The ‘R’ Rule: Fact or Fiction?


Shellfish is one of the tastiest and most popular sea foods landed and sold through Plymouth Fisheries, and continues to grow in popularity as a year-round menu pleaser at some of our many leading restaurants and cafes in the city.

But did you know that a random piece of folklore suggests we are supposed to stop eating shellfish this month - and not tuck in again until the start of September - because of something called the ‘R’ rule?

The ‘R’ rule states that shellfish, such as oysters and mussels, should only be eaten in months with an ‘r’ in their title, otherwise they are unsafe to consume. It sounds like a lot of nonsense but we were curious to know if this myth stemmed from any healthy eating sense we should pay heed to, or if it was safe to dismiss it completely.

The ‘R’ rule apparently dates back to 1599, where the advice appeared in a cook book by Henry Buttes called Dyets Dry Dinner. Although, it’s not confirmed that this was the first appearance of the rule, or where it originated first, some historians trace this advice back to an ancient Latin saying.

The warning originally applied to oysters but it has gradually evolved to include all shellfish. Typically, shellfish is out of season and harder to come by in the warmer months of the year, when they spoil more easily, and summer is their spawning season.

It’s said that the ‘R’ rule allows shellfish to repopulate during this time. It’s also stated that spawning shellfish can taste quite different compared to at other times of the year, as they tend to be soft, watery and less flavourful. So, does this mean that the advice is about quality over safety?

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Well, another reason behind the rule could be because during summer months, high concentrations of an algae that is toxic to humans can be absorbed by shellfish. Bacteria that is dangerous to humans has a greater likelihood to flourish under warmer weather, and so consuming shellfish which has absorbed toxins could cause severe sickness, known as paralytic shellfish poisoning.

This shouldn’t put you off eating any shellfish during the summer period altogether though. Some types of shellfish are harder to come by from May to August as they are out of season, but for those that are readily available, the warning mainly applies to those harvesting the shellfish themselves.

Technology and equipment available today means fisherman can closely monitor and test for high levels of dangerous toxins, and thus protect diners from any risk. Seafood sold at fish markets are also thoroughly inspected to make sure they are safe and of high quality. This safeguarding in place means that bad shellfish are highly unlikely to be sold commercially.

So does this mean the ‘R’ rule no longer applies?

In principle, yes – though the ‘R’ rule can still be used as a guideline when talking about the quality of wild shellfish, and you’d be wise to rely instead on choosing from a high-quality selection available from a variety of fish markets.

If you do have any concerns about buying shellfish during the summer months, it’s best to talk to a fishmonger who can answer any queries or questions you may have.

Here you can find contact details for the fish merchants based at Plymouth Fisheries who are the best people to speak to.