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home : features : science and nature July 22, 2014

Hidden Worlds - Crabby Songsters
Atlantic Ghost Crab
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Atlantic Ghost Crab

The singers I admire most (and can never listen to enough) are Kiri Te Kanawa, Placido Domingo and Frank Sinatra at his peak. Music is a world I don't fully understand, yet I pause in awe at its threshold. But I also thrill to songsters from a less familiar world -- a biological background enables me to make a little sense out of their voices.

I've heard a skylark sing as it ascends almost vanishing into the sky during its spiraling flight before it drops to rise again. Trailing an underwater hydrophone I've listened to the songs of Pacific humpback whales in the Maui channel. Our Northeast Kingdom hermit thrush sings as sweetly as any angel. Song is everywhere in the animal world, even if we consider much of it clamorous, no match for Kiri or the nightingale and perhaps inaudible to our poor ears. We enjoy the complex and lovely music of wolves and whales, but don't think much of a gibbon's ear-splitting howls or a bellbird's clangs. The hisses and guttural utterances of reptiles and amphibians hardly qualify on our subjective scale. We have difficulty interpreting sounds emerging from croaking fish and trilling tree crickets.

Everyone knows insects are versatile noise makers. Flies buzz and hawkmoths thrum. When I pick up a Bess Bug beetle, it cheeps and creaks loudly enough to surprise me -- I've seen a person drop the harmless creature in alarm when it starts complaining.

A female mosquito whines with scales on the trailing edge of her beating wings. Record the sound and play it back over a loudspeaker and amorous male mosquitoes soon arrive. Bushy, direction-finding antennae of the males vibrate only in tune with a female of the same species (if you want to corral all the harmless, non-biting male mosquitoes in your neighborhood, send out an artificially generated electronic signal between 300 and 350 Hz).

Crustaceans are good sound-makers. Atlantic spiny lobsters squeak loudly and mantis shrimp produce burring, rasping sounds by rubbing body parts against one another. One kind of tiny shrimp, less than half an inch long, produces an audible whisper when two specially equipped legs scrape against each other.

Crustacean songsters live nearby. The chimney crayfish burrows deep into Vermont's marshy soils where the bottom half of its burrow can be flooded with water. It communicates with others of its own kind by rubbing parts of its armored skeleton together. Listen carefully and you'll hear a rasping noise arising from the marsh mud. This is important, because when creatures first moved from sea to land many millions of years ago and remained buried in sand and mud, their primary means of communicating with one another was by sound.

By rubbing its wrists together an Atlantic Stone Crab sounds exactly like a grasshopper. Clusters of small tubercles on the side of its otherwise smooth body resemble the surface of a coarse wood file. The inner edge of one of its claws has several rows of sharp ridges that rub over the corresponding rough patch to create a scraping sound. This is called stridulation, from the Latin stridulus, meaning a harsh creaking. (Our word "strident" has the same origin.)


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.

©2012 William H. Amos

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