Imagine, for a moment, that you are a small planktonic crustacean floating in the tropical ocean. Your world is vast, but its physical geography at your scale is relatively simple. Light and warmth are above, dark and cold are down. Most of the information you use—to decide when to feed, to avoid predators—comes either from those diffuse light cues, or the vibrations of the water immediately around you, felt through the tiny hairs, or setae, on your antennae and body. To your tiny body, the water feels more like syrup than flowing liquid, and your world has no boundaries or hard edges…for the most part, anyway.
Because there are hard boundaries around the fringes of land. And, depending on what kind of planktonic crustacean you are, those boundaries are either your only chance at survival or a near-certain death trap. If you are a larva of an animal that lives on a reef as an adult, you need to find a suitable reef to settle on, or you will die floating in the open ocean. But if you are a holoplanktonic animal, one who lives its entire life in the water column, the reef is an alien world, full of hard rocks covered with stinging polyps and surrounded by hungry fish and other predators. You want to stay as far away from them as possible.
A brand-new study in PLoS ONE tests a hypothesis about how small planktonic crustaceans either find or avoid the Great Barrier Reef in Australia. The researchers hypothesize that they do it by listening for the sounds of the reef. This idea makes a lot of sense. Sound is pretty much the only way to gather information from far away underwater, which is why dolphins and fishermen use it to find food, navies use it to find submarines, and whales use it to communicate across ocean basins. And, as anyone who has swum, snorkeled, or dived on a reef will tell you, they are noisy places. Snapping shrimp, scraping urchins, grunting fish, and breaking waves all combine to make a distinctive background snap, crackle, and pop. Some reef fishes have been shown to respond to reef noise, and there is evidence from temperate waters that crab larvae might as well.
The researchers tested their hypothesis by placing light traps in the Great Barrier Reef lagoon, the wide strip of water in between the reef and the mainland. Light traps are relatively simple devices, with a lighted chamber open to the water through a narrow slit. Animals are attracted to the light, swim through the slit, and can’t get back out. To one of these light traps they attached an underwater sound system, playing back a looped recording of reef noise. The traps were deployed at dusk and retrieved at dawn, and the procedure was repeated on 34 nights over three months.
The results, after counting 691,000 (!) crustaceans from the traps, did in fact support the hypothesis. Crustaceans that live in the open water or emerge from soft sediments at night to feed were found preferentially in the silent traps. Crab larvae at the zoea stage, obligated to find a reef to settle on to grow into adults, were found preferentially in the trap with the reef sound playback.
This study confirms what earlier ones had suggested, that crustaceans use acoustic cues to either locate their adult habitat, or to avoid the “wall of mouths” that is a coral reef to a small zooplankter. More generally, it suggests just what a broad cross section of marine life uses sound to orient and find its way in the marine world. Human activities appear to be changing the underwater soundscape in many places around the world, and may be having effects on dolphins and whales, which are very acoustically oriented. This research raises the prospect that changes in the sonic seascape may have implications for many other animals, as well. As always, more research is needed.
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
Montgomery JC, Jeffs A, Simpson SD, Meekan M, & Tindle C (2006). Sound as an orientation cue for the pelagic larvae of reef fishes and decapod crustaceans. Advances in marine biology, 51, 143-96 PMID: 16905427
Stanley, J., Radford, C., & Jeffs, A. (2009). Induction of settlement in crab megalopae by ambient underwater reef sound Behavioral Ecology, 21 (1), 113-120 DOI: 10.1093/beheco/arp159