When night comes, baby Cardinal fish always know where home is — because they can feel the Earth’s magnetic field. New research has shown that the animals have an internal ‘compass’ that can help them orient even when there’s no sun or stars shining to guide them.

Image credits Serge Melki / Flickr.

Professor Mike Kingsford from the ARC Centre of Excellence for Coral Reef Studies at James Cook University wanted to know why it is that baby Cardinal fish can always find their home at night. So he teamed up with colleagues from Germany to study the fingernail-sized little critters.

“This study is the first clear demonstration that reef fish larvae possess magnetic senses to orient them at night,” says Professor Kingsford. “Up until now, we only knew adult birds, marine mammals, sharks and boney fish have this in-built sense of direction.”

The team collected Cardinal fish less than one centimeter in length from Great Barrier Reef’s One Tree Island. They tested how well the fishes were able to orient in total darkness in the same magnetic field as the reef’s. As Kingsford explained, the fish normally orient to the south east but when the team shifted the magnetic field 120 degrees clockwise, the fish changed the direction they swam in — they all turned west, confident they were still on track. This shows that the animals can feel magnetic fields and use them to orient themselves.

“We know from our previous research that once they start to get closer to their target, a ‘homing process’ begins, where the larvae rely on odor, sounds and landmarks to find and settle on a reef,” Kingsford added.

Reef fish, such as the Cardinals, hatch from eggs in the reef as larvae. They then spend a few days up to months in the open ocean while they grow and look for a different reef to settle or return home. But once they do reach a reef, they generally stay there for life.

“The study tells us these baby fish actually have brains. They know where they are going and are strong swimmers. As a result they have some control over the reef they end up on. It’s not just about being led by the currents.”

“Knowing this, we can develop more accurate models of where larvae go to determine the best way to protect and maintain sustainable fish stocks.”

The full paper “A magnetic compass that might help coral reef fish larvae return to their natal reef” has been published in the journal Current Biology.