in the endoderm contract isometrically providing the Claus fibres with a firm foundation. C_LI 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.