Novel organization and development of copepod myelin. i. ontogeny.

Nerve impulse conduction is greatly increased by myelin, a multilayered membranous sheath surrounding axons. Best known from and most extensively investigated among vertebrates, a few invertebrates, including some superfamilies of copepod, have functionally and structurally similar myelin-like sheaths surrounding their axons. We examined the development of myelin ultrastructure in Bestiolina similis, a paracalanoid copepod. Development occurred in a novel way: initial myelination always appeared first as a partial layer, which in later stages came to encircle an axon completely. This partial myelin first appeared in a single pair of reidentifiable fibers, at the second naupliar stage. Two additional pairs of reidentifiable fibers also became partially myelinated by the third naupliar stage. The number of myelin layers in this trio of axon pairs increased with development, but, at any one stage, each axon had the same number of layers along its entire length. These axons disappeared after the copepodite metamorphosis. After metamorphosis, the fiber that took over as largest in the nerve cord became the most heavily myelinated and was identified as the lateral dorsal giant fiber. The rate of myelination was also characterized in the antennular nerve as a representative of the peripheral nervous system. As axons became larger, they were more likely to be partially, and then completely, myelinated, the latter having a lower ratio of axon core to fiber diameter than the former. Copepod myelin is an instructive example of convergent evolution, with far-reaching consequences for nervous system functioning and the behavior that nervous systems subserve.