The adage of eating oysters only in “r” months goes back (at least) to 1599, when it appeared in an English cookbook, “Dyets Dry Dinner.” What it really means is: Say no to raw oysters in the summer. And there’s some merit to that.
During the summer, unrefrigerated (or un-iced) oysters spoil more easily. The bacteria that cause food sickness are more rampant, and oysters are spawning. Dr. Meritt said the cookbook was probably referring to European flat oysters, Ostrea edulis, which brood their larvae inside the shell. During spawning season, a European oyster may come with a surprise crunch: a sac of baby oysters in tiny shells.
But the native oysters on America’s East Coast, one of the most common being Crassostrea virginica, don’t reproduce that way. Instead, males and females broadcast their sperm and eggs out into the water, where the eggs become fertilized and develop.
During summer, many oysters also become “snot bags,” Dr. Meritt said. An oyster’s gonad — its largest organ — makes up 30 to 40 percent of its body mass when fully developed. But as seasons change, warming water may induce oysters to spawn, releasing the contents of their gonads, either eggs or sperm.
This leaves oysters watery and thin, like a bag of pudding that has been replaced with water. Some say this affects the taste, but not Dr. Meritt: “Nothing is wrong with them.”
If you just can’t deal with the gonads, there are oysters called triploids that don’t have them. An extra set of chromosomes makes them sterile. They don’t spawn, and they stay fat all summer.
No oyster is immune from dirty water, though. Pollution is always a problem, but certain types of bacteria that can make you sick or even die may be more prevalent in the summer, and there’s a bigger chance an oyster can become infected.
To address this concern, the National Shellfish Sanitation Program enforces strict guidelines to ensure safe handling for human consumption. Part of that is getting oysters on ice quickly.
“There are strict icing laws in the summer to instantly reduce temperatures at harvest,” Mike Osinski, who runs the Widow’s Hole Oyster Farm in Greenport, N.Y., wrote in an email. “However, I simply do not ship to the city in July or August.”
Dr. Meritt said other oyster farmers also played it safe: If one person gets sick on oysters, the whole market suffers.
So “r” month or not, feel free to keep slurping — but proceed with caution.