Beach-dwelling ghost crabs create three entirely different tones with a stridulating instrument, the patch of which consists of ridges making up a "keyboard," much like a violin with three strings across which the claw "bow" is drawn. We can hear those three tones, but a crab modulates the angle of rubbing as well as its rapidity and vigor, so its "vocabulary" may be much greater than we know. Individuals of an Asiatic land crab relative live in colonies hundreds of miles apart; they look and behave exactly the same way, but speak different dialects by which they can be distinguished.
Atlantic ghost crabs live close to one another but never get in each other's way. If a crab-eating gull flies overhead when they are out foraging on the sand, each darts into its own burrow, then calls out its presence so another doesn't enter. If a burrow is silent, it must be unoccupied and offers safe haven to a fleeing ghost crab too far from its own home.
Different crab species stridulate for different reasons, the most obvious being establishing and protecting territory, warning away rivals and attracting females. There may be "subliminal" meanings as well, for the sounds created by a large robust crab possess a different character than those made by a smaller member of the same species. Age and size differentiation in sound production can give warning of aggressive intent, so combat is avoided by a little guy who is likely to be beaten up or dismembered by a Rambo of his kind.
How did such communication evolve? Possibly through a crab's ability to sense the slightest vibration in its surroundings. A large crustacean not only has nerves constantly telling the position of its legs and body, but sensory nerves that are attached to bundles of delicate bristles extending into the environment. Using a form of communication that consists of tapping the ground, fiddler crabs sense one another's presence and activity with these bristles by detecting vibrations in the mud.
One of the loudest crustaceans anywhere lives in warm shallow seas. It's called a pistol or snapping shrimp and is recognized by its one greatly enlarged claw whose movable finger has a small knobbed projection that fits snugly into a socket on the opposing fixed finger. Once it was thought the sharp sound was created by popping out the knob, like a toy popgun's cork, but it is actually just the reverse: the snapping shut of the movable finger against the fixed one. The force used is so great that if a hard object is placed between the two fingers, one of them shatters.
Occasionally while diving I try what many marine biologists have done when coming across a big tropical barrel sponge. Knowing it contains hundreds of snapping shrimps in its labyrinthine galleries, I'll give the solid sponge a punch with my fist. The sound bursting forth from alarmed pistol shrimps is enough to damage one's hearing and I always retreat. I once spent a near-sleepless night with a single lonely, unhappy pistol shrimp in a nearby porcelain bowl. The repetitive sound was so loud I thought the ceramic sides would shatter and not wanting to end up in a soaked sleeping bag, I tossed the little firecracker out into the bay and got some sleep.
Not many people in the world are interested in singing crustaceans, but a few who work at the United States National Museum have recorded hundreds of identifiable sounds. Scientists elsewhere have added more, once again proving seas, lakes and shorelines are far from silent. The dozens of crab, shrimp and crayfish songs I've heard may not have been melodious, but they were distinct and undoubtedly filled with meaning -- if only I could understand them.
Â©2012 William H. Amos