Myelin for the Masses
Scientists always thought that
myelin sheaths, nature's way of insulating nerve cells, were a unique invention of vertebrate organisms. But a report in this week's issue of Nature shows that some species of tiny marine crustaceans called copepods have independently evolved the same sophisticated feature, which may help them elude predators and thus may explain their prevalence all over the world.
Numbering some 10,000 species, copepods are abundant in all the oceans, despite being obvious prey for whales, jellyfish, and many other ocean animals. Researchers have speculated that the critters, less than 3 millimeters in diameter, do so well because they respond quickly to approaching predators: A copepod darts away from an attack within milliseconds. To better understand how the crustaceans do this, Petra Lenz and Daniel Hartline of the University of Hawaii, Manoa, and their colleagues decided to examine the animals' sensory organs.
The researchers examined the nerve fibers of 11 copepod species of the Calanoida order under a transmission electron microscope. To their surprise, they found that four of the species' nerve fibers were swathed in multiple layers of myelin. In vertebrate animals, myelin helps speed up electric signals by preventing the electrical charge from bleeding off into surrounding tissue. Myelin seems to do the same in the copepods: In lab tests, species with myelin sheaths responded 30% to 50% faster to the sudden movement of a fake predator than those without. Moreover, the team found that the fleeter species are the ones that have evolved more recently and are distributed more widely in the oceans.
Besides overthrowing the dogma that myelin is for vertebrates only, says Edward Buskey, an ecologist who works with copepods at the University of Texas, Austin, the study shows how copepods may have colonized far and wide: "Species with nonmyelinated nerves might be relegated to safer environments, like the deep sea," he says, while the newer variants developed ways to survive without going into hiding.
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© 1999 The American Association for the Advancement of Science
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