Friday, September 24, 2010

The Moon moves the current and the current moves the shrimp

The quote below is Excerpted from Bayou Farewell by Mike Tidwell.

A fascinating and prescient title. But don't take my word for it. Here is a 2005 interview with Mother Jones.
Or this review by PeaceCorps writer William Siegel.

"In mid-autumn in the Gulf of Mexico, when the first serious cold fronts drift down from the nation's midsection and push haltingly across Louisiana's subtropical cost, the temperature of the ocean water begins to drop considerably. Miles offshore inside the minuscule brain of Louisiana's female brown shrimp - aka Farfane penaeus aztecus - this sudden Gulf cooling acts as an ancient trigger.

The female shrimp has already mated. She did so months earlier, back in the marsh, just after molting, when she was still sexually immature. She had carried the male sperm in a sac in her head all this time, and now, in the cooling weather of the open ocean, at a depth of at least thirty feet, she begins to spawn. From ovaries barely the size of peas, inside a body maybe six inches long, a half a million eggs somehow emerge. The eggs enter the cool water and the drift, subsumed in the vast ocean environment, sprayed only with a protective coat of jelly as a parting maternal act, then left totally on their own.

By November the water off Louisiana's coast is a veritable soup of trillions and trillions of microscopic shrimp larvae squirming with new life. So many are there that is all the larvae of just three successive generations of Farfante penaeus aztecus were to survive to adulthood and reproduce, the resulting shrimp would be equal to the volume of the sun in less than two years. A few more and every inch of the universe would be filled with shrimp, the mass of crustaceans expanding outward at the speed of light.

But of course most larvae don't grow to be adults. They'll be devoured by a fantastic array of predators, or they'll lose their way during the long migration as epic and perilous as that of the Pacific salmon and the common eel.

Afloat in the Gulf current, the spherical shrimp eggs hatch into first-phase larvae less than a millimeter in size, having not the look of shrimp but of grotesque monsters worthy of battle with Odysseus. They have only one eye, their bodies opaque and pear shaped, with triangular spines and whiplike appendages for swimming. A series of successive molts give the larvae the even stranger look of multi-limbed space aliens, with dozens of barbed digits protruding from bifurcating arms, a video-game villain from a galaxy far, far away.

But after two or three weeks, these odd changes stop and the final product emerges at last: tiny shrimp just half and inch long, but with the familiar stalked eyes and armoured tail and protruding antennae longer than the body itself. Now in order to survive, these newly morphed creatures must somehow get themselves out of the big ocean and into the estuarine coastal marshes, a long way to the north.

For help, the infant crustaceans, roughly the length and width of grains of rice, turn to a spherical body 92 million miles away in outer space, a G2 dwarf star otherwise known as our sun. Twice a month this fiery body of hydrogen gas nearly a million miles in diameter joins forces with the earth's moon, a mere 238,000 miles away, to create a combined gravitational and centrifugal force of enormous power. This force generates ocean tides on earth -- so-called spring tides -- which are much greater than the tides occurring daily throughout the rest of the month. Every two weeks, when the moon shows itself to the earth either as a barely visible new moon or as a blazing full moon, the phenomenon is at work: the moon and the sun have fallen into a straight line relative to the earth, reinforcing each other's gravitational tug, pulling the earth's oceans into two bulging masses of liquid on opposite sides of the globe. These fantastic waves, these great heaping ridges of water, are brought into collision with the earth's landmasses twice a day as the planet rotates. This, in the simplest terms, is how tides happen, and spring tides are the bimonthly champions. So strong is the combined pull of the sun and moon during this period that even the earth's atmosphere bends outward and parts of the continents bulge slightly.

For the trillions of minute shrimp struggling down there in the earth's Gulf of Mexico, this celestial force is plenty strong enough to bear them landward in a high incoming tide that squeezes between Louisiana's barrier islands and sweeps the shrimp up into the food-rich interior marshes. By March thanks to the spring tides, all the autumn-spawned brown shrimp lucky enough to have survived the marathon journey -- perhaps one in a hundred - are now inside the estuary nursery grounds where they begin to feed ravenously.

Brown shrimp, as they grow up, a famously indiscriminate eaters. They are what biologists call "encounter feeders" and "opportunistic omnivores", meaning they'll eat pretty much whatever organic decaying matter they stumble into: decaying plants, animals, algae, fecal pellets, amphipods. Sometimes each other.

This ability to ingest a wide range of food accounts for their rapid growth rate, especially beginning in April with the temperature starts to rise toward 70 degrees Fahrenheit in the coastal marshes and the shrimp grow as much as an inch per week. If both temperature and water salinity remain favorable, the fast growth rate continues and combines with the initial fecundity of the spawning females to produce skyrocketing results. In collective size and numbers, the shrimp, in the spring, reach massive proportions.. It's a shrimp explosion."

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