Key Points
- As global oyster populations decline and fisheries collapse, archaeologists may be able to inform effective management with valuable, long-term perspectives of the human-oyster connections stretching back millennia.
- Oyster shells found in archaeological middens are difficult to measure accurately because of their irregular shapes and tendency to break apart. Traditionally, researchers only measure whole left valves and ignore the fragments.
- In a new study , researchers from the Florida Museum of Natural History and the Smithsonian National Museum of Natural History show that shell fragments matter. Including them in calculations can improve archaeologists' insight into past oyster populations.
We've feasted on them, built economies around them and in some places nearly erased them from our coasts. Today, 85% of the world's oyster reefs are gone. Many fisheries are collapsing, and those in Florida are no exception: Many estuaries in the state have lost up to 90% of their oyster reefs. Scientists and conservationists are racing to rebuild sustainable oyster populations, something that Indigenous communities were able to steward for millennia. By looking at ancient oyster populations, archaeologists may be able to guide this restoration in the present.
"We know what happens in the past shapes the present and the future, whether that was five minutes ago, five years ago or 50 million years ago. There are tangible and intangible links through time, and these archaeological perspectives of oysters can act as a baseline for the present," said Michelle LeFebvre, associate curator of South Florida archaeology and ethnography as well as Caribbean archaeology collections at the Florida Museum of Natural History.
The eastern oyster (Crassostrea virginica), found in brackish waters along North America's eastern coast, is a modern delicacy but was once a staple food for Indigenous peoples along Florida's Gulf Coast. The evidence of this remains in middens — heaps of shells, bone and other debris built by ancient communities — dotting Florida's landscape. By studying these middens, archaeologists can gauge the health of oyster populations at the time the shells were tossed into the pile hundreds or thousands of years ago.
One of the most basic metrics archaeologists look to is shell size, which can act as a substitute for age (a more complicated and labor-intensive metric to collect). By tracking changes in size over time, scientists can interpret how intensively communities harvested oysters. A reduction in average shell size, for example, may indicate that oysters were being gathered faster than their populations could sustain themselves.
But this method does have its limitations — primarily, due to the shells themselves. The eastern oyster shell, a pale white to gray color and gnarled with ridges and bumps, tends to be teardrop shaped but doesn't develop uniformly.
"Part of what makes them so interesting is that they're ecologically plastic," said Torben Rick, curator of North American archaeology at the Smithsonian Institution. "They're able to mold to their environment, so the way they grow is influenced by the currents, sediment, habitat and the nutrients in the water. All this variation can throw a wrench into the way people measure and think about oysters."
This isn't a problem when the shells are found intact, but that often isn't the case for archaeologists.
Whether during processing or after spending millennia in a midden pile, shells often end up cracked or crushed. If oyster shells developed uniformly, archaeologists could simply measure a broken shell, plug the information into a mathematical model and estimate how large the full shell would have been. But because of the irregularity of the oysters' shapes, scientists have not been able to find a correlation between the broken shell and its once-whole size.
Without this model, archaeologists facing a pile of broken shells have traditionally tried to avoid the problem by only collecting and measuring the shells that remain intact. But doing so may cause an entirely different set of challenges.
"Archaeologists take the whole shell and just ignore the issue of fragmentation. But with a lot of these collections, more than half of the assemblage is broken. That means you're working with a sample that's not even close to complete. We wanted to know if that is a biased sample," said Neill Wallis, curator of Florida archaeology, bioarchaeology and the Ceramic Technology Laboratory at the Florida Museum of Natural History.
There is a risk that by focusing only on the whole shells that "survived" intact, scientists are overlooking important data in the fragments. In reality, size and breakage might be correlated. It's possible, for example, that larger shells are more likely to remain intact, while smaller ones tend to break apart and are left out of the analysis.
"It's like we're trying to reconstruct a 1,000-piece puzzle, but we only have five of them to work with. There are a lot of holes," Rick said.
To test whether fragmentation does affect research outcomes, archaeologists conducted a case study using oyster shells excavated from two midden sites in Florida: Calusa Island Midden and Garden Patch.
Calusa Island, composed largely of its midden, is a small stretch of conservation land north of Pine Island. This area — Pineland — was once home to one of the largest Calusa communities in southwest Florida. With deposits here dating back about 4,000 years, it provides a unique look at the Late Archaic period all the way until the most recent deposits, around A.D. 1200.
While in some cases, middens were simple refuse heaps to toss waste, others held strategic or ceremonial importance. At some sites, the Calusa used shells to build elevated platforms on which they constructed their homes. Living atop these shell mounds would have offered both a defensive vantage point and protection from storm surges.
Farther north in the state, Garden Patch served as a ceremonial center for about three centuries. The early village features several mounds arranged in a horseshoe shape around an open plaza. The 6-foot-tall midden analyzed in this study was formed about 1,600 years ago and seems to have been used seasonally. For some 200 years, local community members and guests likely gathered for winter feasts and rituals, tossing shells and bone scraps into the pile.
At both sites, archaeologists collected samples from the middens, systematically digging bagfuls of whole and fragmented shell to take back to the lab for analysis. During fieldwork, one practical reason for favoring whole shells — a metric that is often collected while still in the field — quickly became obvious. Bags of midden deposit are heavy, cumbersome and time consuming to analyze. Researchers hauled the samples nearly 2 miles from the Garden Patch site to a paved road, sometimes through shin-deep water. The Calusa Island midden is surrounded by water, which meant researchers had to juggle weight limits while cramming onto small boats. Getting the shells back to the lab was one thing; analyzing them was another.
"The amount and volume within an oyster shell midden can be remarkable. We took years to sort through and do the physical analysis," LeFebvre said. "Logistics and capacity are big challenges. It may sound obvious, but it takes time, money and people power to do everything."
After two years of meticulously sorting through the samples — identifying the eastern oysters, counting whole shells and fragments with hinges, and weighing everything for each site — the researchers could begin their calculations. The goal was to determine whether including fragmented shells would significantly alter the average size of the oyster population compared with calculations based solely on whole shells. To test this, they found the sample's average shell size following the traditional method of only measuring whole shells. Then, they did the calculations again, this time using both the whole shells and the fragments (plugging the weight of the fragments into a mathematical model gives scientists an estimated average size of the shells before they were broken).
In the end, their suspicions were confirmed. Including the fragments shifted the sample's average shell size, which means whole shells alone weren't enough to accurately represent the entire population. In one case, the difference was significant enough to flip the interpretation on its head. Rather than increasing, oyster size had decreased over time. If archaeologists only analyzed whole shells at the site, they would draw false conclusions about how past communities were managing oysters.
"This is the first study I know of that has taken the question of fragmentation head-on, showed some concrete results and pointed to the future," Rick said. "We all measure size. We know it's an important metric, but we also all know it's flawed. Now we're going to take a crack at trying to understand those flaws and how bad they might be."
The results are a call to action for archaeologists. "What we came up with is not a solution to the problem. It's a way to recognize the problem," Wallis said. "It is a call for archaeologists to listen up, discuss, reflect and get to work on better metrics."
Oysters are keystone species, actively shaping their habitat and supporting fellow marine life. In optimal conditions, a single oyster can filter nearly 2 gallons of water per hour, removing bacteria, minerals and excess nutrients. Clustered together in reefs, oysters form complex structures that provide habitat for fish and other animals. Over 30 species classified as Florida's Species of Greatest Conservation Need are linked to the habitat or food sources provided by oysters. They play an outsized role not only in their ecosystem, but also in the culture and economy of the communities that rely on them. Their reefs also serve as natural barriers, anchoring the shoreline and protecting towns from powerful hurricane swells. In some of these towns, oyster harvesters have worked the reefs for generations; their jobs and income disappear along with the animal.
Restoring oyster populations can mean cleaner bays, thriving wildlife, sustainable fisheries and a future in which oysters can be both enjoyed and preserved. In some cases, it can also provide an opportunity to restore the stewardship of Indigenous communities whose ancestors once lived along the same coasts and estuaries now being studied by archaeologists.
"For many communities, their ancestors have histories of harvesting oysters for thousands of years," Rick said, whose recent research has primarily focused on Chesapeake Bay. "At these sites, there is an effort to ensure these communities are involved not only in the preservation of their history but also in the management of oysters going forward. These sites and the oysters within them can help empower tribal nations to be stakeholders in environmental management and conservation of those areas."
But recovering oyster populations is no small feat. The work demands collaboration among conservationists, local communities and scientists — guided, in part, by archaeological research.
Still, Rick is optimistic. "I see signs of success in something really simple: People are listening, and they care. If archaeology can inspire people to be better stewards and to see themselves as part of this much bigger system, then, in some ways, we've already succeeded."
This study was published in the Journal of Archaeological Science.
Cristina Oliveira, Alisa Luthra, Jennifer Green and Aditi Jayarajan of the Florida Museum of Natural History are also coauthors of this study.
