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Fairy Shrimp of Ephemeral Ponds

Howdy, BugFans,

The BugLady had so much fun taking last May off that she’s going to take this one, too. She will post weekly, seasonally appropriate (and somewhat enhanced) episodes from the archives, lest your in-boxes blow away. She hopes you enjoy this episode from 2009.

While some may look to the robin to announce the arrival of spring, many wetland enthusiasts mark it by the appearance of that poster child of vernal ponds, the fairy shrimp. Or, in the words of Dr. Mary Linton, wetlands ecologist. “Nothing kicks the slop out of a nasty case of Seasonal Affective Disease like the first fairy shrimp of spring.”

Neither insects nor insect-wannabes, the tiny fairy shrimp have some mighty big names in their pedigree—they are in the phylum Arthropoda (jointed legs), sub-phylum Crustacea (from the Latin crusta, “hard or brittle external covering”), class Brachiopoda, subclass Phyllapoda (“leaf-footed ones”), order Anostraca (“without a shield,” a nod to the fact that they have no hard carapace). These are probably in the family Chirocephalidae and the genus Eubranchus. There are about 800 species in the order, and their Crustacean relatives include lobsters and shrimp, as well as sowbugs and cyclops. Fairy shrimp live in the Antarctic, in the Andes, in ponds, and even in the desert.

They are translucent; their color ranges from whitish through blue and green to orange and red, and color can vary within a species based on age and diet and the bacterial content of the water. In her Field Book of Ponds and Streams, Ann Haven Morgan notes that a fairy shrimp’s tail is often colored red with hemoglobin, and that its back is so transparent that it is possible to see the beating of its long, tubular heart.

Tiny (half-to-three-quarter-inch) fairy shrimp move, belly up, through the world below the water’s surface. Adults have compound eyes set on stalks and two pairs of antennae. Eleven to 19 pairs of leaf-like, multi-tasking “gill-feet” are found just south of the head. These enable locomotion and breathing. Described as having “chewing bases,” the legs’ rhythmic movements also move/manipulate food (really tiny stuff like algae, diatoms, protozoans, bacteria, and detritus) toward the fairy shrimp’s mouth. Look closely at the “head shots” of the male and female fairy shrimp, and you’ll see green bits of algae sifting through the thorax. Some species nibble at the edges of amphibian eggs and dead organic matter; others scrape algae off of hard surfaces. Females outnumber males.

Because of their short life cycle (16 days, in some cases) and early appearance, fairy shrimp in ephemeral ponds dodge many predators, but they are eaten by amphibians, predaceous diving beetles and the larvae of caddisflies. They avoid becoming fish food because the vernal pools they inhabit are generally fish-free, and their populations peak before migrating birds can make a meal of them. Fairy shrimp with other lifestyles are important food for waterfowl. Their relatives, the brine shrimp are eaten by flamingos and have been consumed by humans.

The Elephant-Man-like bulges below eyes of male fairy shrimp are mating organs—his second pair of antennae have been modified into claspers. Though mating is brief, the pair swims in tandem for a few days afterwards. A female fairy shrimp carries eggs in a brood sac/pouch on the underside of her body, releasing 10 to 250 eggs at a time every 2 to 6 days. She can produce two kinds of eggs—“summer” eggs and “winter” eggs. Summer eggs, which are a response to low numbers of males in the population, have thin walls and hatch quickly.

Winter eggs—thick-walled, designed to weather both the seasonal drying of the pool and the cold of winter—wait in the litter of the pool’s floor, the embryos within, dormant. It is in this stage that fairy shrimp “travel,” blowing from pond to pond or moving in the stomachs of other animals. They can wait as long as two decades for favorable conditions to return, and can tolerate both drying and extremes in temperature. In fact, the species that inhabit ephemeral ponds require drying and re-hydration before they can hatch. Some species may hatch as early as January if water is present and may be seen under the ice. Other species don’t hatch till May. Fairy shrimp populations may wax and wane in the same pool from year to year. Oxygen is at a premium in ephemeral ponds, and fairy shrimp are able to regulate their use of it, but when oxygen concentrations are very low, the fairy shrimp are smaller in size.

Fairy shrimp present a delicious “chicken-or-the-egg” conundrum. Does this group of animals survive in ephemeral ponds because they have developed adaptations that allow them to wait out the dry spells? Or do they pick ephemeral ponds because these ponds have the dry spells that the shrimp require?

Sharp-eyed scrutinizers of the “Vernal Pond water” shot will see caddisfly cases, mosquito larvae (wigglers) and pupae (tumblers), fairy shrimp, damselfly nymphs, and others.

 
The BugLady