Choral Reefs

Coral Reef Ecologist Mark Vermeij of the CARMABI Foundation
Coral larvae, which are the size of fleas and hairy with neuromasts, spend their early days, sometimes months, wafting with plankton in the open ocean.

To find established reefs on which to settle, they use light, touch, and scent. But these cues only work if they find themselves both down-current and nearly home. Now scientists out of Carmabi Foundation, Willemstad, Curaçao are finding that settlement-stage larvae are attracted to reefs from meters, possibly even kilometers away - by sound.

The scientists, whose previous work found that coral reef noise provides auditory orientation cues to young reef fish and crustaceans, write here that sound in the sea propagates such tremendous distances that the role of coral reefs "as a beacon for pelagic life stages of marine invertebrates deserves critical attention".

In their elegant study, when biologists moved submerged plexi-glass-boxed speakers playing day and night reef sounds "which consisted of fish calls and grunts and the continuous crackling sound of snapping shrimps" around submerged, coral larvae-filled chambers, the larvae consistently moved toward the reef noise; in one trial, by positioning the speakers above the larvae chamber, the test over rode coral larvae's tendency to swim down to a reef.

"This is the first description of an auditory response in the invertebrate phylum Cnidaria, which includes jellyfish, anemones, and hydroids," write the investigators, and they suggest that the findings present profound implications for connectivity models that can no longer consider larval dispersal as passive. Further, because in open ocean, habitat sought by settlement-stage fish, crustaceans, and coral is so far-flung and "patchy", alleviating anthropogenic marine noise pollution takes on greater urgency.