Pre-settlement coral-reef fish larvae respond to magnetic field changes during the day.

Observations of coral-reef fish larvae have revealed remarkably consistent orientation behaviour while swimming offshore, requiring large-scale orientation cues. However, the mechanisms underlying this behaviour are still being investigated. One potential large-scale cue for orientation is the Earth's geomagnetic field. Here, we examined the effect of magnetic field manipulations on the orientation behaviour of coral-reef fish during the pelagic larval phase. In the absence of visual cues, individual larvae responded to a 90 deg shift of the horizontal component of the magnetic field within a Helmholtz coil with a comparable shift in orientation, demonstrating that they use a magnetic compass for orientation. Our findings suggest that geomagnetic field information guides swimming behaviour of larval fish in the pre-settlement phase. The ability to use large-scale sensory cues allows location-independent orientation of swimming, a behaviour that influences dispersal and connectivity of fish populations, which has important ecological implications for anthropogenic development of marine areas.