March 5, 2002 (11:30 PM)
Riverside Fairy Shrimp
SPECIES CODE: K03F I01
STATUS: Endangered throughout its range- U.S.A. (CA) & Mexico (58 FR 41391, August 3, 1993). Recovery plan completed in 1998 (Recovery Plan for Vernal Pools of Southern California).
SPECIES DESCRIPTION: Mature males are between 13 and 25 millimeters (0.5 to 1.0 inch) long. Mature females are between about 13 and 22 millimeters (0.5 to 0.87 inch) in total length. Fairy shrimp are free‑swimming filter feeders, feeding primarily on bacteria, algae, rotifers, Protozoa, and bits of detritus (Pennak 1989). No specific studies have been done on the feeding habits of the Riverside fairy shrimp. The Riverside fairy shrimp are “osmoregulators” that maintain constant internal chemical concentrations, but cannot tolerate wide extremes in sodium or bicarbonate concentrations (USFWS 1998).
REPRODUCTION AND DEVELOPMENT: A key adaptation of the fairy shrimp is the production of drought-resistant eggs. When the vernal pools dry, the eggs remain on the surface of the pool or embedded within the top few centimeters of soil. There they survive the hot, dry summers and cold, wet winters that follow until the vernal pools and swales fill with rainwater and conditions are right for hatching (Geer and Foulk 1999/2000). With hydration of eggs, time to hatching is usually between two and 25 days (Hathaway and Simovich 1996). Riverside fairy shrimp will not hatch in pools that receive cool waters from early winter rains (Eriksen in litt. 1992), such as those pools on the Santa Rosa Plateau, nor will they hatch in shallow pools. Shrimp eggs tend to hatch or germinate at cool temperatures, with species-specific differences in responses that are related to temperature regime. Lack of hatching at higher temperatures (greater than 25 degrees Celsius; 77 degrees Fahrenheit) protects the Riverside fairy shrimp from the infrequent summer storms that might otherwise be sufficient to stimulate development, but inadequate for the organisms to complete their life cycles. Maturation to reproductive age from hatching is over 2 months for the Riverside fairy shrimp. The time period is compressed or expanded, depending on ambient water temperatures (Hathaway and Simovich 1996).
RANGE AND POPULATION LEVEL: The Riverside fairy shrimp occurs in vernal pools from southwestern Riverside County and western San Diego County, California, to northwestern Baja California, Mexico. One population is known from Orange County. The northern range of the Riverside fairy shrimp is defined by Skunk Hollow and the Santa Rosa Plateau in Riverside County and coastal sites in San Diego and Orange Counties. The Riverside fairy shrimp is known from four vernal pools in a 37 square mile (91 square km) area in western Riverside County, in an area about 13 by 7 km, between elevations of 348 and 413 m, near Temecula and Rancho California (Eng et al. 1990). The crustacean is documented from one complex on MCAS Miramar, throughout MCB Camp Pendleton, and eight complexes on Otay Mesa. In Baja California, Mexico, it has been found in Valle de las Palmas, and at Bajamar north of Ensenada (Brown et al. 1993).
Of the four remaining pools supporting the fairy shrimp in Riverside County, only the Skunk Hollow vernal pool is greater than 1 acre in size. The Skunk Hollow vernal pool is within a planned development. Other sites supporting the fairy shrimp may lack some of the typical vegetation of vernal pools, but that condition probably reflects impacts from past agricultural activities. Another pool that contains the Riverside fairy shrimp is located partially on private land and partially on the Pechanga Indian Reservation. The portion on private land was cultivated during 1990. The region’s drought conditions over the last 2 to 3 years may have rendered the pool dry enough to be plowed (USFWS 1993).
HABITAT: The Riverside fairy shrimp has very narrow habitat requirements. This species is only found in deep, cool lowland vernal pools that retain water through the warmer weather of late spring (Clyde Eriksen, Claremont College, in litt. 1992; Jamie King, University of California, Davis, in litt. 1992). Minimum habitat size is 750 square meters, with a minimum depth of 30 cm at maximum filling. Total dissolved solids (tds), alkalinity, and chloride were very low, conditions corroborated by pH at neutral or just below. This species does not appear until later in the season, so it can be considered a warm water species. (Eng et al. 1990).
Vernal pools are unique seasonal wetlands that support a wide variety of wildlife, from waterfowl to amphibians– all of which rely on the protein-rich food sources found in these ecosystems (Geer and Foulk 1999/2000). The animal is also occasionally found in depressions (road ruts and ditches) that support suitable habitat.
PAST THREATS: The continued existence of the Riverside fairy shrimp is threatened by habitat loss and degradation due to urban and agricultural development, off-road vehicle use, cattle trampling, human trampling, livestock grazing, trash dumping, invasion from weedy non-native plants, drainage or watershed alterations (often due to adjacent urban development), road development, military activities (e.g., driving equipment), water management activities, mowing or plowing, highway construction, fire, fire suppression activities, and drought.
The habitat and range of these four species has been greatly reduced. Many pool groups were entirely eliminated and replaced with urban or agricultural developments.
Pools have also been degraded due to the use of off-road vehicles which compact soils, crush plants when water is in the pools, cause turbidity, and leave deep ruts. The damage may alter the microhydrology of the pools. Dirt roads that go through or adjacent to pools are widened as motorists try to avoid the inevitable mudpuddles. Thus, pools are gradually destroyed by vehicles traveling on dirt roads. Vehicle access and damage has occurred on virtually all remaining vernal pool complexes (USFWS 1993).
Habitat trampling, and in some cases trampling of the organisms, due to livestock grazing, occurs on several vernal pool complexes in Otay Mesa. Organisms within the pools may be trampled and killed by livestock prior to reproduction. Soil may become compacted or eroded, and water may be impacted with sediment. Otay Mesa is a common area for travel from Mexico to the United States; hence, habitat and plants are threatened with trampling by humans. Also, the Immigration and Naturalization Service has proposed several projects and construction activities at the international border, including border lighting, that could result in direct adverse impacts to vernal pools on Otay Mesa (USFWS 1993).
Trash dumping also degrades vernal pools. Chunks of concrete, tires, refrigerators, sofas, and other pieces of garbage or debris were found in pools containing the Riverside fairy shrimp. This trash crushes or shades vernal pool plants, disrupts the hydrologic functions of the pool, and in some cases may release toxic substances (USFWS 1993).
Competition and predation by non-native species pose a serious threat to the Riverside fairy shrimp and other vernal pool species. Interim management, such as controlled burns (Simovich et al. 1996; Pollak and Kan 1996), may be necessary to reduce or eradicate nonnative species to levels that do not adversely affect the native species. Bullfrogs are recognized predators (Morey 1996; Simovich et al. 1996) of vernal pool species and eradication of larvae, post-metamorphic, and adult bullfrogs should be a task item in all vernal pool management plans.
The vernal pool habitat upon which this species depends is also vulnerable to destruction due to alteration of the watershed. In some cases, an increase in water volume due to urban run-off has led to more prolonged periods of inundation, and at the other extreme, some pools have been drained or blocked from their source of water (USFWS 1993).
CURRENT THREATS: The Riverside fairy shrimp has the most limited range of any endemic California fairy shrimp and is currently threatened by agricultural and urban development, off-road vehicle use, trampling, trash dumping, invasion from weedy non-native plants, drainage or watershed alterations (often due to adjacent urban development), and drought.
Bauder, E.T. 1986. San Diego vernal pools: recent and projected losses; their condition and threats to their existence 1979-1990. Prepared for: Endangered Plant Project. California Department of Fish and Game, Sacramento, California.
Brown, J. W., M.A. Wier, and D. Belk. 1993. New records of fairy shrimp (Crustacea: Anostraca) from Baja California, Mexico. The Southwestern Naturalist 38 (4): 389-390.
Eng, Larry L., Denton Belk, Clyde H. Erikson. 1990. California Anostraca: Distribution, habitat, and status. J. of Crustacean Biology. 10(2): pp. 247‑277.
Geer, K., and P. Foulk. 1999/2000. Endangered: Vernal Pools and Fairy Shrimp. Waterfowl 2000. 12(3):28.
Hathaway, S.A. and M.A. Simovich. 1996. Factors affecting the distribution and co-occurrence of two southern Californian anostracans (Branchiopoda), Branchinecta sandiegonensis and Streptocephalus woottoni. Jour. Crustacean Biol. 16(4): 669-677.
Morey, S. 1996. Amphibian life history attributes and implications for vernal pool conservation practice. C.W. Witham, E. Bauder, D. Belk, W. Ferren, and R. Ornduff(Editors). Ecology, Conservation and Management of Vernal Pool Ecosystems- Proceedings from a 1996 Conference. California Native Plant Society, Sacramento, CA.
Nature Serve Explorer. An online encyclopedia of life [web application]. 2001. Version 1.6. Arlington, Virginia, USA: Nature Serve. Available: http://www.natureserve.org/explorer (Accessed: February 28, 2002).
Pennak, R. W. 1989. Freshwater invertebrates of the United States: PROTOZOA to MOLLUSCA. 3rd Edition. John Wiley & Sons, Inc. 628 pp.
Pollak, 0. and T. Kan. 1996. The use of prescribed fire to control invasive exotic species at Jepson Prairie and Vina Plains Preserves. C.W. Witham, E. Bauder, D. Belk, W. Ferren, and R. Ornduff (Editors). Ecology, Conservation and Management of Vernal Pool Ecosystems - Proceedings from a 1996 Conference. California Native Plant Society, Sacramento, CA.
Simovich, M.A., Boudrias, M., and R. Gonzalez. 1996. Vernal Pool Faunal Survey Naval Air Station Miramar. Prepared by the Branchiopod Research Group, University ofSan Diego, for the Naval Air Station Miramar, San Diego, California.
U.S. Fish and Wildlife Service. 1993. Endangered and Threatened Wildlife and Plants; Final Rule; Determination of endangered status for three vernal pool plants and the Riverside fairy shrimp. Federal Register 58:41391.
U.S. Fish and Wildlife Service. 1998. Vernal Pools of Southern California Recovery Plan. U.S. Fish and Wildlife Service, Portland, Oregon. 113+ pp.