This is the story of a real-life horror creature from beneath the sea and the brave little fish that have learned to fight back.

The protagonists of this tale are called monocle breams, colourful tropical fish that live in and around coral reefs in shallow Indo-Pacific waters. Only around ten to 20 centimetres (4-8 inches) long, the breams find protection among the surrounding coral or by gathering in small groups. 
Unfortunately for the fish, their home is also inhabited by a terrifying predator who cares little for such protective tactics: the bobbit worm. (If Pixar ever decided to make a horror version of Finding Dory, this would be their monster: the largest one ever measured, found in 2009, was three metres long!)

The bobbit worm (Eunice aphroditois). Image: Shutterstock

Most of the time, the bobbit worm lurks in a burrow, with its long, segmented body hidden beneath the sandy ocean floor and only its camouflaged head protruding slightly. The predator's colourful antennae are very visible to fish swimming by – but its lightning-fast bear-trap jaws are not.

The strategy is all about patience: the worm waits for prey to investigate its waving antennae, which look like tasty smaller worms. When a fish gets too close, the worm's head comes shooting forward, its jaws snap shut, and in an instant, the hapless victim disappears beneath the sand. Sometimes the worm strikes with so much power and speed that its prey is cut in half:

The match-up between worm of death and tiny fish may sound one-sided, but new research has found that the breams can fight back with a winning combination of smarts and teamwork.

Biologists from the University of Basel in Switzerland observed this incredible defensive strategy in Indonesia's Lembeh Strait. When a bream spots a bobbit worm – or when it sees a fellow member of its species getting captured – it initiates a watery blitzkrieg, other fish quickly joining in. Since the enemy is mostly concealed underground, the fish assault it by firing sharp jets of water from their mouths, disturbing the worm and the sediment around its burrow. This harassing behaviour is called "mobbing".

The fish probably aren't hurting their target, but they're certainly hurting its chances of more food. Once the worm's been spotted, not only have the fish discovered its hiding spot, but they're also broadcasting its location to other fish in the area by making such a big fuss. Eventually, the predator retreats, its hopes of further ambush dashed.

The researchers have observed that at least two species of monocle bream engage in mobbing behaviour, and it's a more complex strategy than most people might expect from regular old fish. "Concerning their mental capacity, fish are for the most part greatly underestimated," said Basel University biologist Daniel Haag-Wackernagel. "Research into their behaviour in their natural habitats continues to reveal big surprises."


Top header image: Shutterstock