Riverside Fairy Shrimp

Streptocephalus woottoni

Description

The Riverside fairy shrimp is a small freshwater crustacean. Both males and females have a red color covering all of the ninth and 30-40% of the eighth abdominal segments. Mature males are 0.56-0.92 in (1.4-2.3 cm) in length. The frontal appendage is cylindrical, double-lobed at the tip, and extends only part way to the distal end of the basal segment of the antenna. The spur of the thumb is a simple blade-like process. The finger has two teeth; the proximal tooth is shorter than the distal tooth. The distal tooth has a lateral shoulder that is equal to about half the tooth's total length measured along the proximal edge. The brood pouch extends to abdominal segments seven, eight, or nine. The cercopods are as in the male. Both males and females have the red color of the cercopods covering all the ninth abdominal segment and some of the eighth abdominal segment.

Behavior

The species begins life from resting eggs from which they are hatched into a larvae stage after which there are dozens of molts before an individual reaches maturity. Even as adults, they continue to molt throughout their lives.

The species is for the most part a filter-feeder. They feed on plankton and algae. Yet, they also may eat particles ranging in size from algae to small crustaceans. Thus, the species is omnivorous.

This species most likely shows seasonal variations in activity related to reproduction and changes in temperature regime.

Habitat

The species occurs in deep, cool pools and occasionally in depressions, such as road ruts and ditches.

Distribution

The species is known from four vernal pools in a 37 sq mi (96 sq km) area in southwestern Riverside County, California, and from one population in Orange County, California. In San Diego County in the fall of 1989, the species was discovered within vernal pools on Miramar Naval Air Station and Otay Mesa. However, since the 1989 discovery of the species in San Diego County, numerous vernal pool complexes in the county have been surveyed without additional populations being found. The species was also found at two locations in Baja California, Mexico in the past.

Threats

The habitat and range of this species has been greatly reduced. Vernal pools, existing as slight depressions on flat mesas, are found in locations that are especially vulnerable to one or more of the following habitat disturbances: urban and agricultural development, off-road vehicle use, cattle trampling, human trampling, road development, military activities, and water management activities. Many pool groups were entirely eliminated and replaced with urban or agricultural developments.

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 pool 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.

Pools have also been degraded due to the use of off-road vehicles, which have impacted the habitats of this species. These vehicles 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 mud puddles. Thus, pools are gradually destroyed by vehicles traveling on dirt roads. Vehicle access and damage has occurred on virtually all remaining vernal pool complexes.

Preliminary designs by the California Department of Transportation for a state route running near this species' habitat include alignments that sever the existing natural connection between two of the largest remaining vernal pool complexes on Otay Mesa. The construction of this new major highway access route into Otay Mesa would further facilitate its development.

An existing local airport is presently being evaluated as a potential site for an international airport servicing San Diego. This proposal includes alternative runway alignments that would destroy portions of one of the two largest remaining vernal pool complexes. A binational airport is also being considered for Otay Mesa, although these plans are too preliminary to allow assessment of potential impacts to vernal pools. An increase in the number of vehicle trips in this area would occur as a result of the airport, and this increased traffic would likely lead to a demand for more roads, which could directly impact the pools.

Habitat trampling, and in some cases trampling of the species itself, due to livestock grazing, occurs on Otay Mesa in areas where several vernal pool complexes collectively contain all four of the proposed species. 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 U.S. Immigration and Naturalization Service has proposed several projects at the international border, including border lighting, that could result in direct adverse impacts to vernal pools on Otay Mesa, due to construction activities.

The species has very narrow habitat requirements. It is only found in deep lowland pools that retain water through the warmer weather of late spring. It will not hatch in pools that receive cool waters from early winter rains, such as those pools on the Santa Rosa Plateau, nor will they hatch in shallow pools.

It is vulnerable to land use changes affecting the small number of pools that meet the species' strict habitat requirements. Of the four remaining pools supporting the species in Riverside County, only one is greater than one acre in size. This pool is within a planned development. Other sites supporting the species may lack some of the topical vegetation of vernal pools, but that condition probably reflects impacts from past agricultural activities. One pool is located within an approved tract for a housing development.

A third pool is on a parcel that is currently proposed for a housing development, adjacent to a golf course. This pool is in an agricultural field and was disced. The Environmental Impact Report prepared by a consultant for the developer of this project failed to acknowledge the existence of the species on the site. Representatives of the landowner expressed a willingness to offer some protection for this site. However, as discussed above, a currently proposed road project would impact the pool.

A fourth pool that contains this species is located partially on private land and partially on an Indian reservation. The portion on private land was cultivated during 1990. The region's drought conditions over the last two to three years may have rendered the pool dry enough to be plowed. A fifth pool was recently converted into a gravel pit. Only one documented population occurs in Orange County.

Other factors have greatly impacted the existence of this species, including introduction of non-native plant species, competition with invading species, trash dumping, fire, fire suppression activities, and drought. The low numbers of vernal pool habitats remaining and their scattered distributions make this species vulnerable to extinction due to future events that are unpredictable, human, or naturally caused.

Many vernal pools on Otay Mesa are dominated by non-native plants such as the common grass Lolium perenne. This species is tolerant of inundation and crowds out the native vernal pool species. Ranchers introduced non-native species into some areas to increase the amount of forage available to livestock. Excessive cover of weedy non-native grasses was noted in six of the pool groups and partially explained two extirpations of P. nudruscula.

Trash dumping also degrades vernal pools. Chunks of concrete, tires, refrigerators, sofas, and other pieces of garbage or debris were found in pools containing this species. This trash crushes or shades vernal pool plants, disrupts the hydrologic functions of the pool, and in some cases may release toxic substances.

Conservation and Recovery

A Recovery Plan has been released for the Riverside fairy shrimp. This crustacean is only known from habitats on privately owned land, all of which are severely threatened by various human activities. The largest of the privately owned habitats should be protected. This could be done by acquiring the land and establishing ecological reserves, or by negotiating conservation easements with the landowners. The known populations of the Riverside fairy shrimp should be monitored, and research undertaken into its biology and habitat needs.

Contacts

U. S. Fish and Wildlife Service
Sacramento Fish and Wildlife Office
Federal Building
2800 Cottage Way, Room W-2605
Sacramento, California 95825-1846
Telephone: (916) 414-6600
Fax: (916) 460-4619

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

References

Thorp, James H., and Alan P. Covich, eds. 1991. Ecology and Classification of North American Freshwater Invertebrates. New York: Academic Press. pp. 765-769.

U.S. Fish and Wildlife Service. 3 Aug. 1993. "Endangered and Threatened Wildlife and Plants; Endangered Status the Riverside Fairy Shrimp." Federal Register 58(147): 41384-41391.

U. S. Fish and Wildlife Service. 1998. "Recovery Plan for Vernal Pools of Southern California." U.S. Fish and Wildlife Service, Portland.