Baby corals find their way to reefs by detecting the sound of snapping shrimps and grunting fish, scientists revealed today.
It had long been assumed that coral larvae drift aimlessly after being released by their parent colonies and almost by chance land back on reefs. But scientists now believe that though they are anatomically very simple, the larvae can pick up the sound of a reef and head towards it.
The discovery is worrying as it is feared the larvae might also be drawn to dangerous man-made sounds in increasingly noisy oceans or struggle to find reefs because human noise masks their sound.
One of the experts involved in the study, Steve Simpson, senior researcher in the University of Bristol's School of Biological Sciences, said: "Until recently we assumed that these larvae drift aimlessly, but this study shows they are able to detect sound and move towards it. This could help them find bustling reef habitats but may also draw them towards human sources of noise such as turbines and drilling operations."
Simpson said a reef was a noisy place but usually when humans dive they hear little but their own breathing. But actually a reef is full of "clicks from snapping shrimps that combine to produce a crackling noise and grunts and chirps produced by fish as they communicate."
Simpson explained how larvae are separated from reefs in the first place: "Coral eggs and larvae are released by their parent colonies (often in dramatic mass spawning events - analogous to a fireworks night on the reef) and then drift out to sea where they spend a few hours to days growing and developing in the plankton.
"When they are competent to settle onto the seabed they seek out suitable solid substrate (eg a bare patch of rock), and then cement themselves and gradually start to lay down their skeleton. Over time they bud - split - asexually, and eventually build a new colony, sometimes metres across."
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Simpson discovered several years ago that baby reef fish use sound as a cue to find coral reefs, but was amazed when his Dutch collaborators working in the Caribbean started finding that coral larvae, which must quickly find a safe place to land or die, can do the same thing.
The team designed a "choice chamber", offering small invertebrates two or more contrasting conditions and allows them to move freely towards the one they prefer.
They played recordings of a coral reef in one area and the results clearly showed that the flea-sized larvae were strongly attracted to the noise.
How the creatures, which look like tiny eggs covered in hairs, detect sound is unknown. Simpson said: "At close range sound stirs up water molecules, and this could waggle tiny hair cells on the surface of the larvae, providing vital directional information for baby corals."
Simpson said the increase of manmade noise in oceans and seas was a worry.
"Anthropogenic noise has increased dramatically in recent years, with small boats, shipping, drilling, pile driving and seismic testing now sometimes drowning out the natural sounds of fish and snapping shrimps," he said.