No Fooling: UH Researchers Find Copepods With Myelin

Time and again, University of Hawai`i electron microscope images of an ocean-dwelling plankton appeared to display an artifact characteristic of improperly prepared specimens. When graduate student April Davis and microscope facility supervisor Tina Weatherby Carvalho excitedly e-mailed researchers Petra Lenz and Daniel Hartline on April 1, 1998, to say that what they were seeing was actually myelin rather than the look-alike artifact, the researchers were sure it was an April Fool's prank.

After all, such a finding ran counter to zoological dogma. The textbooks hold that, with a few exceptions (notably earthworms and shrimp), only vertebrates have the competitive advantage of myelin-coated nervous systems.

Once Lenz and Hartline took a serious second look, they realized something else-the plankton species displaying myelin under the microscope were the same ones that exhibited significantly faster responses in separate behavioral studies.

The findings, confirmed by further observations, are reported in the April 15 issue of the journal Nature, just over a year after the initial, internal e-mail. The authors credit the lucky convergence of studies to the availability here at UH of one of the nation's better core electron microscopy facilities (see BEMF piece).

The Copepod Connection

Copepods are the most prevalent form of zooplankton-crustaceans that dwell almost everywhere that water is available. They constitute the biggest source of animal protein in the oceans, the critical link in the marine food chain between the phytoplankton on which they feed and the krill, fish and whales that feed on them.

Among the copepods studied under a grant from the National Science Foundation by Hartline and Lenz, researchers at the UHM Pacific Biomedical Research Center, are calanoid copepods. Probably the most abundant animals on Earth, they are more prevalent than insects, but less is known about them because of the inaccessibility of many oceanic habitats.

Hartline and Lenz study structural morphology, physiology and behavioral characteristics of copepods. "You'd be hard pressed to sort them into major subgroups by general looks alone," says Hartline. About 3 millimeters long or less, all have oval bodies, swimming legs and T-shaped antennae. Species respond differently to predators, however.

"Remember," says Lenz, "the food web doesn't work if the predator always wins or if the predator always loses." To escape predators, most species dart away. Curiously, Undinula vulgaris, a species found from the shallow waters of Kane`ohe Bay out into the open ocean, reacts amazingly fast compared to its cousins occupying more restricted habitats. The difference in its reaction time is as consistent as it is pronounced. Undinula vulgaris appears to have engineered a snappier reaction to keep ahead of its predators.

Carvalho tries to find morphological clues to such behavior, using scanning electron microscopy (SEM) to look at external details in 3-D images and transmission electron microscopy (TEM) to examine cross-sections of internal structures. Davis, a master's degree candidate in animal sciences who works for zoologists Hartline and Lenz, does the time-consuming and painstaking job of preparing specimens for the microscope-a procedure so sensitive that it can be sabotaged by vibrations of a passing shuttle van outside or temperature changes caused by an extra person entering the room. Davis, her mentors say, seemed born to the demanding task.

Supervisor of the UH Biological Electron Microscope Facility (BEMF) as well as a research associate, Carvalho had set aside her own attempts, disappointed by what she thought were fixation artifacts that kept appearing in some of the TEM samples, to work on other projects requiring her attention. When chemical fixation of TEM sections is compromised, artifacts with concentric rings, like the layers of an onion, appear on the image. Davis persevered. She, too, observed the onion-layer spots in her sample, but reported that the rings were wound like ribbons around axons.

The Magic of Myelin

Long strand-like processes of nerve cells, axons carry nerve impulses. In vertebrates, these nerve fibers are coated with a fatty sheath called myelin, the "white matter" clothing the spinal cord and making up the core of the brain. The lipid works like insulation on a wire, protecting the integrity of the signal as nerve impulses zip along. Myelin may create a competitive evolutionary advantage by speeding signals across the vast distance of vertebrate nervous systems, allowing large organisms to react quickly enough to survive. When the insulation is messed up, the nervous system doesn't work correctly, resulting in diseases like multiple sclerosis.

In TEM cross-section, myelin looks like the concentric circles of a sliced onion or a tree stump…or the chemically introduced fixation artifact. Davis was sure her "ribbons" encircled axons-indicating an abundance of lipids in the membranes, possibly derived from myelin.

To test her observation, Davis and Carvalho prepared specimens from the "rapid-response" groups using an ultra-rapid freezing process that instantly fixes the sample without introducing chemical anomalies. The rings, completely absent from the more sluggish groups, remained. Davis' perseverance and attentive observation won her co- authorship on the scientific correspondence to Nature, an unusual feat for one so young.

Luck? "She made her luck," Lenz says of Davis. Fascinated by the electron microscopy images in her undergraduate biology textbooks, Davis found the BEMF operation at UH and pestered the scientists for a chance to work there. She cheerfully pursued her assignments once hired, whether gathering copepod samples by hauling fine-mesh nets behind a Boston whaler on the bay or hunching over a microscope for hours while moving specimens with one of her eyelashes mounted on a toothpick. Davis still plans to go to veterinary school, but she also plans to earn a PhD. "I just can't imagine not doing research," she says. "Copepods just sucked me in. There's always something new."

The rest of the team plans to pursue their findings. "Copepods react a hundred times faster than humans. Because they are small, they were thought not to need the advantage of the myelin mechanism," Hartline observes. Evidently this is not so. The presence of myelin appears to have evolved independently in some calenoids; continuing studies may explain the phenomenal success of copepods in the world's oceans, as well as the way they divide up oceanic territory between myelin "haves" and "have nots."

Visit www.pbrc.hawaii.edu/~petra/copepod.html for more information on copepod research at UH. Copepod images are available at www.pbrc.hawaii.edu/bemf/microagela/pleuro.htm and the copepod myelin at www.pbrc.hawaii.edu/~petra/myelin.html.