Many people enjoy eating seafood, but popular shellfish like clams and mussels can sometimes be harmful to humans. Each year, outbreaks of "red tide," a sudden growth in toxic algae, shut down parts of Canada's shellfish industry by contaminating popular shellfish like clams and oysters with dangerous toxins.

Outbreaks of organisms that can produce toxins like Alexandrium tamarense, which causes paralytic shellfish poisoning, are not only a health hazard, but they are also an economic problem – a serious concern for businesses that rely on shellfish farming.

Dinner Table Danger

Red tide occurs when there is a sudden algal bloom – a rapid increase in the growth of algae (microscopic plants) in a body of water. Algae become more concentrated and appear to change the water's colour, hence the name "red tide", although some blooms may be green or brown or even cause no water discoloration at all.   

Some algal blooms produce toxins which can accumulate in clams, oysters and other "filter feeders" that eat plankton by filtering large volumes of water that they pump through their gills (feeding organs). These toxins can sometimes harm sensitive shellfish, but other shellfish are resistant to the toxins and can quickly build them up in their tissues. When humans eat shellfish poisoned with toxins, they can become quite sick and experience symptoms ranging from stomach ache to paralysis.

Monitoring Shellfish Safety
  
In 1987 a particularly bad occurrence of a new shellfish toxin led to what is now called amnesic shellfish poisoning. The outbreak caused three deaths and hundreds of people became sick. The east coast shellfish industry was completely shut down. Working for four days, NRC scientists in Halifax identified the toxin responsible for the outbreak and then developed a method that has been used by the Canadian Food Inspection Agency to keep track of toxin levels.

Efforts like these have made it possible to monitor shellfish farms and issue warnings and closures when toxin levels are too high. Warnings are intended to keep poisoned shellfish out of the food supply and are very effective in protecting the public so that it eats shellfish only when they are safe. 

NRC is now developing analytical technologies that can provide early warning of upcoming toxin events through detection of toxic algae in the water and toxins in shellfish. Shellfish farmers may one day be able to use such methods on-site, which would allow harvesting to be stopped well before shellfish become toxic. If the shellfish are left in the ocean, they eliminate the toxin once the toxic algae is gone, so no product would actually be lost.

Filter Feeders in the Future

NRC has also made a significant step towards preventing human and economic suffering caused by red tides and understanding how toxins are transferred through the food chain. 

Scientists have recently identified a genetic mutation in softshell clams that increases their immunity to toxins and results in them accumulating high toxin levels in areas that have been exposed to red tides for many years. Other softshell clams that do not have this mutation accumulate much lower toxin levels.

Hopefully, this genetic characteristic leading to low toxin accumulation can be transferred to other shellfish by controlled breeding in hatcheries in the future. This would have a positive impact on both maritime fisheries and human safety.