Illinois Natural History Survey - University of Illinois

Fairy Shrimp

Susan Post

 

“This woodland pond is temporary, created by snowmelt. By midsummer, evaporation will have claimed its contents and only a low spot in the forest will recall the pond of April.” Larry Weber Backyard Almanac1996

Temporary pools, variously called vernal, ephemeral, or seasonal, are a type of wetland once common in the landscape. These pools developed from the scouring process of the ebb and flow of flooding rivers, streams, and lakes; through wind action; or from depressions created by trees that tip over. These pools are shallow, temporary, and separated from streams and rivers. Their defining characteristics are that they periodically dry up and do not contain fish. While the drying may occur annually or only in drought years, by late summer they are usually dry. It is this temporary aspect of these wetlands that make them valuable.

The wet-dry cycle prevents fish from becoming established, allowing for critical breeding and rearing habitat for amphibians, insects, and crustaceans, including the fairy shrimp. Fairy shrimp are crustaceans with gills, head and thorax fused into a cephalothorax, and two pairs of antennae. In temporary pools fairy shrimp are easily identified. They are 0.5 to 1.5 inches long, with stalked compound eyes, two sets of antennae, and 11 pairs of leaflike swimming legs. Fairy shrimp can range in color from translucent whitish to blue, green, or red-orange. Their coloration is determined by the food supply in the pool. Microscopic organisms such as algae, bacteria, and protozoa, along with bits of detritus are the main food supply. The movements of the shrimp’s legs serve not only as a means of obtaining food, but also aid the shrimp in taking up oxygen from the water.

Fairy shrimp move along the bottom or glide about gracefully (usually upside down) by beating their legs, resulting in a wavelike front-to-back motion. Sometimes they rest on the bottom or drift slowly; other times they dart rapidly. Reproduction begins when the male clasps the female with his antennae. The pair may swim clasped together for several days, but once mating occurs the male dies. A female can produce two types of eggs— summer and winter. Summer eggs are produced if there is a shortage of males. They are thin-shelled and hatch rapidly.

The young from these eggs populate the pool the same season they are laid. Winter eggs are thick-shelled and once released, fall to the bottom of the pool. These eggs can withstand unusual amounts of heat, cold, and desiccation. When the pool refills in the spring, these eggs usually hatch in 1–2 days after being exposed to water. Winter eggs can be carried from pool to pool by traveling animals or by the wind if the pool dries out completely. The egg hatches as a nauplius (a type of crustacean larvae) and develops in a series of instars. The fairy shrimp continues to molt until it reachs 20 segments. The speed of development tends to reflect the amount of time water remains in the pool, but is usually around 16 days. Due to their ephemeral nature, fairy shrimp have few predators. The limiting factor in fairy shrimp populations is the need for 1 part per million dissolved oxygen in the water they inhabit and, of course, the temporary pools.

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