Aug 052011

Photo by David Velk

Recent heavy rains have begun to fill the ephemeral pools on the prairie. The ones that remain longest will soon throng with aquatic life–some of extremely ancient lineage. Clam Shrimp and Tadpole Shrimp both lived here when the trees in the Petrified Forest stood upright, leafed and alive. More than 200 million years later, both shrimp species remain here still, even though the rainforest rivers have been replaced by Shortgrass Prairie and Painted Desert badlands. Their eggs are drought resistant. Lying dormant in the cracked mud of dry lakes, ponds and puddles, they can endure for years before summer monsoons bring enough water to trigger a hatching response. Clam Shrimp resemble small freshwater clams, but the shell halves open to reveal legs similar to conventional shrimp, and so they are free-swimming. The Clam Shrimp of today are commonly olive in color, although bright scarlet ones are sometimes observed. Tadpole Shrimp are larger; full adults can reach thumb size. Resembling tadpoles at a distance, closer examination will show typical shrimp-type swimming legs. They propel themselves by rowing, or sculling, instead of using their tails like tadpoles. Tadpole shrimp have a distinctive feature–their heads come with three eyes, hence the scientific name, “Triops.” When other food is scarce, their usual scavenging behavior is abandoned. Becoming predatory, they make a specialty of hunting down mosquito larvae, devouring them before they emerge as the familiar winged annoyance. Both living-fossil shrimp remain active only as long as the waters linger, but before the pools succumb to evaporation in the fall, new eggs will be laid to await the next monsoon season.