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Scientists 'Shocked' At Shrinking Size Of Colossal Oysters In Florida

The top shell is the maximum size colossal oysters reach in Crystal River today. The bottom shell is the max size from about 1,000 years ago.
Jessica Meszaros
The Florida Channel
Leon County Judge John Cooper on June 30, 2022, in a screen grab from The Florida Channel.

Colossal oysters, which once thrived in Florida's northern Gulf Coast, are shrinking in size and in numbers. Scientists at the University of South Florida published a study in the journal Biology Letters on Feb. 5 that examines this loss. 

For University Beat, WUSF's Jessica Meszaros spoke with the head researcher, Stephen Hesterberg.

Meszaros: Can you tell us about your latest study and what it revealed?

Hesterberg: Sure, we had kind of two major findings. We collected thousands of prehistoric and modern oysters from the Crystal River area, and the first thing we found was we just see a dramatic reduction in body size of these oysters-- both decreases in maximum size and average size.

Click here to view the published article.

The second thing that we found is we actually investigated the chemistry of these large shells, and we were actually able to show that the size reduction is due to slower adult growth, as well as early mortality of these large individuals. So that kind of gives us some clues as to what's kind of stressing the population today.

Meszaros: What would they need to thrive that might be missing?

Hesterberg: Well, freshwater is incredibly important for oysters. Probably most people think of oysters as a marine organisms, but they're actually estuarine. So that means they're adapted to freshwater, really big changes in freshwater, and if there isn't enough fresh water entering into the estuaries, things like parasites and predators are increasing abundance and often disproportionately affect and kill those large oysters.

From left to right: maximum size of colossal oysters 1,000 years ago in Crystal River, average size from 1,000 years ago, maximum size oysters found today in the river, and average size today.
Credit Jessica Meszaros / WUSF Public Media
The Florida Channel
From left to right: maximum size of colossal oysters 1,000 years ago in Crystal River, average size from 1,000 years ago, maximum size oysters found today in the river, and average size today.

Meszaros: What does the decline of colossal oysters and Crystal River tell us about climate change and the state of Florida's water bodies?

Hesterberg: Well, I think it directly ties into that latter part, which is changes in our watersheds. We can't pinpoint an exact cause for this decline yet, but it would be consistent with reductions and freshwater input that we've been seeing in our estuaries more broadly in Florida.

Meszaros: Is there any chance of getting these oysters- number one- back in numbers, but also, can we get back to this larger size?

Hesterberg: Well, there's been a lot of effort in oyster restoration- so returning those numbers- but what we haven't really focused on is returning that quality of a habitat, which would be described by the size. Now, in terms of that, it might be very challenging.

To return to a time where we have oysters as large as you know, five to seven inches, and that could be a variety of reasons why, but one of them is warming climate. As temperature goes up, oysters actually need more oxygen. Consequently, when temperature goes up oxygen levels decline in our waters, so it's kind of a double whammy for oysters that might limit their size recovery.

Meszaros: Why did USF get interested in studying this issue?

Hesterberg: I was taking a class with a co-author, Dr. Greg Herbert in geosciences, and through that class, he introduced me to other archaeology grad students. We compared information on oyster size, and we were really shocked at the size of oysters that they were pulling out from these archaeological deposits in Crystal River.

If it wasn't for that class, I don't think we would have done this project. It's truly an example of interdisciplinary research that I think is really important on moving forward.

Copyright 2020 WUSF Public Media - WUSF 89.7

Jessica Meszaros is a reporter and host of All Things Consideredfor WGCU News.
Jessica Meszaros
Jessica Meszaros is a reporter and host of Morning Edition at WUSF Public Media.