Many animals with backbones -- vertebrates, such as you and me -- insulate much of the electrical wiring of their nervous systems by wrapping it in multilayered sheaths of a fatty material called myelin. This insulation allows electrical nerve impulses to be conveyed over long distances much more rapidly. Considering its advantages, nerve myelination is thought to be rare amongst invertebrates, and it has been suggested that the small size of most invertebrates makes it not worth their while to invest resources in making myelin sheaths.

This notion is firmly rebutted by Petra H. Lenz from the University of Hawaii, and colleagues, who describe in the 15 April issue of Nature how even the tiniest invertebrates can benefit from myelinating their nerves. They have found that the myelinated nerves possessed by some species of copepods--minute crustaceans that inhabit the oceans--give them a distinct edge on their confrères when it comes to a quick getaway from a predator.

Copepods are the most abundant and successful of the small animals living in the plankton. On average a few millimetres long, they pick up signs of disturbance in their watery world through sensitive antennae. They shoot away from danger at remarkable speed, taking just a few milliseconds to reach speeds of around 200 body lengths per second.

Lenz and colleagues found that a copepod species with myelin sheaths around the antennal nerves took just 1.5 milliseconds to initiate this escape behaviour, compared with 6 milliseconds for a species that only has unmyelinated nerves. Not a great difference, but quite handy when you are no more than a few millimeters long.

And it seems that even such a small advantage does have adaptive value. Copepods with myelinated nerves are widespread throughout the oceans, dominating the plankton of the more hazardous open ocean environment. Copepods with non-myelinated nerves, on the other hand, tend to skulk in the deep oceans, where there are fewer predators.

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