Angry crabs conceal a secret weapon to repel their enemies and protect predators when their nails are engaged in battle, scientists say.
Ghost crabs, named for their sandy-pale bodies and night bulbs, use their teeth in their stomachs to "growl" at aggressors, leaving their nails free for attacking maneuvers and waving together.
This is the first known evidence of an animal using the sounds of its abdomen to communicate, researchers say.
"It's an advantage if a predator is nearby," says Jennifer Taylor, a biologist who studies crayfish communication at the Scripps Institute of Oceanography in La Jolla, California. "They can pull out their nails and be ready to dress, but they still make those sounds."
Most crabs make noises called strictures by rubbing their tongs together. Raping comes from ridges or bristles that go up and down each nail.
But while ghost crabs make strictures in the normal way, Taylor noticed further aggressive noises coming from crustaceans when they mock at rods, robotic toys called Hexbugs, or dead crab bodies.
"I was really intrigued by the sounds they made," Taylor told the Guardian. "I looked at them to see if they were moving anything other than their nails, but there was nothing outward."
So the researchers looked inside. The first attempt ended when a cancer smashed a tiny endoscope placed in his mouth. The following approach was more effective. Using a technique called laser Doppler vibrometry, scientists have bounced a laser from different parts of aggravated cancer. The reflections revealed that the strongest vibrations came from the animal's stomach region.
Crabs have an unusual apparatus that helps them process food. Inside their stomachs are small teeth that together form what is known as a gastric mill. In ghost crabs, the mill has a central tooth surrounded by neighbors who wear their fine teeth similar to a comb. The intricate arrangement of the muscles and nerves sends the mill in motion, allowing the crabs to cut into food that has already passed into their stomach.
Taylor suspects that the mill was involved in the protection of crustaceans. To find out, the team drove their crabs to a medical center, where they were given x-rays as they struggled with sticks and other objects. After several attempts, researchers shot footage of steel mills to generate warning sounds. The study was published in Proceedings of the Royal Society B.
"Ghost crabs have developed specialized structures for their nails to produce sounds, but they also take this second method as a backup," Taylor says. "If their nails are already occupied, they can make the same sounds inside."
Crabs can grow as a hand and are a common sight on the tropical shores where they live in holes dug in the sand. Animals breathe air through humidified gills, which occasionally moisten themselves by sitting in moist sand or running headlong into the surf.