Anyone who indulges in wild oysters has surely heard the advice: Eat them only in months with an R. In other words, skip them during the summer. Now, researchers from the Florida Museum of National History have found evidence that the practice was in play more than 4,000 years ago, reports Vice. The scientists analyzed ancient, empty shells off the coast of Georgia's St. Catherines Island and concluded that inhabitants back then harvested them only from late fall to late spring. The research team figured this out with some scientific sleuthing: They measured the length of small parasitic snails that had attached themselves to the oyster when it was still alive. Because these snails have a 12-month life cycle, their length at death functions as a "tiny seasonal clock," explain the researchers in a news release.
Why the inhabitants avoided the oysters during the summer is trickier to explain. It may have been for the same reasons that people avoid them today: Oysters taken in warm waters may be more susceptible to bacteria in the water and thus make you sick, explains the Digital Journal. What's more, because oysters spawn in the summer, they tend to be watery and less appetizing during that season. In their study at PLOS One, the researchers raise another intriguing possibility: People on the island may have realized that harvesting oysters during spawning season would deplete their numbers. Thus, this might be "one of the earliest records of sustainable harvesting." Whatever the reason, the team says this technique might be able to shed light on the health of coastal ecosystems past and present. (Read more oysters stories.)