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
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
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
"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
for more stories from this issue of UMaine Today Magazine.