RIVERSIDE FAIRY SHRIMP } Streptocephalus woottoni
DESCRIPTION: The Riverside fairy shrimp is a small, freshwater crustacean between 13 and 25 millimeters long, with females being slightly smaller than males. The animal’s frontal appendage is cylindrical and bibbed at the tip; its body features cercopods, jointed appendages that enhance the rudder-like function of its abdomen. The rosy color of Riverside fairy shrimp is primarily provided by their red-hued cercopods.
HABITAT: The Riverside fairy shrimp is restricted to deep vernal pools and ponds with chemistry and temperature conditions specific to nonmarine and nonriverine waters. All known vernal pool habitat lies within annual grasslands, which may be interspersed with chaparral or coastal sage scrub vegetation. Vernal pools form when winter rains fall on relatively flat lands whose soils are underlain by naturally impervious layers, usually dense clay layers. Because the water can’t easily percolate down into the ground, above-ground pools form in the vernal months of late winter and early spring, evaporating in late spring. A suite of uniquely adapted plants and animals like the Riverside fairy shrimp depend on these seasonal pools for life.
RANGE: Known from California’s Ventura County, Los Angeles County, Riverside County, Orange County and San Diego County, the shrimp has also been found at two locations Valle de las Palmas and south of El Rosario in Baja California, Mexico.
MIGRATION: Fairy shrimp are nonmigratory and have limited movement. Any dispersal of the species is likely caused by animal vectors wading or walking through wet or dry pools inhabited by fairy shrimp adults or eggs. Because of the long distances between the few remaining pools, natural gene flow is greatly if not completely reduced.
BREEDING: Riverside fairy shrimp deposit eggs or cysts (organisms in a resting stage) in their pool’s soil to wait out dry periods. The hatching of the cysts usually occurs from January to March within seven to 21 days after the pool refills, depending on water temperature. “Resting” or “summer” cysts are capable of withstanding temperature extremes and prolonged drying; thus, the soil may contain egg banks that are the result of several years of breeding.
LIFE CYCLE: “Resting” cysts may lie dormant and hatch years after being deposited; non-resting eggs — thin-shelled eggs that are produced if there is a shortage of males in the population — will hatch during the same wet season in which they are laid. There is no information regarding Riverside fairy shrimp survivorship, but since the species is dependant on an ephemeral and unpredictable resource, it may be assumed that there is often 100 percent mortality in hatched shrimp when pools dry.
FEEDING: Nearly all species of fairy shrimp feed on algae, bacteria, protozoa, rotifers and various bits of organic matter.
THREATS: This shrimp is imperiled by a huge variety of threats, including urban sprawl, agribusiness, off-road vehicles, livestock grazing, wetland draining, pollution, invasion of nonnative plants, fire and fire-suppression activities.
POPULATION TREND: The Riverside fairy shrimp has the most limited range of any endemic California fairy shrimp. Unfortunately, the species’ population numbers are dwindling primarily because the vernal pools it needs to survive are being destroyed at an alarming rate in Southern California. Sometimes Riverside fairy shrimp are overlooked during vernal pool surveys because they hatch out and swim later in the season than many other fairy shrimp, and if they aren’t detected, the pool is often plowed up. Vernal pools occur on relatively flat land that is highly desirable for agriculture and housing development. Now, with the looming threat of global climate change, vernal pools may very well be threatened with extinction due to decreased rainfall predicted in southern California. Not only the Riverside fairy shrimp but a whole suite of rare species dependent on vernal pools for reproduction could slip closer to extinction. About 45 vernal pool complexes, containing up to 200 vernal pools, are estimated to exist.