Tradition dictates only eating wild oysters in months with names containing the letter 'r' - from September to April - and a new study suggests some people were following this practice 4,000 years ago. Analysis of a large shell midden off the southeast USA coast reveals the ancient inhabitants of Saint Catherine's Island limited their oyster harvest to the non-summer months.
     Snails known as 'impressed odostomes' are common parasites of oysters. Because the snail has a 12-month life cycle, its length at death offers a reliable estimate of when the oyster died. Scientists analysing oysters and snails from a 4,300-year-old, 70 metre diameter shell ring and comparing them with live oysters and snails found that the island's ancient inhabitants were primarily harvesting oysters during late fall, winter, and spring.
     The seasonality of the shell ring may be one of the earliest records of sustainable harvesting. Oysters in the southeast of North America spawn from May to October, and avoiding oyster collection in the summer may help replenish their numbers.
     Using impressed odostomes to gauge what time of year oysters were harvested offers an independent way to assess ancient patterns of oyster gathering, complementing other methods including stable isotope analysis and shell growth rings, and the method could be applied to other marine invertebrates, whether living or extinct. Oyster populations are currently in decline worldwide as the health of coastal and other ecosystems is in question.
   
Edited from EurekAlert! (20 November 2019)