Only a handful of sea creatures are known to be able to hear. The list includes marine mammals, sea turtles, fish, and some shrimp. Now a new study blows open the club doors, adding a diverse assortment of crustaceans to the ranks of the auditorily gifted. Not only can these tiny invertebrates hear, they apparently rely on sound to find a comfortable, watery home.
Coral reefs are noisy places, with shrimp snap-crackle-popping away in reaction to predators and prey and fish chiming in with communicative grunts and chirps. But until recently, researchers hadn’t bothered to check whether invertebrates or the larvae of various organisms respond to these sounds, largely because the animals have no known mechanism for detecting them.
Marine biologist Stephen Simpson of the University of Bristol in the United Kingdom has started chipping away at the question. He and colleagues began by showing that larval reef fish just a few days or weeks old use sound to find their way from the open water to coral reefs, where they spend their adult lives. Then last May, he co-authored a paper showing that coral larvae do much the same. Meanwhile, similar evidence was accumulating for larval crabs and rock lobsters, prompting Simpson’s team to look back at a diverse by-catch of crustaceans they’d preserved from an early fish study on Australia’s Great Barrier Reef.
To test whether larval fish were attracted to reef noise, they set two light traps on 34 nights in waters some distance away from the reef. To one they attached an underwater sound system that broadcast recorded reef noise, to the other a silent dummy sound system. They caught and preserved all the crustaceans that wound up in the traps. For the present study, published online last month in PLoS ONE, the team hired a graduate student to identify and tally all 700,000 tiny pickled crustaceans under the microscope. “He ... got very good at it and almost addicted to it. Perhaps it was the fumes of the alcohol,” Simpson jokes.
The team found that seven of the eight most common taxonomic groups—including larval crabs, shrimp, and crustaceans called copepods, ostracods, hyperiids, mysid shrimp, and gammarids—showed small but significant differences in the number of individuals caught in the two traps, indicating that the crustaceans can detect and respond to sound.
Most preferred to avoid it: the silent trap attracted more crustaceans than the noisy one, providing the first evidence that some organisms avoid reef sounds, probably to steer clear of predators or habitat unsuited to their particular lifestyle. Only the larval crabs, which settle on reefs, showed up more often in the noisy trap. The six types of organisms caught more often in the silent trap live not on reefs but mainly in the open water or in sea-floor sediment.
Simpson and colleagues say their findings suggest that increasing ocean noise from ships and other human activities may cause problems not only for marine mammals but also for innumerable fish and invertebrates seeking food and shelter.
The ocean is a "profoundly acoustic place," says biologist Sheila Patek of the University of Massachusetts, Amherst. All the same, she says, many crustaceans make sounds, so it would be surprising if they couldn’t also detect them. The study's importance, Patek says, lies in helping to overturn a pervasive misconception that the ocean is essentially silent.