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 be eating shellfish when there’s an ‘R’ in the month”. In truth, my gran didn’t much care for seafood 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 organisms. Many of them are autotrophic, which means they gain sustenance by photosynthesising the sunlight that filters down through the water column. Others are heterotrophic and need to consume prey to survive and a number of others, called mixotrophs, do a bit of both. This microscopic micro-plankton community sits at the bottom of the intricate food web that extends throughout our seas and oceans and shellfish, both wild and farmed, feed on it.
Diarrhoea, memory and paralysis
While the vast majority of these organisms are perfectly harmless, there are a few that are capable of producing a range of different toxins that can get into these shellfish and cause food poisoning – it isn’t just that shellfish have “gone off”. And these toxins can’t simply be removed by cooking in high temperatures.
The most familiar of these, diarrheal shellfish poisoning, struck 70 unfortunate mussel eaters in south-east England in July. Symptoms include diarrhoea but also common are nausea, vomiting and cramps that can begin within half an hour of eating infected shellfish.
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. What starts as a tingling in your arms and legs can lead to paralysis of the lungs and in the worst cases can kill in a couple of hours.
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 death.
Luckily, these forms are more rare and most outbreaks of shellfish poisoning feature more diarrhoea than paralysis. The July outbreak was traced back 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 – okadaic acid and its derivative dinophysistoxins (DTX’s) – are produced by an attractive species of winged dinoflagellates, a group of marine plankton and persistent offender also known as Dinophysis.
They are ubiquitous around our coasts, but of the 100 different species of Dinophysis or so that have been 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 as under the right conditions, their numbers start to increase. Concentrations of these organisms become what are known as harmful algal blooms. And they can occasionally reach levels high enough to discolour the water, turning it red, brown or green, giving rise to the term “red tide”.
Not all species have to be present in large numbers to produce toxicity in shellfish. The number of Dinophysis found around Shetland this summer wasn’t enough to discolour the water but was much higher than usual, probably because unusual wind patterns in Shetland in June and July blew them onshore where they accumulated in the inlets and bays along the coast. And so the stage was set for the unfortunate poisoning finale over a thousand miles away.
So should you only eat shellfish when there’s an “R” in the month? Well, the majority of the dinoflagellate species that are 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 float on top of the cooler, denser water below. Perfect for marine plankton to thrive.
So, in general, there is likely to be more risk of a harmful bloom in summer and, as a rule of thumb, in the past abiding by the old aphorism may have saved you from some unpleasant stomach upsets.
Warmer seas and worrying blooms
Unfortunately, as sea temperatures continue to rise, the conditions favouring dinoflagellate growth are becoming more common and around the globe harmful algal blooms are perceived to be on the increase.
Warmer seas also mean that these favourable conditions can begin earlier in the year and end later, sadly, extending their growing season into months that do have an “R” in them.
Much of the shellfish produced in UK waters is farmed and as the industry continues to grow, the threat of closure due to harmful algae blooms is a worrying one.
Fortunately in the UK, research to find better prediction methods is ongoing, and the Food Standards Agency regularly monitors shellfish growing areas to ensure that the shellfish arriving on our plates is safe to eat. And while some areas may occasionally be forced to close there will always be others free from toxic algae where the shellfish can be safely harvested whatever month of the year.