A harmful algal bloom (HAB) is an algal bloom that causes negative impacts to other organisms via production of natural toxins, mechanical damage to other organisms, or by other means. HABs are often associated with large-scale marine mortality events and have been associated with various types of shellfish poisonings.

Brevetoxins, which are brain-changing compounds synthesized by some harmful algal blooms, have now been shown to affect a free-ranging marine species. In this case, researchers focused on lemon sharks, but they believe many other types of sharks could fall victim to the toxins.

"Sharks are exposed via consumption of brevetoxin-contaminated water and food, such as shellfish," co-author Niladri Basu explained to Discovery News, mentioning that the toxins can easily cross the shark's blood-brain barrier that otherwise protects the brain.

"Once inside the brain, brevetoxins bind very strongly to a protein that controls sodium flow," added Basu, an assistant professor of environmental health sciences at the University of Michigan School of Public Health. "By disrupting sodium flow in the brain, nerve cells will over-fire and cause hyperexcitability and ultimately result in death."

For the study, Basu and his colleagues analyzed the brains, other organs and tissues of 30 juvenile lemon sharks from a site directly off Cape Canaveral in east-central Florida.

Nursery habitat
"The area we studied represents a recently discovered nursery habitat for lemon sharks, and it may serve as one of the most valuable lemon shark nurseries in U.S. waters," co-author Douglas Adams of the Florida Fish & Wildlife Conservation Commission told Discovery News.

The well-known "Florida red tide" that occurs in the Gulf of Mexico is a HAB caused by Karenia brevis, another dinoflagellate which produces brevetoxin.

Some of the sampled sharks were exposed to Florida red tide events, while others were not. At least one of the sharks perished as a result of a toxic algal bloom. The exposed sharks showed high levels of the poisonous substances in their brain, liver and gills, with the brain exposure causing significant brain chemistry changes.

In a separate study, Leanne Flewelling of the Florida Fish and Wildlife Conservation Commission, and her team linked one red tide event in October 2000 to mass mortality of blacktip sharks and Atlantic sharpnose sharks in northwest Florida.

Even if a shark survives such a harmful algal bloom, the long-term impact to its population remains unknown, since "the presence of brevetoxins in shark embryos raises questions about the effects these toxins may have on the reproductive success of sharks," concluded Flewelling and her colleagues.

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