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For the paddle crabs crawling around the tank in biologist Ashley Flood’s lab, feeding times were noisy. As the crustaceans scampered eagerly around a small pouch of squid, designed to let them smell but never access what was inside, they would erupt in a chorus of rasps seeming to emanate from deep inside their stomachs. Flood and her colleagues at the University of Auckland in New Zealand recorded the cacophony with a hydrophone. Later, when they played the tapes to a different group of paddle crabs without introducing any food, something remarkable happened—the crabs reacted in almost exactly the same way, excitedly foraging around their tank.
This rasping noise is one of three distinct sounds recently discovered by Flood and her team. The findings reveal the depth and variety of communication by an invertebrate that, until recently, has been considered mostly silent.
Scientists have known since the 1960s that some crabs communicate using sound, primarily by rubbing their claws against their legs—a technique known as stridulation. However, communication has only been proven in a handful of semiterrestrial species. Flood’s study is the first to demonstrate that crustaceans living beneath the waves can be just as loquacious. “These sounds may play crucial roles in many life events, such as finding certain habitats, breeding success, and finding food sources,” Flood says.
Paddle crabs were perfect candidates for investigating underwater communication because they spend their breeding season in the opaque waters of New Zealand’s harbors and estuaries where acoustic communication would be more useful than visual cues.
To test how well the crabs produce sound, and to analyze associated behaviors, Flood’s team placed paddle crabs in a water tank with a video camera and a hydrophone.
The team discovered that of the three distinct sounds produced by the paddle crabs, two—the zip and the bass—are only made by adult males near a sexually receptive female and in the presence of other males. The researchers believe these sounds are used to ward off sexual competitors. The accompanying behavior is another clue: the crab alternates between rubbing a walking leg against its claw and swaying its body. The overall display, says Flood, resembles a war dance.
The rasp sound, meanwhile, was produced intermittently by all crabs in each of Flood’s experiments, but increased dramatically in rate during feeding. The biologist assumes that the crabs produce this noise internally, likely by mashing their gastric teeth. This opens up the possibility that different species of crustaceans that we assume are silent might also be capable of sound production.
The rasp’s pitch is also correlated with the size of the individual crab, something that Sophie Mowles, a behavioral ecologist from ARU in England who wasn’t involved in the research, finds particularly fascinating. As the crab’s size increases, the sound frequency decreases, resulting in a lower pitch. “That’s very similar to what you would find in other animals,” she explains.
Mowles is also intrigued by the crab’s listening habits. The feverish reaction of paddle crabs to the rasp noise during Flood’s playback experiments suggests the species is prone to eavesdropping on other crabs, listening for clues that individuals nearby have found a new food source. “That might be detrimental to one that’s found food, but sometimes feeding in numbers is safer,” says Mowles.
The study is also timely, says Mowles, given that we are only just coming to grips with the impact of anthropogenic noise at sea. Understanding that human-induced sounds may disrupt animals’ subtle communications might make us think twice about how our species asserts itself in the oceans.