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Callum Whyte: Primrose has a thing or two to teach my grandmother about when it is safe to eat shellfish

Callum Whyte: Primrose has a thing or two to teach my grandmother about when it is safe to eat shellfish

As I was growing up, any time my mother suggested buying mussels or cockles for dinner, my gran would pipe up with the old adage that you should only eat shellfish when there’s an ‘R’ in the month.

In truth, my gran didn’t much care for shellfish in any month of the year but is there any validity in that old saying?

The waters around our coasts abound with different species of microscopic, single celled organisms. Shellfish feed on this microscopic micro-plankton community that sits at the bottom of the intricate food web within our seas and oceans.

While the vast majority of these organisms are harmless, a few are capable of producing toxins that cause us food poisoning through the shellfish we eat. These toxins are stable at high temperatures and cannot simply be removed by cooking.

While the UK has a robust shellfish monitoring programme, unusual weather patterns in the summer of 2013 led to an outbreak of Diarrhetic Shellfish Poisoning (DSP) that struck 70 unfortunate mussel eaters in southeast England and London. Symptoms included diarrhoea, nausea, vomiting and cramps.

The 2013 outbreak was traced to mussels that had come from the coast of Shetland, which resulted in the closure of the fishery and several harvesting sites for several weeks. The responsible toxins in this case are produced by a dinoflagellate, a group of marine plankton, and a persistent offender known as Dinophysis.

There are three other forms of shellfish poisoning, each caused by different toxins and found in varying areas of the world. Paralytic shellfish poisoning caused mainly by saxitoxin can, as you probably guessed, cause paralysis; neurotoxic shellfish poisoning causes muscle ache and pinprick sensations; and the final type, amnesic shellfish poisoning, caused by the toxin domoic acid can cause permanent short-term memory loss, brain damage and, in rare cases, death. Fortunately, these forms are less common.

Dinophysis are ubiquitous around our coasts, but of the 100 or so different species identified, only eight are confirmed to produce toxins. They are generally found in relatively low concentrations and present very little risk to the public. However, as with many of the other toxin-producing micro-plankton, problems can arise under the right conditions when their numbers start to increase exponentially, causing an event known as a harmful algal bloom (HAB).

As global sea temperatures continue to rise, the conditions favouring dinoflagellate growth are becoming more common and HABs are on the increase. Warmer seas also mean these favourable conditions can begin earlier in the year and end later, extending their growing season into months that do have an ‘R’ in them.

So, should you only eat shellfish when there’s an ‘R’ in the month? Well, the majority of the dinoflagellate species capable of producing toxins are happiest when the top layers of the waters where they grow are stratified – when water with different properties form separate layers and levels of nutrients. This tends to happen during the summer months as the sun heats up the surface layer of the sea, lowering its density and allowing it to sit on top of the cooler, denser water below.

There is therefore likely to be more risk of a harmful bloom in summer and, in the past, abiding by the old aphorism may have saved you from some unpleasant stomach upsets.

Much of the shellfish produced in UK waters is farmed and, as the industry continues to grow, the threat of closure due to HABs is a worrying one. That is where marine scientists can play their part to inform industry and protect the public. I am currently part of a team of researchers at the Scottish Association for Marine Science, UHI, along with European colleagues, working on an EU Interreg-funded project known as Primrose to ensure that it is, in fact, safe to eat shellfish at any time of year.

We use information from a range of sources including marine robotics, satellites, in situ sensors and teams of experts who identify and count the microorganisms in water samples collected from sites around Europe. We then develop and improve a range of ‘bulletins’ and forecasts that inform farmers of the risk of harmful algae in their waters.

Using these forecasts, alongside testing carried out by the farmers on site, they can then make informed choices of when to harvest their shellfish. As consumers, we can therefore be confident that the shellfish we eat is safe. Since SAMS UHI began issuing weekly bulletins at

there have been no incidents of amnesic, paralytic or diarrhetic shellfish poisoning in Scotland.

However, we can never become complacent. That’s why projects like Primrose are important in helping us increase our knowledge of these toxic species and provide better, longer and ever more accurate forecasts to the industry.

Callum Whyte is a phytoplankton ecologist at the Scottish Association for Marine Science, University of the Highlands and Islands


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