CHAMPAIGN, Ill. -- While scientists aboard Sunburst, the Smithsonian Institution's ocean-going vessel, stalk the elusive giant squid in its natural deep-water habitat, researchers combing the California coast say they have unlocked a possible reason for a squid's ability to avoid capture.
In a study comparing the nervous systems of the squid and octopus (cephalopods) with a variety of snail-like sea slugs (gastropods), it was found that cephalopods have more rapid-acting electrical firing systems than their more sluggish molluskan kin. The adaptation enables the squid's high-speed, jet-propelled swimming behavior and may partly explain the intelligence shown by octopuses.
The key element of the findings, say University of Illinois and Stanford University scientists, involves sodium channels proteins in nerve-cell membranes that generate the electrical impulses that buzz along axons in the brain during decision-making and along nerve fibers for driving muscular responses. The more rapidly sodium channels open and close, the faster electrical activity is turned on and off, and the faster impulses can be transmitted, much as with a high-speed modem.
That sodium channels in motor neurons used in jet-propelled swimming are faster than those of other types of nerve cells in squids was reported in 1992 by William F. Gilly, a marine neurobiologist at Stanford. In the 1970s, other researchers noted that sodium-channel activity seemed unusually slow in some snails. In 1993, Gilly's group cloned a squid gene, suspecting it was a fast-type squid sodium channel; its structure was similar to that found in vertebrates, including humans.
Acting on a hunch, Rhanor Gillette, a U. of I. physiology professor, worked with Gilly and Stanford doctoral student Matthew McFarlane to use new technology to re-examine the fast-slow sodium channel issue. They looked at how the channels are controlled by electric fields over a wide voltage range, asking how the fields may affect the rapid firing of nerve impulses.
In May's Journal of Neurophysiology, they will report that sodium gateways are much slower in non-squid mollusks in a normal voltage range in intact, living nerve cells. At extreme voltages, the rates are similar. The work was funded by National Institutes of Health and a Ford Foundation fellowship.
Gilly, who studies membrane-channel proteins in the signaling of nerve and muscle cells, was surprised by the findings: "I was trained to think sodium channels were all big and fast. It was difficult to think anything as noble as a sodium channel could have anything to do with sluggardly behavior, even in slugs."
A squid's faster sodium channel probably evolved to give it fine control of rapid swimming for capturing prey and for escape, said Gillette, an affiliate of the U. of I. Beckman Institute for Advanced Science and Technology, who worked with Gilly at the Hopkins Marine Station while on sabbatical.
"Squids evolved from quite sluggish, heavily shelled mollusks into somewhat faster-swimming shelled polyps, like the chambered nautiluses that bob moronically around the abyss," he said. "Eventually, they evolved into the sleek, modern versions that hunt in packs."