Sneaky fish use sharks like moving shields while hunting

Some fish are not just fast – they’re smart. A team of scientists, including researchers from the University of Edinburgh, has revealed a never-seen-before hunting trick in the Mediterranean Sea. Small fish called blue runners hide behind sandbar sharks to sneak up on prey.

This behavior stands out. Fish often swim near sharks to avoid threats or shed parasites. But hiding behind a shark to launch a hunt? That’s a first.

Fish use sharks as cover

Until now, scientists had noticed that fish sometimes trail sharks. This was usually for protection or parasite removal. Some even used sharks as moving shields to escape larger predators. But the new footage captured around Lampione Island in Italy shows something more calculated.

Over several years, divers and remotely operated vehicles recorded 34 separate cases of blue runners sneaking behind sharks. Each time, a lone blue runner shadowed a sandbar shark for about 30 seconds. Then, without warning, it broke away to dart at smaller fish, especially damselfish.

What changed wasn’t just the fish’s timing – but their approach. Blue runners typically hunt in groups. But these solo hunts seemed far more effective when combined with the shark’s distraction.

Shark cover boosts hunting success

The team analyzed hours of underwater footage to understand the success of this hunting trick. The findings were clear. When blue runners used sharks as cover, prey detected them only 10 percent of the time.

In contrast, when they hunted as a group – without shark cover – their targets noticed them over 95 percent of the time.

The difference changed everything. Instead of fleeing or forming tight defensive schools, the damselfish and other prey stayed relaxed. The shark’s presence seemed to lull them into a false sense of security, never realizing another predator swam in its shadow.

The blue runners also appeared to gain other benefits. Scientists believe that swimming behind the shark may help conserve energy, thanks to reduced water resistance. It may also offer safety, as predators are less likely to attack a fish moving close to a large shark.

A rare gathering of sharks

This remarkable behavior occurs in one of the few places in the Mediterranean where sandbar sharks still gather in groups.

Every summer, they return to the waters around Lampione Island, creating what scientists call an “aggregation.” This kind of natural event is rare in these seas, especially as shark populations decline due to overfishing and habitat stress.

Because of the consistency and uniqueness of this site, the researchers had the chance to observe complex behaviors that would likely go unnoticed elsewhere.

Shark loss could affect hunting behavior

“The study highlights how interactions between species can drive alternative hunting tactics in marine ecosystems. Understanding these dynamics enriches our knowledge of marine biodiversity and underscores the importance of conserving high-level predators, like sharks,” said Professor Fabio Badalamenti, of the University of Edinburgh and CNR-IAS Palermo.

“This unique interaction emphasizes the ecological importance of the few remaining shark aggregations, which can influence the structure and function of ecosystems,“ noted Dr. Carlo Cattano, of the Stazione Zoologica Anton Dohrn.

“Lampione Island is one of only two known aggregation areas for the sandbar shark in the Mediterranean Sea. The ongoing decline in large shark populations due to overfishing could jeopardize associations like these, which could have knock-on effects on other species.”

Hidden dynamics of fragile ecosystems

This behavior isn’t just a cool trick of nature – it carries serious implications. The blue runners’ strategy depends on the shark’s presence. If sharks vanish, so might these unique hunting methods. As top predators decline, ripple effects may shift entire ecosystems in ways we don’t yet understand.

Understanding these hidden dynamics reveals how fragile marine systems can be. Every player in the food web, no matter how strong or small, shapes the behavior of others.

Preserving biodiversity, then, isn’t just about saving one species – it’s about keeping the game of life in motion, with all its surprises intact.

The study’s success resulted from collaboration across institutions. Scientists from the University of Edinburgh, Stazione Zoologica Anton Dohrn, the National Biodiversity Future Center, CNR-IAS, and the University of Palermo all played roles.​

Support from the Blue Marine Foundation and the National Geographic Society enabled long-term observation, which proved key in documenting this behavior over multiple seasons.

The research is published in the journal Ecology.

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