Structure and function of the nervous system in the stem of the siphonophore Nanomia septata : its role in swimming coordination

The multiple swimming bells, or nectophores, of the colonial hydrozoan Nanomia septata are capable of coordinated avoidance swims in both forward and reverse directions. Individual nectophores also contribute to slower forms of swimming during foraging. Communication between a nectophore and the rest of the colony is at cone-shaped structures in the colony’s stem. The stem provides an attachment point for the nectophores and houses the simple nervous system responsible for their coordination. The stem nervous system, revealed by immunocytochemistry, has three main components: two giant axons, a distributed, polygonal nerve network and a set of FMRFamide-immunoreactive nerve tracts. Whereas the nerve network is distributed throughout the stem, the nerve tracts link specific contra-lateral nectophores. Action potentials in the giant axons spread excitation rapidly along the stem, but their connection with individual nectophores is by way of the nerve network. Anatomical evidence is provided for the location of two connecting pathways between the nerve network and the nectophore; one excites an epithelial impulse and leads to reverse swimming; the other provides excitation for forward swimming by feeding into a ganglion-like cluster of nerve cells. Excitation passes to the swimming muscle epithelium by way of a single nerve axon and a nerve ring at the nectophore margin. The work presents physiological evidence for mechanisms, such as facilitation and summation, operating within a multifunctional, bidirectional nerve network, responsible for coordinating epithelial and neural signals in an early-evolved nervous system containing both condensed and distributed units.