4,000-Year-Old Adage About How To Avoid Bad Shellfish Still Rings True


Madison Dapcevich


Madison Dapcevich

Freelance Writer and Fact-Checker

Madison is a freelance science reporter and full-time fact-checker based in the wild Rocky Mountains of western Montana.

Freelance Writer and Fact-Checker


Modern seafoodies avoid eating shellfish during the warmer months and, as it turns out, so did their ancient predecessors. Yuliya Koshchiy/Shutterstock

Any coastal food junkie likely knows the old rule of thumb that holds shellfish lovers should only consume certain mollusks in months that contain the letter ‘r’. New research now finds that coastal inhabitants have been abiding by the adage for at least 4,000 years.

Months that don’t contain an ‘r’ in their name, May through August, tend to be hotter in temperatures, increasing the likelihood of potentially fatal, and oftentimes undetectable, algal blooms. Some microscopic algae naturally produce a biotoxin known as paralytic shellfish poison (PSP), which affects the nervous system and can contaminate filter feeders like shellfish. In turn, humans that eat infected mollusks can become infected with PSP and suffer severe illness and even death, according to the Washington State Department of Health.  


To come to their conclusion, researchers from the Florida Museum of Natural History analyzed a 4,300-year-old shell ring, or deposit, off the coast of Georgia state. Tossed out alongside these oyster shells were parasitic snails, or odostomes, that latch onto the shell for the majority of its 12-month life cycle. The snail’s length at the time of its death allows researchers to determine when the host oyster likely died. This seasonal clock allowed the team of scientists to determine when ancient inhabitants of the region were harvesting and eating oysters.

The impressed odostome, Boonea impressa, is a tiny marine snail that parasitizes oysters by piercing their shells and sucking their insides. These snails can be found by the hundreds on individual oysters throughout the year. Kristen Grace/Florida Museum

“People have been debating the purpose of these shell rings for a very long time,” said Nicole Cannarozzi, study lead author and Florida Museum environmental archaeology collection manager, in a statement. “Were they everyday food waste heaps? Temporary communal feasting sites? Or perhaps a combination? Understanding the seasonality of the rings sheds new light on their function.”

Located on Georgia’s St. Catherines Island, the shell ring measures 70 meters wide (230 feet) and was once used by inhabitants as a place to seasonally dispose of shells. Researchers compared rings from the site with live oysters and modern snails (Boonea impressa), and found that early inhabitants were collecting oysters from late fall to early spring over the course of several months.

Because oysters typically spawn from May to October, the scientists note in PLOS One that this harvesting strategy suggests that "archaic populations may have opted out of consuming summer oysters to focus on other resources, avoid unpalatable food, decrease pathogen risks, or ensure sustainable harvesting.” Additionally, limiting harvests to a specific window of time may have helped the mollusks to replenish their populations in what appears to be one of the earliest examples of sustainable harvesting, which could, in turn, help modern oyster connoisseurs to develop environmentally viable practices.  

This model shows the outline of the St. Catherines shell ring, with darker areas indicating more density. Matthew Sanger et al./Southeastern Archaeology

“It’s important to look at how oysters have lived in their environment over time, especially because they are on the decline worldwide,” said Cannarozzi. “This type of data can give us good information about their ecology, how other organisms interact with them, the health of oyster populations and, on a grander scale, the health of coastal ecosystems.”

Using species with “consistent growth patterns and predictable spawning behavior” can also inform how archaeologists study ancient marine-dependent civilizations in other marine invertebrate studies. 

Using the impressed odostome is a cost-effective way to supplement other methods of dating shell specimens from archaeological sites and can help researchers understand the health of the region’s coastal ecosystems over time. Kristen Grace/Florida Museum


  • tag
  • oysters,

  • st catherines island,

  • oyster farming,

  • paralytic shellfish poisoning,

  • psp,

  • biotoxin,

  • oyster shell ring,

  • ancient oysters