Vernal Pool Tadpole Shrimp

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Vernal Pool Tadpole Shrimp

Lepidurus packardi

ListedSeptember 19, 1994
FamilyTriopsidae (Freshwater tadpole shrimp)
DescriptionLarge shield-like carapace that coversmost of the body, and a pair of long cercopods.
HabitatVernal pools containing clear to highly turbid water.
FoodDetritus, fairy shrimp, and other invertebrates.
ReproductionSix clutches of eggs totaling about 860 eggs in the female's lifetime.
ThreatsUrban development, agricultural conversion, sterilization by parasites.


Lepidurus packardi (vernal pool tadpole shrimp) adults reach a length of 2 in (50 mm). They have about 35 pairs of legs and two long cercopods. This species superficially resembles the ricefield tadpole shrimp (Triops longicaudatus ). However, Lepidurus possess a flat paddle-shaped supra-anal plate that is entirely lacking in members of the genus Triops.

Tadpole shrimp have dorsal compound eyes, a large shield-like carapace that covers most of the body, and a pair of long cercopods at the end of the last abdominal segment.


Tadpole shrimp climb or scramble over objects, as well as plow along in bottom sediments. Their diet consists of organic detritus and living organisms, such as fairy shrimp and other invertebrates. The females deposit their eggs on vegetation and other objects on the bottom. Vernal pool tadpole shrimp populations pass the dry summer months as diapaused eggs in pool sediments. Some of the eggs hatch as the vernal pools are filled with rain-water in the fall and winter of subsequent seasons.

The life history of the vernal pool tadpole shrimp is linked to the phenology of the vernal pool habitat. After winter rainwater fills the pools, the populations are reestablished from diapaused eggs that lie dormant in the dry pool sediments. Eggs found in one pool are hatched within three weeks of inundation and mature to sexually reproductive adults in another three to four weeks, although one scientist reported sexually mature adults occurred in another pool three to four weeks after the pools had been filled. A female surviving to large size may lay up to six clutches of eggs, totaling about 861 eggs in her lifetime. The eggs are sticky and readily adhere to plant matter and sediment particles. A portion of the eggs hatch immediately and the rest enter diapause and remain in the soil to hatch during later rainy seasons. The vernal pool tadpole shrimp matures slowly and is a long-lived species. Adults are often present and reproductive until the pools dry up in the spring.


The vernal pool tadpole shrimp inhabits vernal pools containing clear to highly turbid water, ranging in size from 54 sq ft (5 sq m) in the Mather Air Force Base area of Sacramento County, to the 89-acre (36-hectare) Olcott Lake at Jepson Prairie. The pools at Jepson Prairie and Vina Plains have a very low conductivity, TDS, and alkalinity. These pools are located most commonly in grass bottomed swales of grasslands in old alluvial soils underlain by hardpan or in mud-bottomed pools containing highly turbid water.


The vernal pool tadpole shrimp is known from 18 populations in the Central Valley, ranging from east of Redding in Shasta County south through the Central Valley to the San Luis National Wildlife Refuge in Merced County, and from a single vernal pool complex located on the San Francisco Bay National Wildlife Refuge in the City of Fremont, Alameda County. Fourteen of the 18 known populations of the vernal pool tadpole shrimp are imperiled, one of which is comprised of less than five pools. The report listed 3,092 "discrete locations" that contained 345 records of the vernal pool tad-pole shrimp.


Urban development and agricultural conversion imperil populations of the vernal pool tadpole shrimp in the San Joaquin valley. At least five pool complexes in the Central Valley of California that were known to contain suitable habitat for the vernal pool tadpole shrimp were eliminated by urban development in the late 1980s.

A 275-acre (111-hectare) site containing vernal pool and swale habitat for the vernal pool tadpole shrimp in the Jepson Prairie area in Solano County was destroyed by discing in October 1992.

Almond and fruit orchards in Stanislaus, Madera, and Fresno Counties continue to be planted in habitat suitable for the vernal pool tadpole shrimp.

In Solano County, an off-road vehicle park adjacent to the Jepson Prairie Reserve owned by the Nature Conservancy could adversely affect populations of the vernal tadpole shrimp.

A 370-acre (150-hectare) area around the Redding Municipal Airport lost 62% of its vernal pools during the period from 1952 to 1992, and 37% of the pools remaining are degraded. The vernal pool tad-pole shrimp has been observed in these pools. Continued residential development and growing Eucalyptus farming around the airport imperils the pools that remain.

In the Chico area, certain areas inhabited by the vernal pool tadpole shrimp recently were ditched and drained, and there are four new proposed developments for this area.

Because of rapid urbanization, several highway projects are proposed that may affect the vernal pool tadpole shrimp. Vernal pools containing this invertebrate in the Sacramento area, in Solano County, and Butte County may be damaged by highway widening and improvements.

The modification of vernal pool areas to create artificial reservoirs, such as the Modesto Reservoir and Turlock Lake in Stanislaus County, have led to the extirpation of the vernal pool tadpole shrimp population that was known to occur in the vernal pools where these reservoirs now lie.

The U.S. Army Corps of Engineers has proposed the Merced County Streams project that would facilitate urban development in many areas that provide suitable habitat for the vernal pool tadpole shrimp.

Introduction of the bullfrog (Rana catesbeiana ) to areas inhabited by the vernal pool tadpole shrimp appears to increase the threat of predation facing this crustacean. These amphibians are voracious predators on many species of native and exotic animals. Large numbers of vernal pool tadpole shrimp were found in stomach content analysis of bullfrogs captured in vernal pools in the Chico area. Although bullfrogs are unable to establish permanent breeding populations in vernal pools, dispersing immature males take up residence in these areas during the rainy season. A number of bullfrogs were observed at Jepson Prairie during the winter of 1992/1993.

Vernal pool tadpole shrimp were found to have been parasitized by flukes (Trematoda) of an undetermined species at the Vina Plains, Tehama County. The gonads of both sexes were greatly reduced in size and their body cavities were filled with many young flukes (metacercariae). Parasitic castration appears to be the major limiting factor affecting reproduction of the vernal pool tadpole shrimp at the Vina Plains. The range and extent of this parasite is unknown.

Conservation and Recovery

Some critical habitats of the vernal pool tadpole shrimp are in the San Luis National Wildlife Refuge, the San Francisco Bay National Wildlife Refuge, and in the privately owned Pacific Commons Project site adjacent to the Don Edwards San Francisco Bay National Wildlife Refuge. These habitats are being conserved. Other critical habitats are privately owned and imperiled by development and other threats. These critical habitats should be protected. This could be done by acquiring the habitats and establishing ecological reserves, or by negotiating conservation easements with the landowners. The populations of the vernal pool tadpole shrimp should be monitored, and studies made of its habitat needs.


U. S. Fish and Wildlife Service
Sacramento Fish and Wildlife Office
Federal Building
2800 Cottage Way, Room W-2605
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
(503) 231-6121


Goettle, B. 1997. "A "Living Fossil" in the San Francisco Bay Area?" U.S. Fish and Wildlife Service,

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Vernal Pool Tadpole Shrimp

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