Over on the Manhattan Board, Blanche asked about the truth of the old adage that you should only eat oysters in months with an "R". I responded earlier today, and then realized that I should have posted my answer on the General Topics Board, since the subject is certainly not limited to Manhattan. Sorry. Here's my response to Blanche's question.
The adage is a somewhat imprecise rule of thumb as to when sea water will be sufficiently warm to induce the oysters to spawn, making them soft and milky, and therefore undesirable to eat.
An exception to an otherwise dismal history of investments was my decision to purchase some property on Hood Canal in Washington State back in the early 1970s when such property was selling at bargain prices. Among the many benefits of this property is a beach loaded with Pacific oysters. As a result of my interest in, and maintenance of, these oyster beds, I've come to learn a little bit about oysters.
There are at least two theories regarding the origin of the notion that you should only eat oysters in months with an "R." One is the belief that, in the days when there was little refrigeration, there was increased risk of oysters spoiling in the hot summer months. I tend to discount this theory. The real problem, which is as true today as in years past, is that oysters spawn in the summer months. Spawning takes place in response to water temperature. As the water temperature becomes warmer in the summer months, the oysters respond by spawning. During spawning, the oysters become soft and milky. There is nothing dangerous about eating them in this condition. They simply aren't very palatable. The exact time of spawning varies year to year, depending on the weather and water conditions. On northern Hood Canal, where my property is located, June is usually a pretty safe month for eating raw oysters, even though it doesn't have an "R" in it. I keep eating oysters until I hit my first milky ones. This is usually in July. Sometime in September, the oysters usually complete their spawning. After spawning, oysters build up glycogen, which provides the appearance of a "fat" oyster. The oysters on my beach are best in the winter when the water is cold and the oysters are both fat and firm. Picture a cool, crisp winter day, a bonfire on the bulkhead overlooking my beach, a good loaf of good bread, a good bottle of Chablis, and buckets full of firm, fat oysters just plucked off the beach. Heaven!
The commercial oyster industry has responded to the lack of oysters during the spawning season in a couple of different ways. One is to individually quick freeze ("IQF") oysters during the peak winter season, so they can be thawed and served "raw" in the summer months. This practice, for example, is noted on the Fanny Bay Oysters website. When eating raw oysters during the summer, you should ask whether the oysters being served have been previously frozen. The other commercial response has been to create triploid or "sexless" oysters. Ken Chew, Associate Dean of the University of Washington College of Ocean and Fishery Science is credited with this invention. For those interested, I recommend the book, "Sexless Oysters and Self-Tipping Hats: 100 Years of Invention in the Pacific Northwest," by Adam Woog.
Oysters are very interesting critters. For example, American oysters are divided into separate male and female individuals and spawn by spewing millions of eggs and sperm into surrounding waters, where external fertilization takes place. Because of this, the water currents can take the larvae far from the oyster beds where the spawning originated. Some years, I have lots of new "baby oysters" on my beach. Other years, because of the fickleness of the ocean currents, I have very few new "babies" on my beach. Another interesting fact is that American oysters can change sex during their lifetime from one to the other and back again. The change is thought to be related to environmental conditions, with femaleness being favored in locations and years with good food supply. Europeans oysters, in contrast to American oysters, are hermaphroditic. Each individual possesses male and female germ cells, and fertilization occurs inside the inhalant chamber of the oyster. The larvae then incubate in the chamber for up to ten days prior to being ejected into the surrounding waters.