February 1, 2002 (4:35 PM)
Vernal Pool Fairy Shrimp
SPECIES CODE: K03G I01
STATUS: Threatened throughout its range (59 FR 48153, September 19, 1994).
SPECIES DESCRIPTION: The vernal pool fairy shrimp ranges in size from 10.9 to 25.0 mm (Eng et al. 1990). Vernal pool fairy shrimp are almost translucent, but can be whitish or have some orange body parts. Fairy shrimp have delicate elongate bodies, large stalked compound eyes, no carapace, and 11 pairs of swimming legs. They swim or glide gracefully upside down by means of complex beating movements of the legs that pass in a wave-like anterior to posterior direction. Nearly all fairy shrimp feed on algae, bacteria, protozoa, rotifers, and bits of detritus (Pennak 1989). Fairy shrimp play an important role in the community ecology of ephemeral water bodies (Loring et al. 1988). They are fed upon by migratory waterfowl (Krapu 1974; Swanson et al. 1974) and other vertebrates, such as spadefoot toad tadpoles (Marie Simovich, University of San Diego, pers. comm., 1991).
REPRODUCTION AND DEVELOPMENT: The second pair of antennae in the adult females are cylindrical and elongate, but in the males’ antennae are greatly enlarged and specialized for clasping the females during copulation. The females carry the fertilized eggs in an oval or elongate ventral brood sac. The eggs are either dropped to the bottom or remain attached until the female dies and sinks. (Pennak 1989). 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). When the pools refill in the same or subsequent seasons some, but not all, of the eggs may hatch. The egg bank in the soil may be comprised of the eggs from several years of breeding (Donald 1983). Although the animal can mature quickly, allowing populations to persist in short-lived shallow pools, it also persists later into the spring where pools are longer lasting (Simovich et al. 1992).
RANGE AND POPULATION LEVEL: The remaining populations of the vernal pool fairy shrimp are restricted to southern Oregon and northern, central, and portions of southern California. Vernal pool fairy shrimp are known from a total of 32 populations located in an area extending from Stillwater Plain in Shasta County through most of the length of the Central Valley to Pixley in Tulare County, and along the central coast range from northern Solano County to Pinnacles in San Benito County (Eng et al. 1990; M. Fugate, pers. comm., 1991; Sugnet & Associates 1993b). Five additional, disjunctive populations exist near Soda Lake in San Luis Obispo County, in the mountain grasslands of northern Santa Barbara County, on the Santa Rosa Plateau in Riverside County, near Rancho California in Riverside County and near the city of Medford, Jackson County, Oregon. Three of these five isolated populations contain only a single known pool occupied by the vernal pool fairy shrimp. Populations of this species are known from Beale, Mather, and Castle Air Force Bases (Eng et al. 1990; Mike Fugate, pers. comm., 1991). Several refuges have been established on Nature Conservancy preserves in the Vina Plains, the Haystack Mountain area, and the Santa Rosa Plateau. The areas surrounding the refuges are cultivated fields (Nature Serve Explorer 2002).
Simovich et al. (1992) reported that the vernal pool fairy shrimp typically is found at low population densities. Only rarely does the vernal pool fairy shrimp co-occur with other fairy shrimp species, but where it does, the vernal pool fairy shrimp is never the numerically dominant one (Eng et al. 1990). The vernal pool fairy shrimp has been collected from early December to early May (Collie and Lathrop 1976).
HABITAT: Vernal pool fairy shrimp populations live in ephemeral freshwater habitats, such as vernal pools and swales. None are known to occur in running or marine waters or other permanent bodies of water. 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 distribution of vernal pools is highly discontinuous and some of the aquatic invertebrates that are found in this habitat occur only in specific geographic areas. Due to local topography and geology, the pools are usually clustered into pool complexes (Holland and Jain 1988). Pools within a complex typically are separated by distances on the order of meters and may form dense, interconnected mosaics of small pools or a sparser scattering of larger pools. This species has a sporadic distribution within vernal pool complexes (Jones and Stokes, 1992, 1993; County of Sacramento 1990; Patton 1984; Stromberg 1933; Sugnet and Associates 1993b) wherein the majority of pools in a given complex typically are not inhabited by the species.
Although the vernal pool fairy shrimp has a relatively wide range, the majority of known populations inhabit vernal pools with clear to tea-colored water, most commonly in grass or mud bottomed swales, or basalt flow depression pools in unplowed grasslands, but one population occurs in sandstone rock outcrops and another population in alkaline vernal pools (Collie and Lathrop 1976). They are ecologically dependent on seasonal fluctuations in their habitat, such as absence or presence of water during specific times of the year, duration of inundation, and other environmental factors that include specific salinity, conductivity, dissolved solids, and pH levels. Water chemistry is one of the most important factors in determining the distribution of fairy shrimp (Belk 1977; Jamie King, University of California, in litt., 1992; Marie Simovich, University of San Diego, in litt., 1992). The water in pools inhabited by this species has low total dissolved solids (TDS), conductivity, alkalinity, and chloride (Collie and Lathrop 1976). The vernal pools the animal inhabits vary in size from over 10 ha to only 20 square meters. The vernal pool fairy shrimp occurs at temperatures between 6-20 degrees C in soft and poorly buffered waters (Eng et al. 1990).
PAST THREATS: This invertebrate species is imperiled primarily by habitat loss and modification. According to Holland (1978), there were an estimated 4.2 million acres (1.7 million hectares) in the Central Valley that possibly supported vernal pools at the time Europeans arrived in California. Holland estimated that between 67 and 88 percent of this acreage was destroyed by 1973, largely by human activities (Holland 1978). The rate of loss of vernal pool habitat in parts of California has been estimated to occur at approximately 2 or 3 percent per year (Holland 1988). The remaining populations of the vernal pool fairy shrimp were threatened by one or more of the following: commercial and residential development, conversion of land to agricultural uses, habitat fragmentation (which leads to the loss of genetic variability and related problems of inbreeding), off-road vehicle use, disposal of garbage into their habitat, water, flood control, highway, and utility projects, changes in the hydrologic patterns of their vernal pool and swale habitat, inadequate regulatory mechanisms that protect sites inhabited by these species, overgrazing and potential extinction by virtue of the small isolated nature of the remaining populations (Jones and Stokes Associates 1987). Habitat loss occurs from direct destruction and modification of pools from filling, grading, disking, leveling, and other activities. Vernal pools also are indirectly affected by modifications of surrounding uplands that alter the vernal pool watershed.
Off-road vehicle (ORV) use imperils vernal pool fairy shrimp inhabiting vernal pools (Bauder 1986, 1987). ORVs cut deep ruts, compact soil, destroy native vegetation, and alter pool hydrology. Fire fighting, security patrols, military maneuvers, and recreational activities have cumulatively damaged vernal pool habitats in many areas (Bauder 1986, 1987).
Other secondary impacts associated with urbanization include disposal of waste materials into the habitat of the vernal pool fairy shrimp (Bauder 1986, 1987). Disposal of concrete, tires, refrigerators, sofas, and other trash adversely affects these animals by eliminating habitat, disrupting pool hydrology, or, in some cases, through release of toxic substances. Dust and other forms of air or water pollution from commercial development or agriculture projects also may be deleterious to these animals.
Human activities that alter the watershed of vernal pools may indirectly affect the vernal pool fairy shrimp. Many of the plants and several of the aquatic invertebrates that occur in vernal pools are dependent upon specific hydrologic patterns that occur during wet winters followed by spring and summer drying. The flora and fauna in vernal pools or swales can change if the hydrologic regime is altered (Bauder 1986, 1987). Activities that reduce the extent of the watershed or that alter runoff patterns (i.e., amounts and seasonal distribution) may eliminate the animals, reduce their population sizes or reproductive success, or shift the location of sites inhabited by these animals.
Lastly, the vernal pool fairy shrimp may be vulnerable to random fluctuations or variation (stochasticity) due to annual weather patterns, availability of food, and other environmental factors. Most of the populations of this species are isolated from other conspecific populations and are distributed in discontinuous vernal pool systems. Such small, isolated populations are vulnerable to stochastic extinction. There are no known diseases affecting the vernal pool fairy shrimp.
CURRENT THREATS: All of the threats which led to the listing of the vernal pool crustaceans still threaten these species and their vernal pool habitat today. Vernal pool habitats continue to be imperiled by a variety of human-caused activities, primarily urban development and conversion of land to intensive agricultural use. Vernal pools also are subject to such threats as invasions of aggressive non-native plant and animal species, gravel mining, fertilizer and pesticide contamination, overgrazing, off-road vehicle use, and contaminated stormwater runoff (Goettle 1997). Illegal and unreported destruction of vernal pools continue to pose a significant threat to the crustaceans. Substantial acres of grasslands with vernal pool habitats supporting vernal pool crustaceans have been filled without authorization from the Army Corps of Engineers. These activities, plus other projects such as certain mosquito abatement measures and water supply and flood control projects, can lead to changes in water table depth, alterations to vernal pool hydrology, and water pollution.
Contamination of vernal pools from adjacent areas may injure or kill vernal pool crustaceans. Toxic chemicals, such as petroleum products, pesticides, herbicides, fertilizers and soap, may wash into vernal pools during development of adjacent areas. Contamination also may result from increased discharge of contaminants such as fertilizers and pesticides into surface waters from golf courses, irrigated agricultural lands, or landscaped residential areas (Petrovich 1990). Fertilizer contamination can lead to the eutrophication of vernal pools, which can kill vernal pool crustaceans by reducing the concentration of dissolved oxygen (Rogers 1998).
Despite protection of the vernal pool crustaceans in conservation areas established since the listing, available information suggests the distribution of these areas is not yet sufficient to provide for their recovery. The existing preserve configuration represents only a small percentage of the remaining vernal pool habitats, and probably is not adequate to ensure the long term viability of vernal pool crustacean populations. There are still substantial opportunities to improve the existing preserve design to include the range of the species, to include additional high quality habitat, and to augment the size and configuration of established preserves to further protect them from indirect effects of adjacent development.
The California Natural Diversity Database, maintained by the Natural Heritage Program of the California Department of Fish and Game (CDFG), estimates that four occurrences of the vernal pool fairy shrimp are threatened by pollution (CDFG 1999). At least 40 occurrences of vernal pool fairy shrimp are threatened by development (CDFG 1999).
Conversion of vernal pool habitats from grazing to intensive agricultural uses also threatens the vernal pool crustaceans throughout their ranges. Grazing lands typically support vernal pools, while other forms of agricultural uses, such as irrigated farmland, do not maintain vernal pool habitats. The CDFG (1999) estimates that 27 occurrences of vernal pool fairy shrimp are threatened by these agricultural conversions. However, although grazing lands may support vernal pools, they may not be managed appropriately to protect vernal pool crustaceans. In many cases, overgrazing may threaten vernal pool crustacean populations. The CDFG (1999) considers 52 occurrences of vernal pool fairy shrimp to be threatened by overgrazing.
The CDFG also estimates that nine occurrences of vernal pool fairy shrimp are threatened by erosion and subsequent siltation (CDFG 1999). Vernal pool crustaceans breathe through leaf-like lobes approximating gills, and may suffocate in pools with high degrees of siltation. Any ground-disturbing activities, such as plowing, grading, or construction-related activities, adjacent to or within the watersheds of vernal pools, can result in siltation when pools fill during the following wet season. Trail and road systems near vernal pools may also cause erosion and result in siltation of vernal pools. At least five populations of the vernal pool fairy shrimp are threatened by human-related disturbances (CDFG 1999).
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