When threatened, Ocypode quadrata—a sand-colored Atlantic crustacean dubbed the ghost crab in a nod to its camouflage skills—emits a series of guttural growls. But as researchers from the University of California report in the journal Proceedings of the Royal Society B, the source of this sound varies depending on the distance between a crab and its competitors: If a potential predator is far away, O. quadrata uses its claws to create noise, but if danger approaches, it switches gears, preparing its pincers for battle and relying on teeth located in the stomach to produce warning calls.
According to the Guardian’s Ian Sample, the scientists’ findings represent the first recorded example of an animal using its stomach sounds to communicate. Lead author Jennifer Taylor, a biologist at UC San Diego’s Scripps Institution of Oceanography, tells Sample that the adaptation offers the crab a second layer of protection during combat. “It’s definitely an advantage if a predator is up close,” she says. “They can pull out their claws and be ready to lunge, but still produce these sounds.”
Ghost crabs owe their rasping stomach growls to a mechanism known as the gastric mill. According to Newsweek’s Hannah Osborne, this feature enables crustaceans to process food without needing teeth in their mouth. As Taylor explains to Osborne, O. quadrata’s three main stomach teeth rub against each other to grind up food in the animal’s foregut.
Taylor and her co-authors—Maya deVries, a former postdoctoral researcher at Scripps who will be teaching at San Jose State University starting in 2020, and Damian Elias, an animal communication and behavior expert at UC Berkeley—decided to investigate ghost crabs’ stridulations, or rasping sounds produced by rubbing body parts together, after realizing the crustacean could emit aggressive noises without moving any external features.
“The first time I heard the rasp, I couldn't believe how clear the sound of stridulation was, even though the crab's claws were outstretched and clearly not producing sound,” deVries says in a press release.
To identify the source of these sounds, the team taunted test subjects with rods, dead crabs and robotic insects. Then, David Shultz writes for Science magazine, the researchers monitored the angry crabs with a combination of X-ray imaging and a vibration-sensing laser. Ultimately, the analysis pinpointed the growls to the animal’s gut: specifically, an exchange between the gastric mill’s comblike teeth and another structure called the medial tooth.
According to Shultz, ghost crab stridulations serve as both a warning and a sign of their maker’s battle prowess. Based on the scientists’ assessment, Newsweek’s Osborne adds, male and female crabs are equally likely to adopt the communication method during aggressive encounters. When threats are situated farther away, the crustaceans use their claws to send a menacing message, but if a potential threat “gets too close,” Taylor says, “then using the stomach apparatus could be advantageous because it allows the crabs to use their claws for fighting while continuing to communicate with sound.”
Moving forward, the researchers hope to determine if other animals produce similarly stomach-based stridulations. As Taylor tells New Scientist’s Michael LePage, it’s possible species equipped with comparable food processing mechanisms (including worms, mollusks and birds) use these features to communicate, too.
Speaking with Osborne, Taylor concludes, “There are many more questions to answer now, including, for example, how these sound signals are acted upon by other crabs and potential predators, how crabs use these sounds in their natural environment, as opposed to a laboratory setting, and how prevalent is this form of communication across crab species.”