Shadowing behaviour displayed by blue runner fish hiding below a sandbar shark before attacking prey. Lampione island, central Mediterranean Sea, 11 July 2022. Credit: Rocco Canella.

Scientists working in the Mediterranean have revealed for the first time that some fish sneak up on their prey by hiding behind sharks.

The previously unknown behaviour was discovered by a team of researchers, including scientists from the University of Edinburgh, studying sandbar sharks in the Mediterranean Sea.

Underwater video footage captured by divers and remotely operated vehicles shows a type of small predatory fish – called blue runners – using the sharks for cover while they hunt.

Previous research has shown that some fish shadow sharks to remove parasites and hide from predators. However, they have never before been seen hiding behind sharks to ambush their prey.

Videos captured off the coast of Italy’s Lampione Island revealed 34 examples of blue runners using this hunting strategy.

On each occasion, a single fish shadowed a shark for around 30 seconds before breaking off to launch high-speed attacks on smaller types of fish, such as damselfish.

Analysis of the footage suggests that the fish – which usually hunt in small groups – are more likely to catch their prey off guard by shadowing sharks, increasing their chances of success, the team says.

Protection

When hiding behind a shark, prey only noticed a blue runner’s approach around 10% of the time. In contrast, when the fish hunted in packs their prey spotted them almost every time – more than 95% of attacks – and got into a defensive schooling formation.

As well as improving their chances of a successful hunt, the shadowing behaviour may protect blue runners from their own predators and help them conserve energy by swimming in sharks’ slipstream, the team says.

The research is based on multi-year observations around Lampione Island, where a rare clustering – known as an aggregation – of sandbar sharks occurs every summer.

Professor Fabio Badalamenti, of the University of Edinburgh and CNR-IAS Palermo, said: “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.”

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

“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 jeopardise associations like these, which could have knock-on effects on other species.”

The study is published in Ecology.