Seafood lovers take note: there’s good news and bad news.

First, there’s the bad news for those who enjoy tuna on rye.

High concentrations of mercury, a neurotoxin that can damage developing brains in fetuses, are found in some kinds of popular fish such as albacore tuna. Swordfish and shark, king mackerel, marlin, orange roughy and tilefish also contain dangerous levels of mercury.

Women of reproductive age and young children are advised to avoid these types of fish and limit overall consumption of all fish to no more than 12 ounces per week, according to the Food and Drug Administration, as it takes months for the body to rid itself of mercury.

The danger from mercury is not just to developing brains. There is evidence to suggest an association between mercury exposure and heart disease, making it dangerous for everyone, but especially those who are already at risk.

The American Heart Association, however, recommends eating fatty fish at least twice a week because it is high in omega-3 fatty acids which are believed to help lower rates of heart disease, reduce hypertension, relieve some arthritis symptoms and prevent cancer. Fatty varieties that are low in mercury include herring, sardines, and wild salmon. Some popular fish that are also good choices include sole, tilapia, clams and oysters.

“It all depends on your diet — you can’t eat a lot of big, wild fish,” said Tim Fitzgerald, a marine scientist for Environmental Defense Fund, who provides health consumption information to Monterey Bay Aquarium’s Seafood Watch.

The problem with large, predatory species like marlin and swordfish is that they contain much higher levels of mercury than small fish, such as anchovies and sardines, because of the way mercury moves up the food chain. “Sharks, marlin, polar bears and people at the end of the food chain have the highest concentration of mercury,” Fitzgerald said.

It’s sometimes difficult for consumers to make seafood choices that are good for their health — and the environment. According to Tim Fitzgerald, “Billions of pounds of imported fish come into the United States annually, and less than one percent is tested for environmental toxins by the FDA.” Because marlin is not a popular dining choice in the U.S., many people are not aware of this. And while the FDA is the regulator body that creates consumer advisories about mercury for pregnant women, they actually do very little testing for this neurotoxin.

Another problem with the advisories is that they are not terribly specific and there’s a lot of room for interpretation, according to Fitzgerald. It’s also difficult for consumers to make the best seafood choices because sometimes what’s best for the environment is not always best for their health, and vice versa. For example, blue marlin and striped marlin from Hawaii are fairly resilient to fishing pressure and are listed as “good” alternatives for the environment on Seafood Watch. But, Seafood Watch also lists a health advisory for these fish, due to high levels of mercury. Monterey Bay’s other regional pocket guides provide further guidance for consumers and note that imported blue marlin and striped marlin should be “avoided.”

So, for U.S. consumers, the situation is “buyer beware — eat with caution,” but certainly not to give up on all fish. Consumers may just need some help from Monterey Bay’s Seafood Watch, which maintains a list of “which seafood to buy and why,” including a comprehensive seafood search, regional seafood guides — and printable pocket-sized guides for your wallet.

And, if you are a tech-savvy-seafood-lover, a “fish phone” may be more of what you’re looking for. Environmental Defense Fund’s Seafood Selector to-go allows mobile web users to look up their seafood guide on a blackberry or iPhone and download the information.

Ultimately, it’s ideal to exercise moderation and caution when eating seafood by taking into account both environmental and health concerns. Fortunately you don’t have to wonder whether the seafood menu at your favorite restaurant is environmentally friendly, the answers to your questions may just be a text-message away.

Blue-Ringed Octopus (Hapalochlaena maculosa)

• Type: Cephalopod
• Family: Octopodidae
• Habitat: Shallow marine waters and tide pools
• Location: Common off the coast of Australia and the western Pacific Ocean
• Diet: Crabs, fish, and mollusks
• Average lifespan in the wild: 2 years
• Size: 5-7.8 in (12.7-20 cm)
• Weight: .92 oz (26g)

With its fascinating coloring and delicate curling arms, the blue-ringed octopus may be a beautiful creature, but this small cephalopod is also deadly. The blue-ringed octopus appears grey or beige with light brown patches when it is at rest, but when agitated its 50 or 60 bright blue rings appear and pulsate with color, as a warning. Inside the salivary glands of the blue-ringed octopus live colonies of bacteria that produce tetrodotoxin, the potent neurotoxin found in pufferfish and other animals. A bite from a blue-ringed octopus can completely paralyze and kill an adult human in a matter of minutes. There is no known antidote. The octopus itself is not affected at all by the toxin-an evolutionary prerequisite for the symbiotic relationship that has developed between the blue-ringed octopus and the toxin-producing bacteria.

The blue-ringed octopus is commonly found in shallow, sandy areas surrounding the coastal reefs of Australia and the western Indio-Pacific. It is most active after dark, and spends most of its day hidden in its nest. Like all octopods, the blue-ringed octopus has no skeleton and is thus very flexible and maneuverable. It can squeeze into tiny crevices and make dens in bottles, aluminum cans, or mollusk shells. The blue-ringed octopus is also known to burrow into sand or gravel to conceal itself.

The blue-ringed octopus feeds primarily on crabs and mollusks, ambushing from behind and enveloping prey with its eight arms. Using its bird-like beak, the octopus bites a hole through its victim’s shell to inject toxic saliva. With its arms and beak, the creature tears soft pieces from the prey, sucking the rest of the meat from the shell once it becomes partially digested by the saliva.

Packets of sperm rest in the grooved tip of the male’s modified third arm, called a hectocotylus. When mating, the male slips this grooved tip under the mantle and into the oviduct of the female through a gill slit, and transfers multiple sperm packets, or spermatophores. The female lays her eggs in several unattached clumps, which she carries in her arms until they hatch. After the young emerge from their eggs, the mother dies.

Did you know: The blue-ringed octopus, like all octopuses, has three hearts and blue blood.

Related Episode: Encountering Sea Monsters

Photo © Gary Bell / Picture Quest

When it comes to research on venom and converting it into useful drugs, studies involving exotic snakes or brightly colored frogs seem to attract the most attention. However, one of the most promising new venom-derived drugs actually comes from a very modest-looking sea snail.

Worldwide, there are more than 600 kinds of cone shells found mostly in tropical waters around the Pacific. Collectors love them because their shells are decorated with an amazing array of intricate patterns.

Biologists, however, have long been fascinated by the behavior of these clever hunters. Some cone shells target other snails, while others like to feast on fish. To sense food, cone shells filter water through a tubelike organ called a siphon, awaiting a whiff of the telltale chemicals emitted by their prey.

To sense food, cone shells filter water through a tubelike organ called a siphon.

Then, when its victim comes near, the cone shell extends a proboscis armed with a harpoonlike tip that injects venom filled with special chemicals called “conotoxins.” These toxins stop nerve cells from communicating with each other, causing paralysis within seconds and, eventually, death. Cone shells have even killed people who pick them up, unaware of the danger. Indeed, cone snail venom is so powerful and painless that victims can die unaware that they’ve even been bitten.

Conotoxins have long interested medical researchers because of their potential painkilling abilities. It turns out, however, that cone shell venom is very complex; each kind contains perhaps 50 or more different chemicals that target the brain and nervous system. Overall, researchers believe that more than 50,000 conotoxins may exist. That diversity has made it hard for them to isolate a specific chemical to work on.

But over the last few decades, conotoxins have begun to give up their secrets. Researchers have published more than 2,500 papers on the chemicals, and have described and identified more than 100 specific toxins which show promise for treating everything from arthritis to cancer. But the first new drug derived from a conotoxin, approved in 2004, targets chronic pain. Researchers estimate that the drug, based on the venom from the delicate gray and ivory magician cone shell, is a thousand times stronger than morphine, the most powerful traditional painkiller.

Even as cone shells show promise for medicine, however, their survival may be at stake. Collectors gather millions of the animals each year for the decorative shell trade. Demand from conotoxin researchers is growing too, since many shells may be needed to produce even small amounts of toxin. And coral reefs, which support more than half of all cone shell species, are under increasing threat from human activities.

To protect cone shells, biologists are asking nations in tropical zones to take new steps to monitor the shell trade and protect reefs. “To lose these species would be a self-destructive act of unparalleled folly,” researcher Eric Chivian of Harvard University in Cambridge, Massachusetts wrote in a 2003 paper published by the journal SCIENCE. “Tropical cone snails may contain the largest and most clinically important pharmacopoeia of any [group of animals] in nature.”

In the dead of night, a small boat slides across Australia’s Gulf of Carpentaria. On board are researchers Bryan Fry and his wife Alexia, both of whom stand ready, equipped with bright spotlights and large nets. They peer into the dark water. But it is not fish they are after — they are searching for sea snakes.

As NATURE’s The Venom Cure reveals, their goal is to collect, study, and catalog the chemical characteristics of venom from sea snakes. They have already identified a powerful anticoagulant that could one day be used to treat potentially fatal coronary conditions.

Eventually, on this night, the Frys catch a host of the sea reptiles. They carefully “milk” each of its deadly venom, then return the slithery creatures to their watery home.

After catching the sea snakes, the Frys carefully “milk” each of its deadly venom before returning them to the ocean.

Bryan Fry has milked thousands of snakes during his career, and has been bitten more than a dozen times. Luckily, he’s lived to tell about it, and all bites aside, he continues to study venom.

“There is something peculiarly fascinating in the use of a deadly toxin as a life-saving medicine,” says Fry, a Deputy Director of the Australian Venom Research Unit at the University of Melbourne. “The natural pharmacology that exists within animal venoms is a tremendous resource waiting to be tapped.”

Once back in the laboratory, Fry will carefully analyze the venom, which is essentially specialized toxic saliva that attacks a prey animal’s organs. Some snake venoms target the brain, shutting it down, while others destroy liver or blood cells.

How did snakes develop such specialized chemical weapons? Fry’s research offers some insights.

In a paper published in the March 2005 issue of the scientific journal GENOME RESEARCH, Fry and colleagues note that the active ingredient in snake venom is often an “evil twin” version of chemical proteins that the snake’s own organs need to function. Over the last 80 million years, snakes “learned” to adapt these proteins and convert them into toxins. So a protein that helps a snake’s liver function became a weapon able to destroy the liver of the snake’s prey.

Overall, Fry’s team found that some snake toxin types originated from proteins normally made by the snake’s brain, eye, lung, liver, or other organs. Gradually, the toxins were produced in tandem with saliva, to create their deadly bite.

“Snakes are incredibly inventive,” Fry says. “The wide-ranging origins of snake venom explains the amazing diversity of ways that snakes can kill their prey and why they have so much potential use in medical research.”

Eventually, Fry hopes that his work will help researchers identify chemicals that could be used to treat everything from liver disease to brain disorders.