Resistance to Red Tide
A newly discovered strain of renegade clams with the ability to ward off paralytic shellfish poisoning could shed light on the nature of the nervous system

Sidebar Safer Shellfish Through Technology Facial numbness, tingling in the arms and legs, nausea, dizziness — and those are the milder symptoms.

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Combed from the coastal muds of New Brunswick, Canada, hundreds of soft-shell clams made their way from basket to boat to pickup, headed south for their last hurrah. The men and women who harvested the mollusks carefully cleaned and sorted their haul, working through their catch with a clamdigger's dexterity and a scientist's eye, prepping the hapless clams for their final demise.

That many clams could have been used to make a big batch of chowder. Instead, they were used to make history.

The clams were the focus of research conducted by an international team of scientists that included University of Maine marine scientist Laurie Connell. The scientists' paper last April, "Sodium Channel Mutation Leading to Saxitoxin Resistance in Clams Increases Risk of PSP," was the culmination of more than eight years of intensive research aimed at achieving a better understanding of a notorious and potentially deadly compound known as saxitoxin. Saxitoxin is the primary culprit in cases of paralytic shellfish poisoning (PSP), the always dangerous, sometimes deadly consequence of the coastal phenomenon known as red tide.

The paper, published in the highly respected scientific journal Nature, spent more than five months at the top of the charts.

"I was shocked to hear that the paper was so popular. It's had more than 60,000 downloads and it's still going," said Connell in January. "I'm not sure what made it so popular, but it does have a very broad appeal."

Saxitoxin's role in PSP has been the subject of considerable interest in recent decades. However, most of the research focused primarily on the effects the poison has on humans and other mammals that contract PSP by eating clams and other filter-feeding shellfish. Filter feeders accumulate saxitoxin in their tissues as they dine on the algae that carry the poison, passing along a concentrated dose to their mammalian predators. The first researchers to take a comprehensive look at the effects of saxitoxin on clams, Connell and her team, including retired UMaine researcher Betty Twarog, found that mollusks suffer many of the same PSP symptoms as humans.

Well, at least some of them do.

Connell, an assistant research professor in the School of Marine Sciences, discovered that not all clams are created equal when it comes to fighting off the effects of PSP. Now she's begun to unravel a microscopic mystery that speaks to the very nature of the nervous system itself.

As it turns out, a renegade strain of red tide-resistant clams has been working its way into both Maine and Canadian clam populations in the last few millennia. Thanks to a mutation in their genetic code, these clams were able to survive and reproduce despite the presence of saxitoxin. This strain eventually became dominant in clam populations that are frequently exposed to red tide.

In fact, Connell and her team of specialists found that the mutant clams were more than 1,000 times more resistant to the effects of red tide than their unmutated brethren, a surprising discovery that has significant implications in both clam management and medical research.

The symptoms of saxitoxin poisoning — numbness, paralysis and even death —result from the compound's ability to interfere with the nervous system at the subcellular level, blocking sodium channels, and interrupting the crackle and blink of nerve impulses moving from cell to cell. Because of its power over the nerve impulse, saxitoxin has been used extensively by medical researchers to study the function of the nervous system and its associated diseases. Connell's comprehensive approach opens new doors to future research by connecting sodium-channel function to specific control sites in the organism's DNA.

Connell's work is likely to affect the clams as well. While the mutant gene itself makes the carrier clam no less edible, the clam's ability to tolerate high levels of saxitoxin increases its potential danger to humans by allowing it to stockpile higher levels of the toxin in its tissues. Discovery that some clam populations are genetically more resistant to red tide poisoning could lead to management changes in the soft-shell fishery.

"Genetically resistant clams are able to continue feeding much longer, accumulating more toxins in their tissues, which take longer to purge. Knowledge of the genetic susceptibility of clams to red tide could help managers make better decisions on what clams to use in seeding programs, how long to close clam beds and other issues," she says.

The project's implications don't stop there. Connell's discoveries are of interest to marine ecologists, public health officials, bioengineers, fishermen — the list goes on and on. The significance of the research in such a broad range of disciplines speaks to its popularity in Nature.

by David Munson
March-April, 2006

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