The mystery behind nail-sized Cardinal fish larvae swimming home miles away to native reefs has been solved. According to a new study, they are guided by an internal magnetic compass in reaching their destination unhindered even at night.

Cardinal fish is predominantly found in marine habitats like coral reefs.

Hatched into larval form, the reef fish disperse and spend months in the ocean. Then comes the "homecoming" to the reef where they settle for a lifetime.

According to researchers, when hatched, currents push larvae to the north-northwest. However, they swim toward the south-southeast direction with ease. A study was conducted on the larvae of Ostorhinchus doederleini — a tiny reef fish.

Suspecting the play of magnetic forces, the researchers tested the fish larvae with a device that can adjust the magnetic field.

The results of the study have been published in Current Biology. It reveals the play of an internal magnetic compass supporting the voyage of larvae at night.

"This study is the first clear demonstration that reef fish larvae possess magnetic senses to orient them at night," said study co-author Michael Kingsford from the James Cook University in Queensland, Australia.

Calling the study the first of its kind in asserting that larvae carry magnetic senses, Kingsford said such behavior was known among marine mammals and adult birds.

"Up until now, we only knew adult birds, marine mammals, sharks and bony fish have this in-built sense of direction," he said.

Reef Fish Research

For the test, researchers collected tiny Cardinal fish from One Tree Island at the Great Barrier Reef and tested their direction in total darkness using a magnetic field.

The researchers noticed that fish moved to the southeast. However, when the magnetic field was altered clockwise by 120 degrees, they all turned further west, thinking they were still on track to their destination, Kingsford said.

According to Kingsford, the results make it clear that larvae are guided by magnetic senses in pointing the right direction during night time.

This was reinforced by the comments of Gabriele Gerlach, lead author and marine biology professor at the University of Oldenburg in Germany. She noted that many other marine fish and invertebrates also display similar behavior. They hatch and are washed out into the ocean for days, weeks, even months.

"And they might have this same magnetic orientation behavior," Gerlach added.

Threat To Coral Fish

Meanwhile, conservation of coral fish has assumed urgency. As valuable natural assets, coral reefs provide an average $375 billion worth of goods and services a year by supporting fisheries and protecting the coasts.

Coral reef conservation is also linked with protecting the plant-eating herbivorous fish. According to a study, heavy fishing has reduced herbivorous fish populations on coral reefs.

Coral fish are crucial for the survival of coral reefs as regulators of competition between corals and seaweeds for space. By eating seaweeds and algal turfs, herbivorous fish prevent the dominance of these organisms into smothering the reefs.

Other threats faced by reefs are habitat damage and climate change including ocean warming.