A new study found that reef noise may act as a guide for zooplankton. Planktonic crustaceans are known to use underwater light as a cue for migration. Other factors that determine orientation underwater are chemical and mechanical responses, and in part, food availability. However, of the myriad of factors that zooplankton respond to, ‘sound’ is something that has been kept fairly unnoticed. Coral reef habitats provide a hard surface for zooplankton in an environment that is, for the most part, without such boundaries, and depending on the kind of planktonic crustacean, those boundaries are either a chance at survival or a dangerous death trap. To a larva that grows into an adult that lives on a reef, finding a suitable reef to settle on is crucial. But to a holoplanktonic animal that prefers the open water, the reef presents a world of danger with stinging coral polyps and resident planktivorous fishes.
The idea that zooplankton either find or avoid reefs by listening for the sounds of the reef makes a lot of sense. Sound travels far underwater, which is why dolphins use to find food and whales use it to communicate across ocean basins (up to 1000 km!). Anyone who has snorkeled or dived on a reef will notice that they are noisy places; a combined contribution by the likes of snapping shrimps, scraping urchins, grunting fishes and breaking waves.
In the study, the researchers placed light traps in the Great Barrier Reef, one with an underwater speaker attached, playing back a looped recording of reef noise, and one without the noise. The traps were deployed at dusk and retrieved at dawn for 34 nights. The researchers found that zooplankton that preferred the open water were more abundant in the silent traps. Crab zoea, which require a reef to settle on to grow into adults, were found preferentially in the trap with reef sound playback.
Now that we know acoustic cues are important for zooplankton to either go to or move away from coral reefs, it opens up the possibility of a wider scope of marine organisms that uses sound to orient and navigate themselves in the ocean. This means a new category of anthropogenic influence is raised as human activities appear to be changing the underwater soundscape in many parts of the world.
Reference:
Stephen D. Simpson, Andrew N. Radford, Edward J. Tickle, Mark G. Meekan, Andrew G. Jeffs (2011). Adaptive Avoidance of Reef Noise PLoS ONE. doi:10.1371/journal.pone.0016625
