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Wednesday, 14 August 2013 13:57

A pistol of a shrimp

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out natcornOnce again, through the gracious hospitality of a dear friend the Hendershot family found itself on the Isle of Palms — one of the South Carolina barrier islands just up the coast from Charleston. We have been here before and I have written about it before. It is always the same; it is always different; and it is always wonderful.

We stay on the marsh side and there is a long dock reaching out into the marsh. It provides a great setting to watch the dramatic and continuous ebb and flow of the tide. There are two high and two low tides and the marsh is always filling or emptying.

When it empties the fiddler crabs take over the mudflats and seem to scurry in waves across the pluff mud. And when walking along the dock at low tide, one is always aware of the smacking sound emanating from the marsh. Now I’ve trudged the marshes of South Louisiana and I lived on Hilton Head Island for a short spell and the popping/smacking sound has always been a part of the marsh. 

I am at a loss to figure where I filed that sound away — who or what I attributed it to. But for some reason on this trip, perhaps because we spent so much time trudging back and forth on the dock — to get in the kayaks, for the girls to go crabbing, and/or to take a peek at the perseids — the popping got under my skin.

I’m sure you pogie-boot wearing, skin-so-soft smearing, clam digging aficionados of all things marshy know what causes that sound. But I had to research it — and I am so glad I did. That ubiquitous marsh popping sound comes from a small crustacean — the bigclaw snapping shrimp or pistol shrimp in the family Alpheidae. Snapping shrimp are worldwide in distribution. There are more than 600 species. They are common in our neck of the woods from Chesapeake Bay down to Florida and the Gulf of Mexico.

Alpheus heterochaelis is the largest and one of the most common bigclaw snapping shrimp of the Southeast. It grows to about 2 inches in total length. And the claw that it is named for may be half it’s body size. This asymmetrical critter may be right-clawed or left-clawed, with the opposite appendage being “normal” with typical pincers. Like most arthropods, if the pistol shrimp loses an appendage it can regrow it. If this shrimp should lose its mega claw the other pincer will grow into its namesake, and the new appendage will be typical.

 

The Big Bang theory

The snapping shrimp’s big claw has two parts. Like the hammer of a pistol, one part cocks at a right angle. The other half is fixed. When the hammer is released it snaps into a niche in the fixed part of the claw. It was thought at one time that this snapping action created the popping sound, but the truth is way cooler. The hammer falling into the niche shoots out a small stream of water at speeds up to 60 miles an hour. As the liquid moves faster the pressure inside decreases and air bubbles form, as the stream slows down, the pressure increases and the air bubbles implode creating the popping noise. The entire process, from the snapping of the claw to the bursting of the bubble, takes about 300 microseconds. It produces a short (10 nanoseconds) intense flash called sonoluminescence as temperatures in the collapsing bubble reach about 4,700 degrees Celsius — nearly the temperature of the surface of the sun. The “shot” is used for hunting and is powerful enough to stun passing crabs and/or kill small fish. 

Snapping shrimp are often found in large colonies in reefs in the ocean. Their snapping is so loud that it can interfere with sonar and other underwater communications. It is one of the major sources of noise in the ocean, rivaling the sounds emitted from beluga and sperm whales.

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