How do fish larvae that have drifted for kilometres manage to return to the reef where they were born? An international team of researchers has investigated the migratory behaviour of coral reef fish. And discovered that the animals use a solar compass to navigate in the open ocean.
Every year in the Australian summer, from December to February, a mass migration of fish larvae takes place in the Great Barrier Reef. Just as salmon migrate to the mouth of their native river to spawn, coral reef fish return to their home reef. Prof. Dr Gabriele Gerlach has been studying this migration behaviour for around ten years. Together with the biologist and neurosensory expert Prof Dr Henrik Mouritsen (both from the University of Oldenburg), the university lecturer in animal biodiversity and evolutionary biology has now been able to demonstrate this behaviour: The larvae use a solar compass to navigate in the open ocean.
Under the title "Sun Compass Orientation Helps Coral Reef Fish Larvae Return to their Natal Reef", Gerlach and Mouritsen, together with Michael Kingsford (James Cook University in Townsville, Australia) and Jelle Atema (Bost on University, USA), have now published the results of their research in the internationally renowned online journal of the Public Library of Science Plos One.
Genetic markers in use
Only a few weeks after the larvae hatch in the reef, they are flushed into the open water at night. For a long time, scientists assumed that it was pure chance where the currents and storms caused the larvae, which are only a few millimetres in size, to drift. In order to track down their migration behaviour, Gerlach examined the larvae using genetic markers. "Just as DNA traces are used for forensic purposes, DNA analysis can be used to determine the larvae's reefs of origin," explains the scientist.
In this way, she was able to show that A large proportion of the larvae drifted more than 15 kilometres. However, 60 per cent also settle back on their home reefs. According to the biologist, this return is particularly important for the survival of very isolated reefs. Without a sense of direction, the larvae would perish in the ocean. So how do the tiny marine creatures find their way back to their home?
Larvae sniff out their reef - up to a distance of two kilometres
Together with Kingsford and Atema, Gerlach discovered that the larvae can orientate themselves by smell. They can sniff out their reef up to a distance of two kilometres - a remarkable ability, but it does not help in the vastness of the ocean. Current models show: Most larvae drift more than ten kilometres to the north-west in the first week. So the question remains: How do the larvae find their way to the natal reef?
Mouritsen, an expert in bird navigation, ventured a hypothesis. It is possible, according to Mouritsen, that the coral reef fish - like migratory birds - use a solar compass. The scientists had observed that the larvae always swam in a south-south-easterly direction on their way back to their natal reef - provided the sky was clear and the sun was visible. "This direction would be optimal for swimming against the prevailing north-north-west current and reaching the home reef," says Mouritsen.
Internal clock enables larvae to navigate
The position of the sun changes with its daily path across the sky. In order to be able to use the sun as a compass, the fish larvae must therefore have an internal clock. To test this, Mouritsen changed the alternation of light and darkness for some animals in a laboratory experiment. He advanced their day-night rhythm by six hours. The result: they swam in the opposite direction - clear evidence of the internal clock that enables the larvae to navigate.
The scientists were thus able to show that The coral reef larvae really do have a time-compensated solar compass orientation, and they use this to compensate for the unavoidable drift to the north-north-west - and to find their natal reef again. Can this solution to a previously puzzling migration be applied to other animals? Gerlach and Mouritsen assume that the abilities they have identified are not only found in coral fish, but in many marine organisms.
Gabriele Gerlach, Henrik Mouritsen, Jelle Atema, Michael Kingsford: "Sun Compass Orientation Helps Coral Reef Fish Larvae Return to their Natal Reef"