San Diego Fairy Shrimp

Branchinecta sandiegonensis

Description

Adult male San Diego fairy shrimp, Branchinecta sandiegonensis, range in length from 0.4-0.6 in (9-16 mm) and the females are 0.4-0.5 in (8-14 mm) long. Mature individuals have a delicate elongate body, large stalked compound eyes, no carapace covering the back, 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 front-to-back direction. Nearly all species of fairy shrimp feed on algae, bacteria, protozoa, rotifers, and bits of organic matter. The second pair of antennae in adult female San Diego fairy shrimp are cylindrical and elongate, but in the males they are greatly enlarged and specialized for clasping the females during copulation. The females carry their eggs in an oval or elongate ventral brood sac. Five other species of branchinectid fairy shrimp occur in southern California. The only other branchinectids in southern California that are similar in appearance to the San Diego fairy shrimp are Lindahl's fairy shrimp (B. lindahli ) and the threatened vernal pool fairy shrimp (B. lynchi ), which occurs in southwestern Riverside County. Male San Diego fairy shrimp can be distinguished from males of other Branchinecta species by the shape of the second antenna. Female San Diego fairy shrimp are distinguishable from other members of the genus by the shape and length of the brood sac and by the presence of paired dorsolateral spines on five of the abdominal segments. The San Diego fairy shrimp, a member of the aquatic crustacean order Anostraca, was recognized as a distinct taxon by scientists in 1990, although it was not described taxonomically until 1993.

Behavior

Adult San Diego fairy shrimp are usually observed from January to March; however, in years with early or late rainfall, the hatching period may be extended. The species hatches and matures within 7 days to 2 weeks depending on water temperature. The San Diego fairy shrimp disappear after about a month, but shrimp will continue to hatch if subsequent rains result in additional water or refilling of the vernal pools. The eggs are either dropped to the pool bottom or remain in the brood sac until the female dies and sinks. The resting, or summer, eggs are capable of withstanding heat, cold, and prolonged drying. When the pools refill in the same or subsequent rainy seasons, some but not all of the eggs may hatch. Fairy shrimp egg banks in the soil may be comprised of the eggs from several years of breeding.

Habitat

The San Diego fairy shrimp is a habitat specialist occurring only in small, shallow vernal pools, which range in depth from 2-12 in (5-30 cm) and in water temperature from 50-68°F (10-20°C). Water chemistry is one of the most important factors in determining the distribution of fairy shrimp. The San Diego fairy shrimp appears to be sensitive to high water temperatures; present data indicates that pool temperatures below 41°F (5°C) and above 86°F (30°C) represent the natural limits for this species. This means that pools located in the inland mountains and in desert regions may be out of this range.

Distribution

The species was first collected in 1962 in San Diego County at Poway and Ramona; it was first described B. sandiegonensis based on collections made at Del Mar Mesa in San Diego County. The species is restricted to vernal pools in coastal southern California south to extreme northwestern Baja California, Mexico. No individuals have been found in riverine waters, marine waters, or other permanent bodies of water. All known localities are below 2,300 ft (700 m) in altitude and within 40 mi (65 km) of the Pacific Ocean, from Santa Barbara County south to northwestern Baja California.

Threats

The San Diego fairy shrimp is imperiled because its vernal pool habitat is being damaged, fragmented, and destroyed by a variety of human-caused activities: urban development and agricultural conversion, water development and flood control projects, highway and utility construction, modifications of surrounding uplands that alter vernal pool hydrology, off-road vehicle activity, and livestock overgrazing. Urban development and agricultural conversion have been the primary causes of habitat destruction. Habitat loss occurs from destruction and modification of vernal pools due to filling, grading, discing, leveling, and other activities, as well as any activity that alters vernal pool watersheds. High livestock densities may result in excessive physical disturbances analogous to vehicle damage, such as trampling and rutting, resulting in altered pool water chemistry and degraded water quality. Trampling of pool margins and thinning of vegetation from overgrazing may increase pasture runoff, leading to erosion and increased siltation of vernal pool habitat.

The continuing rapid urbanization of areas containing vernal pools poses a significant threat to the San Diego fairy shrimp. Nearly all of the vernal pools that occurred throughout the range of the species from southern Santa Barbara County to extreme northwestern Baja California have been eliminated: 838 vernal pools comprising 698 acres (283 hectares) were eliminated by urban development between 1979 and 1986. San Diego County, where most of the remaining vernal pools are, is one of the fastest-growing counties in the nation, with a population increase of 349% between 1950 and 1990. The population growth rate that is predicted could further fragment and degrade the remaining vernal pool habitat of the San Diego fairy shrimp.

There remain only a modest number of very small and widely scattered vernal pools that are fit habitat for the San Diego fairy shrimp. The San Diego County locations are in Tijuana Slough National Wildlife Refuge, Proctor Valley, Otay Mesa, Otay Lakes, Sweetwater Reservoir, Mission Trails County Park, Kearney Mesa, Del Mar Mesa, Lopez Ridge, Mira Mesa, Carlsbad, Marine Corps Base Camp Pendleton, Poway, Ramona, and San Marcos. The largest remaining number of vernal pools and the largest block of contiguous habitat occurs on Miramar Naval Air Station.

Approximately 65 acres (26 hectares) of vernal pools are here, 70% of the extant total. These pools exhibit a wide variety of conditions from disturbed to pristine, and vary greatly in size, depth, type and number of cobbles, soil type, hydrological characteristics, and species composition. The San Diego fairy shrimp has been estimated to inhabit 80% of the vernal pools at the base. The Marine Corps has proposed construction of additional helicopter landing fields, ammunition bunkers, and other facilities that may adversely affect areas containing habitat for the San Diego fairy shrimp. The vernal pools at Montgomery Field occur within the approach path of the airport in a heavily urbanized area. Three separate areas of airport land encompass the watershed containing 138 vernal pools

The construction of a sludge processing facility and mounding of excess dirt at the Miramar Landfill, as well as on-going landfill maintenance have eliminated vernal pools inhabited by the San Diego fairy shrimp. The proposed extension of Nobel Drive would damage or eliminate the vernal pools containing habitat for the species.

The San Diego fairy shrimp is especially vulnerable to alterations in hydrology. Its vernal pool habitat is also vulnerable to indirect destruction due to the alteration of supporting watersheds. Development projects adjacent to vernal pools are often responsible for adverse alterations in drainage. Hydrological alterations can result from urban or agricultural development or a combination of these activities. An increase in water due to urban runoff leads to increased inundation, making the pools vulnerable to invasion by marsh plant species that outcompete obligate (restricted to) vernal pool taxa, resulting in decreased abundance of obligate vernal pool taxa. At the other extreme, some pools have been drained or blocked from their source of water and have shown an increased domination by upland plant species. Alterations in vernal pool hydrology may adversely impact the San Diego fairy shrimp due to changes in the maximum and minimum water temperatures. At least three of these parties likely intended to alter the elevations of the site to eliminate one or more of the parameters used by the U. S. Army Corps of Engineers to define a wetland according to their 1987 jurisdictional manual. Similar deliberate activities that are damaging or destroying vernal pools are likely occurring throughout the range of the San Diego fairy shrimp.

The primary threat to the San Diego fairy shrimp of habitat loss due to human activities is reinforced by another factor: extraordinary increases in the human population and associated pressures from urban development have rendered existing regulatory mechanisms inadequate.

No state or local laws exist that adequately protect the San Diego fairy shrimp, and other regulations aimed at the conservation of its vernal pool habitat have also proven ineffective and insufficient.

Secondary impacts associated with urbanization also pose a very significant threat to the continued existence of this species. These include disposal of waste materials, trash, and toxic substances into habitat for the San Diego fairy shrimp. Solid waste can disrupt pool hydrology; and malathion, herbicides, laundry detergent, household plant fertilizer, and motor oil are known to be fatal to the San Diego fairy shrimp through outright poisoning or by the formation of an asphyxiating barrier to gas exchange on the vernal pool surfaces. Dust and other forms of air or water pollution from commercial development or agricultural projects may also be injurious to the shrimp. Off-road vehicles crush fairy shrimp eggs, cut deep ruts over dried pool beds, compact soil, destroy native vegetation, and alter pool hydrology.

Fire fighting activities, security patrols, military maneuvers, and recreational activities have cumulatively damaged vernal pool habitats in many areas within the range of the species. Livestock grazing can ultimately lead to pool siltation and trampled pool margins that compromise water quality.

Conservation and Recovery

The continued survival and recovery of the San Diego fairy shrimp can only be assured at this time by the preservation and enhancement of extant vernal pools and their associated watersheds.

Contacts

Regional Office of Endangered Species
U. S. Fish and Wildlife Service
Eastside Federal Center
911 N. E. 11th Ave.
Portland, Oregon 97232
http://pacific.fws.gov/

Carlsbad Fish and Wildlife Office
2730 Loker Ave. W.
Carlsbad, California 92008-6603
Telephone: (760) 431-9440
Fax: (760) 431-9624

References

U.S. Fish and Wildlife Service. 1997. "Determination of Endangered Status for the San Diego Fairy Shrimp." Federal Register 62(22):4925-4939.