The Scary Story Fossil Shells Told Us About The Health of the Adriatic Sea

The Changing Balance of Predator-Prey Interactions in the Adriatic Sea

Back in high school, I spent a lot of time with my local Scout troop. To say that I loved it was an understatement. There, I went from a shy and introspective guy to a funny and cheerful teenager.

One of my favorite trips was to an area with multiple beaches, where we spent countless hours playing, cleaning, and making cool crafts out of shells and driftwood.

One of my favorite memories from that trip was the troop girls collecting shells to make necklaces for everyone. But they didn’t collect just anything they saw. Instead, they took shells with perfectly circular holes.

Later, while studying Invertebrates for my Bachelor’s degree, I learned that those holes were so common because they were made naturally. Two groups of sea snails make those predate on other shelled organisms (mollusks). Cool, right?

Well, imagine my delight when I started my PhD and one of my labmates was studying exactly this: the holes made by snail predators. Studying those holes can infer a lot about the health of an ecosystem, and my friend greatly took the opportunity to study them.

But how? Well, a recent study offers some perspectives on this methodology. But first, watch this video to learn more about these holes in the fossil record.

For millennia, the Adriatic Sea supported a thriving community of marine life, with predators like snails hunting mollusks. However, recent research has uncovered a reversal of fortunes, where this previously balanced ecosystem is falling apart at an alarming rate, largely due to… you guessed it: human impact.

A study led by paleontologist Dr. Martin Zuschin and colleagues documents a sharp decline in predator-prey interactions in the northern Adriatic Sea. The research team’s findings build on decades of studying fossils and current marine communities to understand how life under the sea has changed. As Dr. Zuschin puts it, “species composition in these environments is much simpler than it used to be.”

This simplification of marine ecosystems signals deeper concerns about the health of this environment, which is being reshaped by human activity.

But how did the authors reach this conclusion? Let’s look at the methods, shall we?

To figure out how predator-prey dynamics have shifted, the researchers took a deep dive into the seafloor’s past by analyzing fossilized shells. Specifically, they focused on “drill holes” left behind by predatory snails. The ones I was telling you about. These holes provide a clear record of the interactions between predators and their prey — almost like nature’s own documentation of who ate whom.

The team collected sediment cores from two regions, the Po and Isonzo prodeltas, to capture thousands of years of history in layers of sediment. By examining the frequency of these drill holes and comparing it with present-day communities, they could track changes in predation patterns over time.

This field of research, called conservation paleobiology, gives researchers a long-term view of ecosystems, helping them understand how human actions have influenced natural processes over centuries. Unlike studies that only look at individual species or population numbers, the team here could assess how relationships between species, like those between predators and prey, have evolved — or, in this case, broken down.

And what did they find?

The research paints a troubling picture. Predator-prey interactions in the Adriatic Sea remained relatively stable for centuries. However, things took a sharp turn around the mid-20th century.

Both predator and prey populations dropped dramatically, with some species disappearing entirely. The common corbulid clam (Varicorbula gibba) began to dominate, replacing the once-vibrant array of marine species with a much simpler ecosystem.

As Dr. Zuschin explained, “This species became more abundant and grows much larger than it did previously because there are fewer predators and less competition from other species.” Essentially, V. gibba thrives in environments that are hostile to most other organisms, making it a marker of an unhealthy, struggling ecosystem.

The decline in drill hole frequency, a proxy for predator activity, mirrors this shift. While the predators in the Adriatic once kept mollusk populations in check, fewer predators today mean less predation overall.

This reduction in predator activity is linked to several human-driven changes, including nutrient runoff from agriculture, bottom trawling, and the warming waters caused by climate change. Together, these factors have led to a simplification of marine communities, making them more vulnerable to further environmental stress.

But why do Predator-prey relationships matter?

Predator-prey interactions are excellent indicators of ecosystem health because they show the strength and complexity of food webs. When predators are active and prey species are diverse, it suggests that the ecosystem is balanced and capable of supporting a wide range of organisms.

On the other hand, when these interactions break down, it’s often a sign that something is wrong. In the case of the Adriatic, the decline in these interactions highlights how human activities have reshaped the natural environment, leading to a loss of biodiversity.

The researchers found that predators like snails were once much more common and engaged in regular predatory behavior. But as nutrient levels rose and oxygen levels dropped — a phenomenon known as eutrophication — predators found it harder to survive.

The overgrowth of algae, which dies and sinks to the seafloor, further reduces oxygen levels as bacteria break it down, essentially suffocating marine life. This situation left room for resilient species like V. gibba to thrive while driving out other organisms that couldn’t cope with the changing conditions.

So, what’s in the future?

While this study’s findings are worrisome, they also offer a clear direction for future action. Scientists’ ability to observe such detailed changes in predator-prey dynamics gives conservationists important tools for restoring marine ecosystems. If we understand what conditions supported a more balanced ecosystem in the past, what we call the basal conditions, we might be able to create policies that restore those conditions. Or we hope so.

Efforts are already underway to reduce nutrient runoff into the Adriatic, and researchers have even noticed a small uptick in predator activity in certain areas, suggesting that the situation might not be entirely hopeless. As the authors point out, “Environmental degradation is extremely expensive. You cannot even quantify it.” Restoration will take time and effort, but it’s clear that without intervention, the health of the Adriatic Sea — and other marine ecosystems like it — will continue to decline.

Let’s hope the past keeps helping us on this scary journey.