FAIRY SHRIMPS (AND VERNAL POOLS)

WE HUMANS DEMAND permanence-in the buildings we build, in the boundary lines we draw, in the relationships we hold. Everything in its place forever, predictably and comfortably true. Yet nature remains her dynamic self, with hurricanes uprooting beachside condos and ponds shriveling up into clayey crusts. And for many non-humans, such impermanence means life everlasting.

This past spring, I visited one such impermanent place, a vernal ("spring") pool tucked into the base of Ruffner Mountain. A truly nasty-looking place, its shallow bottom oozed with tomato-red mining waste, the drying, goopy edges shining bright metallic. Ah, but such abundant life! The water's surface fairly boiled with wriggling tadpoles and squirming salamanders, while hordes of dragonflies cruised above. But the weirdest inhabitants, and the objects of my visit, were the fairy shrimps. (No, ye English majors, that last word is not a mistake: biologists use "shrimps" and "fishes" to indicate multiple species or forms, while chefs [and Forrest Gump's best friend, Bubba] prefer the singulars-even though they may actually refer to plurals!)

Classified as phyllopods ("foliage feet") due to the leaf-like appearance of their legs, fairy shrimps "bloom" in mineral-rich temporary ponds, reaching ridiculously dense concentrations. There they gracefully glide about, ventral sides up, gently propelled by the wavelike, anterior-posterior beating of those legs. Such movement also serves for food-getting, with algae, plankton, and bits of detritus strained from the water and concentrated in a median groove running the length of the body.

As a group, phyllopods demonstrate a wide range of reproductive strategies: "regular" male-female interactions (similar to the human version described in sixth-grade health class); mixed reproduction involving males and hermaphrodites; and parthenogenesis, with females producing offspring entirely on their own. (Males may be few, or absent altogether.) No matter how they are produced, eggs remain for one to several days in an elongated brood sac, developing rapidly-so rapidly, in fact, that larvae often hatch before release. They either drop to the pond's bottom or remain attached until their mother dies and sinks.

And two kinds of eggs result: thin-shelled "summer" eggs (which, if water is available, will grow out into reproductive adults in just a few weeks) and thick-shelled "winter" eggs or cysts. Cysts tide over a population from one spring to the next, their shells preventing desiccation, mechanical injury, and sunlight damage as the pond slowly shrinks. Blown about as dust or hitchhiking on water birds, they also effect widespread distribution.

Which brings us to the Amaxing Sea Monkeys long advertised on the back pages of comic books.... Fellow phyllopods, brine shrimps inhabit briny Western lakes-to such an extent, in fact, that each year billions of their cysts are harvested for fish food or sold as "Sea Monkey eggs" to naive, biology-challenged youths. Researchers find that brine and fairy shrimps share a "bethedging" strategy in these precarious habitats, with only a small percentage of cysts germinating each year-the same risk-spreading routine followed by desert-dwelling plants. I mean, why risk all of your gene pool in one shot, when that pool may soon dry up?

So do "extremophiles" like these prefer hot, nasty, isolated, mineral-laden waters to all others? (I can hear Bubba now: "We could have Shrimp Alkaline, or Shrimp Saline, or Shrimp Temperatura....") Absolutely not! Instead, they "choose" such places because (1) they're best adapted to them and (2) something's missing, namely fish(es). Vernal pools, with their widely fluctuating water levels and chemical concentrations, arc just too small, too salty, too alkaline, and/or too temporary to support predators. (For similar reasons, frogs and salamanders develop in shallow pools and ditches, hopping or scurrying away from their evershrinking natal waters.)

Fairy shrimps provide a lesson in the ephemeral nature of nature, with fleeing creatures and fleeting relationships, where permanence means certain death. In direct contrast to us, they demand a pulsed life cycle, with no contact between generations-no way for a shrimp mom to teach her babies a better backstroke, or her best plankton recipe, or how to recognize (and avoid) the shadowy specter of a ravenous duck swimming overhead.

But rejoice, brothers and sisters! It's only a temporary "death" in a temporary pond, to be resurrected with next spring's rain.

Larry Davenport is a professor of biology at Samford University, Birmingham, Alabama.

Copyright University of Alabama Press Fall 2003
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