Shark fins contain high levels of a

Shark fins contain high levels of a potent neurotoxin that scientists have linked to neurodegenerative diseases like Alzheimer and Parkinson.

The study was the first to find the toxin, BMAA, or Beta-methylamino-L-alanine, in sharks, a researcher at the University of Miami. Previous research by the same group found significant quantities of the substance in several varieties of fish, crustaceans and shellfish on the Florida coast.

The study provides another reason not to eat shark fins or shark fin soup, an expensive delicacy prized in Asia for its taste and supposed health benefits. Growing demand for the product drives a global hunt that kills an estimated 73 million sharks a year; the animals are often brutally definned and tossed back into the water to slowly die.

Several species are on the brink of extinction, and the loss of so many sharks spells trouble for marine ecosystems.

The trade continues despite growing evidence of its health risks and the negative environmental impacts as well as legislation against the practice. Four states — Hawaii, California, Oregon and Washington — have banned the trade and possession of shark fins, and a bill introduced last month to do the same in New York is working its way through the state Legislature.

“Everybody would be happy if this study has some impact on conservation,” Paul A. Cox, a researcher with the Institute for EthnoMedicine in Jackson Hole, Wyo., who was not involved in the study, said in an interview.

The researchers took tiny clippings from the fins of seven different species of sharks off the coast of south Florida before releasing the animals and then analyzed their tissues for the toxin. They found it present in high levels in each of the seven species tested, without any apparent link to the animal’s size, diet, age or habitat. Blacknose, bonnethead and hammerhead sharks contained the greatest concentrations.

BMAA is produced by virtually every known species of cyanobacteria, a ubiquitous algae-like microbe present in freshwater and saltwater worldwide.

The study suggests that the toxin can accumulate up the food chain, increasing in concentration as one animal eats another.

That is worrisome because sharks, like humans, are apex predators, and if BMAA can accumulate in shark tissues, the same could possibly happen in humans. “We could be exposed to BMAA over our life span, and it could accumulate in our tissues as well,” Dr. Mash said. But exactly what foods contain BMAA — or what concentrations could cause problems — has not been thoroughly investigated.

A growing body of research suggests there may be a connection between exposure to the toxin and the development of neurodegenerative diseases like Alzheimer’s, Parkinson’s and Lou Gehrig’s disease, also known as amyotrophic lateral sclerosis, or A.L.S.

Working in Guam, Dr. Cox and the neurologist Oliver Sacks hypothesized that the toxin might be partly responsible for a mystifying degenerative disease amongst the Chamorro people called lytico-botig whose symptoms resemble those of other neurodegenerative diseases. The pair suggested that the disease could result from a heavy dose of BMAA related to dining on the Guam flying fox, or fruit bat, which has since been hunted to extinction.

Tests showed that these animals biomagnified BMAA from the cycad seeds upon which they subsisted; the seeds get it from symbiotic cyanobacteria in the plant’s roots.

More recent research has found high levels of BMAA in the brains of some people who died from Alzheimer’s and Parkinson’s, but not in the brains of other people with similar backgrounds and physiology who died from other conditions, said Douglas C. Lobner, a Marquette University researcher who was not involved in the study. These concentrations are similar to those seen in the Guam fruit bat — and now, in shark fins.

The toxin has been shown to incorporate itself into brain proteins in animal studies, causing protein tangles like those seen in neurodegenerative conditions, Dr. Mash said.

Dr. Lobner said BMAA also acts synergistically with other neurotoxins like mercury; research has shown that when sublethal amounts of the two are combined, they become exponentially more deadly. That is of particular concern because shark fins are often contaminated with mercury and other heavy metals.

Dr. Cox has found BMAA in other foods, including a type of Peruvian soup and a species of fish in Japan. In each case — as with the Guam bats and shark fin soup — people generally describe the taste as delicious and are willing to pay a lot to obtain it. This may be because the toxin binds to glutamate receptors, possibly like those found in “umami” taste buds, said Dr. Cox and his colleague, Sandra Banack.

Dr. Cox emphasized that this remains a hypothesis, however, as does the link between BMAA and neurodegenerative conditions. But that does not mean it is necessarily smart to dine on shark.