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 nectosome stem. The stem provides an attachment point for the nectophores and houses the simple nervous system responsible for their coordination. As revealed by immunocytochemistry, the nectosome stem has three main components: two giant axons, a distributed nerve network and a set of FMRFamide-immunoreactive nerve tracts. Whereas the nerve network is distributed throughout the stem, the nerve tracts link specific contralateral 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 suggests a location for the two pathways connecting the nerve network and the nectophore: one excites an epithelial impulse and leads to reverse swimming, and the other provides excitation for forward swimming by feeding into a ganglion-like cluster of nerve cells. The two-way exchange of neural information between the stem and the nectophore is by way of this terminal ganglion and a single nerve leading to a nerve ring at the nectophore margin. This 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-branching nervous system containing both condensed and distributed units.