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Fairy Shrimp

Fairy Shrimp of the Merced Vernal Pools and Grassland Reserve
(Information Sources: US Fish and Wildlife Service Endangered Species Program; D.Christopher Rogers, 2009. Branchiopoda (Anostraca, Notostraca, Laevicaudata., Spinicaudata, Cyclestherida); Encyclopedia of Inland Waters, Vol 2, pgs. 242-249); Wildlife and Rare Plant Ecology of Eastern Merced County’s Vernal Pool Grasslands. 2002. John Vollmar, editor. Vollmar Consulting, Berkeley, Calif.;  Fairy Shrimp of California's Puddles, Pools, and Playas. 1999. Clyde Ericksen and Denton Belk. Mad River Press, Arcata, Calif.).
Fairy shrimp are small crustaceans (related to crabs, lobsters and krill) in the class Branchiopoda, order Anostraca. These are considered among the most primitive of living crustaceans. Fossil remains which look quite similar to modern day fairy shrimp have been discovered in marine sediments that date to the Devonian Period about 400 million years ago.  California hosts 25 species of fairy shrimp. Most are endemic (found only in) to vernal pools - small ephemeral wetlands that fill seasonally with rainwater but are dry most of the year.  Pools that support shrimp are devoid of fish, for fish would quickly gobble up the shrimp.
Fairy shrimp have delicate elongate bodies, stalked compound eyes, no carapace, and eleven pairs of swimming legs. They glide gracefully upside down, swimming by legs that beat in a wavelike motion that passes from the front to the back of the body. The legs are multi-tasking appendages - they're used for swimming, feeding, and breathing. The shrimp range in size from a few millimeters to 17 cm, but most are about 1.0 to 5.0 cm in length.
Most fairy shrimp are omnivorous filter feeders, filtering small particles from the water, including  algae, bacteria, protozoa, rotifers and detritus. They don't discriminate in prey selection; all food items of a suitable size are taken in.
The life cycle of fairy shrimp is fairly simple. When pools fill with winter rains, the cysts ("resting" eggs) from previous years' reproductive effort hatch and a very tiny nauplii emerges. The "baby" shrimp grows quickly, undergoing a serious of molts in which it gains a new exoskeleton with each molt until it develops into a sexually mature adult. This process is fast - from one to three weeks.  Adults live for only one season; none survive to live a second season. During their brief life the adults swim, eat, mate, lay eggs, and die. Toward the end of their life, females produce thick-shelled cysts. The cysts are so tiny they can't be seen with the naked eye. During late spring and summer as the pools dry, these cysts become embedded in the dried bottom mud, and during the winter they may freeze for varying periods. They lay dormant, but viable, in the mud for decades and possibly centuries. Cryptobiosis describes this unusual condition: a state of life entered by an organism in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all metabolic processes stop, preventing reproduction, development, and repair. Yet the organism is alive! Cysts hatch when the rains come again in winter or early spring of the following year. If a drought hits, they may not hatch for several years. When conditions are right and hatching takes place, only a small fraction of viable cysts in a pool actually hatch. The majority of cysts rest - quiesent - waiting so it seems until it is "their" turn to hatch. This "bet hedging" adaptation ensures that popuations persist through time and that they don't die out if pools were to fill and dry too quickly before reproduction could occur. It appears that prior freezing and/or drying is a necessity for cyst hatching. Therefor, long term population survival is accomplished through the production of cysts, which are very resistant to long periods of desiccation, heat, and anoxic conditions. Cysts are even resistant to stomach acids, such as those of migratory birds which might ingest the cysts while feeding in vernal pools. Cysts carried aboard the U.S. Space Shuttle were subjected to the vacuum of space and this did not affect their viability. According to a study carried out by Hildrew of vernal pools in Kenya, cyst density can be as high as 9,600 per square meter!

According to Denton Belk and Michael Fugate, California is home to 43% of the total North American fairy shrimp fauna (24 of 56 species). Of the species that have narrow geographical ranges, those with a known area of occurrence 100,000 sq km or less, 38% live in California (11 of 29 species).

Many Fairy Shrimp species are now federally protected largely because the habitat they depend on – vernal pools -  have been lost to agriculture and development. Holland (1978) estimated that roughly 1,600,000 hectares (4,000,000 acres) of vernal pool habitat existed in the Central Valley during pre-agricultural times. Only a small fraction of these pools remain today.  Based on careful mapping, only 5% of the vernal pools in California present at the time of European contact are still in existence today.  Because fairy shrimp depend on vernal pools, if this habitat is threatened so are the shrimp. Researchers wishing to study or handle fairy shrimp must have federal and state permits, have many hours of field experience, and must pass a rigorous identification exam.

Four Fairy Shrimp, one tadpole shrimp, and a clam shrimp are found in the Merced Vernal Pools and Grassland Reserve:
  1. Conservancy Fairy Shrimp (Branchinecta conservatio). Endangered.
  2. Vernal Pool Fairy Shrimp (Branchinecta lynchi). Threatened.
  3. Mid-Valley Fairy Shrimp (Branchinecta mesovallensis). Not listed at this time.
  4. California Fairy Shrimp (Linderiella occidentalis). Not listed.
  5. Vernal Pool Tadpole Shrimp (Lepidurus packardi). Endangered.
  6. California Clam Shrimp (Cyzicus californicus). Not listed.
  • The Conservancy Fairy Shrimp (Branchinecta conservatio), Mid-Valley Fairy Shrimp (B. mesovallensis) and Vernal Pool Fairy Shrimp (B. lynchi) are in the family Branchinectidae.
  • The California Fairy Shrimp, (Linderiella occidentalis) is in the family Linderiellidae.
  • The Vernal Pool Tadpole Shrimp (Lepidurus packardi) is in the family Triopsidae.
  • The Clam Shrimp (Cyzicus californicus) is in the family Cyzicidae.
The Conservancy Fairy Shrimp (Branchinecta conservatio) is one of the rarest species of fairy shrimp in California.  This species is currently known only from several disjunct populations: the Vina Plains in Tehama County, south of Chico in Butte County; the Jepson Prairie Preserve and surrounding area in Solano County; the Sacramento National Wildlife Refuge Complex in Glenn County; Mapes Ranch west of Modesto; San Luis National Wildlife Refuge, the Flying M Ranch, and UC Merced Reserve in Merced County; and two locations on the Los Padres National Forest in Ventura County. It is found in our area in large, clay-bottomed, turbid pools associated with Mehrten geological formations. This species co-occurs in pools with the endemic, endangered vernal pools grasses, Colusa grass (Neostafia colusana) and San Joaquin Orcutt grass (Orcuttia inaequalis).
Photo by Dwight Harvey
The Conservancy Fairy Shrimp is one of the largest fairy shrimp found in the Central Valley. They range in size from about 1.25 cm (1/2 inch) to 2.5 cm (1 inch) long. It is quite easy to identify this species from similar shrimp because of the female’s elongate, spindle-shaped brood sac. It has a relatively long maturation period (36 days) and reproduction period (46 days). It has been observed in vernal pools from November to early April. The recorded overall longevity of the population within a vernal pool is 114 days, as measured from first hatching to the death of the last individual within a pool (Helm 1998).
The Vernal Pool Fairy Shrimp (Branchinecta lynchi) occurs only in cool-water pools. Individuals hatch from cysts during cold-weather winter storms; they require water temperatures of 50oF or lower to hatch. Time to maturity and reproduction ranges from 18 to 147 days (mean of about 40 days), and depends on water temperature. It has the widest geographical distribution of any federally-listed fairy shrimp species in California. The geographical range extends from disjunct locations in Riverside County and the Coast Ranges, north through Central Valley grasslands to Tehama County, and then to a disjunct area of remnant vernal pool habitat in the Agate Desert of Oregon. There are about 220 known occurrences in California.
In general, the vernal pool fairy shrimp has a sporadic distribution within a vernal pool complex, with most pools being uninhabited by the species. Helm (1998) found vernal pool fairy shrimp in 16.3% of pools sampled across 27 counties, while Sugnet and Associates, in 1993, found the species in only 5% of 3,092 locations sampled over much of the range. In one relatively small locale, the 2000-acre University of California Merced Planning Area (prior to campus construction), the species is documented from almost 90% of pools, although abundance of shrimp within pools was not recorded (Jones and Stokes 2007). The species is typically associated with smaller and shallower vernal pools (typically about 6 inches deep) that have relatively short periods of inundation (Helm 1998) and relatively low to moderate total dissolved solids (TDS) and alkalinity. It often co-occurs with other fairy shrimp species, but is never the numerically dominant species.
The Mid-Valley Fairy Shrimp (Branchinecta mesovallensis) was very recently described, in 2000 by Belk and Fugate. The type locality where the first specimens were collected is on the Merced Vernal Pools and Grassland Reserve (formerly the Virginia Smith Trust lands)! Specimens had been collected as early as 1989. The species is named for its limited range in the Central Valley of California. This species was proposed for federal listing, but at this time it has no special protection. The reasoning is that the vernal pools where this species occurs also contain fairy shrimp species that are listed so the species's habitat would already protected.
Male Mid-Valley Fairy Shrimp are very similar in appearance to Conservancy Fairy Shrimp (Belk and Fugate 2000) and only trained experts can distinguish these species. These two species are distinguished by the subtle differences in the shape of the tip of their antennae. The Mid-Valley Fairy Shrimp's antennae are bent such that the larger of the two humps possessed by both species is anterior, whereas the larger hump is posterior in the Conservancy Fairy Shrimp. Females of these two species differ in the shape of their brood pouches.
Mid-Valley Fairy Shrimp are endemic to a small portion of California’s Central Valley. Helm (1998) found them in less than 0.5% of the vernal pools he examined. Based on the few known occurrences, the species’ distribution is apparently limited to southeastern Sacramento, the southern Sierra foothills, and San Joaquin and Solano-Colusa vernal pool regions. In the Southeastern Sacramento region, most occurrences are clustered around the City of Sacramento and Mather Air Force Base in Sacramento County. In the Southern Sierra Foothills and San Joaquin Vernal Pool Regions, the Mid-Valley Fairy Shrimp has been documented in the vicinity of the Virginia Smith Trust property in Merced County and from
isolated occurrences in San Joaquin, Madera, and Fresno Counties. However, because this species was described only recently, it is likely additional occurrences will be found in the future.
B. mesovallensis
The Mid-Valley Fairy Shrimp inhabits small, short-lived vernal pools and grass-bottomed swales ranging from 4 to 663 square feet (0.37 to 61.6 sq m) in area and averaging less than 4 inches (10 cm) in depth (Helm 1998).

The Mid-Valley Fairy Shrimp is adapted to habitats that are inundated for short periods and they can complete their life cycle (cyst to adult with fertilized eggs) in as little as 4 days, especially under extreme circumstances, such as years with below-average rainfall (D.C. Rogers, in prep.).  The ability to rapidly complete its life cycle allows the Mid-Valley Fairy Shrimp to use habitats that are extremely hydrologically unstable (i.e., fill and dry quickly).

The species has been collected from pools on a volcanic mudflow landform of the Merhten Formation in Pentz Gravelly Loam and Raynor Clay soils. The Mid-Valley Fairy Shrimp has also been found on San Joaquin Silt Loam soils on the Riverbank formation on Low Terrace landforms. They have only been collected with one other fairy shrimp, the Vernal Pool Fairy Shrimp (Eriksen and Belk 1999). They may occupy habitats that are not inundated long enough for other species to inhabit.

Of the 53 Mid-Valley Fairy Shrimp occurrences in the California Natural Diversity Data Base (2003), roughly 19 (36%) are directly threatened by proposed development projects, while 22 (41.5%) are on protected lands. The protected lands include two National Wildlife Refuges, several vernal pool mitigation banks, a California Department of Fish and Game ecological reserve, and several Nature Conservancy conservation easements. Sacramento and Merced Counties have the most threatened occurrences, with seven and five, respectively. Threats in Sacramento County mostly involve urban development projects, while the primary threat in Merced County was the proposed construction of the University of California, Merced, campus. These lands are now protected within the Merced Vernal Pools and Grassland Reserve (UC Merced). Merced County also has the highest number of protected occurrences, with a total of 14 occurrences located on lands that have been set aside for the conservation of vernal pool species.  These lands function as conservation areas to offset the direct, indirect, and cumulative effects of the new university campus. Three ranches containing conservation easements held by The Nature Conservancy (totaling about 9,900 hectares [24,500 acres]) contain known Mid-valley Fairy Shrimp sightings. The easements are permanent, are managed by The Nature Conservancy, and cannot be extinguished by selling the land to a new owner.
The California Fairy Shrimp (Linderiella occidentalis) is the most widely distributed fairy shrimp in California.  The ubiquitous nature of its occurrence across the state means that this fairy shrimp receives no special federally protected status. This species is smaller than those in the family Branchinectidae and is the smallest species in California. Male California fairy shrimp are about  0.9 cm (1/3 inch) long, and females are about 1.0 cm (1.2 inch) in length.  It is also distinguished by its red eyes. The California Fairy Shrimp occurs in a wide range of vernal pool habitats in the Central Valley of California. It is likely that the historical distribution of this species coincides with the historical distribution of Central Valley vernal pools.  The California Fairy Shrimp occurs in the vicinity of Jepson Prairie in the Solano-Colusa Vernal Pool Region. In the Central Coast Vernal Pool Region the California fairy shrimp occurs on private property and at Fort Ord and Fort Hunter Liggett in Monterey and San Benito Counties. In the San Joaquin Vernal Pool Region the California Fairy Shrimp is known from the Grasslands Ecological Area in Merced County and from a single occurrence in Stanislaus County. In the Southern Sierra Foothills Vernal Pool Region the species is known from the Big Table Mountain Preserve and private land in Fresno County, from Bureau of Reclamation and private lands in Madera County, and from a few scattered locations on private land in Merced, and Stanislaus Counties. According to the California Natural Diversity Database (2005), 42 occurrences of California Fairy Shrimp are threatened by development, and 13 occurrences are threatened by agricultural conversion.
The California Fairy Shrimp is uniquely adapted to the astatic (temporary, ephemeral) conditions of vernal pool habitats. This species is the longest lived of the Central Valley fairy shrimp species (Eriksen and Belk 1999). Helm (1998) found that the California Fairy Shrimp required a minimum of 31 days and an average of 43 days to reproduce, and was observed to live as long as 168 days. California Fairy Shrimp eggs can hatch when temperatures drop below 20 C (68 F), although optimum hatching may occur at 10 C (50 F) (Eriksen and Belk 1999). The California Fairy Shrimp may have relatively small clutch sizes. Dodds (1923) reported that brood pouches he examined never contained more than six eggs. They have been observed in pools with 4 to 16 week durations, and mortality was caused by pool drying (Gallagher 1996). When pools almost dried, Gallagher (1996) observed California Fairy Shrimp surviving in the pool bottoms, suggesting they may be tolerant of high temperatures and low levels of dissolved oxygen.
Helm (1998) found the California Fairy Shrimp in pools ranging in size from 1 to 52,500 square meters (from 10.8 square feet to13 acres). Other studies have also documented them in vernal pools ranging widely in size (Syrdahl 1993, Alexander and Schlising 1997). However, the California Fairy Shrimp tends to be in deeper pools (Platenkamp 1998). They are tolerant of a wide range of water temperatures, and have been found in pools with temperatures from 5 to 29.5 C (41 to 85 F) (Syrdahl 1993). They are often found in pools with clear to turbid water with pH ranging from 6.1 to 8.5, low (13 to 170 ppm) alkalinity and low (33 to 273 ppm) total dissolved solids (Eng et al. 1990, Syrdahl 1993, Eriksen and Belk 1999). California Fairy Shrimp have been found in vernal pools ranging in elevation from 10 to 1,159 meters (30 to 3,800 ft.) above sea level (Eriksen and Belk 1999).
The Vernal Pool Tadpole Shrimp (Lepidurus packardi) is quite different from fairy shrimp, though it is often placed in this general group. They can reach a length of 5 cm. The shield-like covering called a carapace makes it appear as a tiny version of the much larger horseshoe crab (actually an arachnid and not a crustacean) that is found along mid-Atlantic ocean beaches and estuaries (especially Delaware Bay). This species has been called an ecosystem engineer because it causes bioturbation producing so much turbidity when it digs through pool sediments that it may alter the ecology of its pool habitat by reducing plant cover.
The Tadpole Shrimp is found in the Central Coast, Sacramento Valley, San Joaquin Valley, and southern Sierra Nevada foothills. The southeastern Sacramento Valley contains about 15% of the remaining vernal pool grassland habitat in the state, and it has about 35% of the known occurrences. 28% of all occurrences are in Sacramento County, California. Other areas with Tadpole Shrimp include the Don Edwards San Francisco Bay National Wildlife Refuge, several preserves in Tehama County, and the vicinity of the cities of Chico, Redding, and Red Bluff. It has been observed at the Sacramento National Wildlife Refuge, Travis Air Force Base, and the Jepson Prairie Preserve. In the San Joaquin Valley it has been noted at the Merced and San Luis National Wildlife Refuges. There are occurrences in the Sierra foothills region in Tulare, Fresno, Merced, and Stanislaus Counties. It also occurs outside California in the Agate Desert of Oregon.
The construction of the UC Merced campus in its original location would have drastically altered the vernal pool habitat in the area. The conservation of over 20,000 acres (8,100 ha) at UC Merced now means that much critical habitat for this species is permanently protected.
The California Clam Shrimp (Cyzicus californicus) looks like a small clam. But it’s not! It is a filter feeding crustacean just like its fairy shrimp relatives. The Clam Shrimp has two shells covering  its body. Fourteen pairs of appendages that look like legs protrude from the shells. The appendages help the Clam Shrimp swim, but they're also used for collecting food and getting dissolved oxygen from the water. The California Clam Shrimp flat,  yellow to brown, and about the size of a dime when fully grown (see photo of 5 in the hand).
Clam Shrimp Photo by Frank PBurka
There are more than 100 species of Clam Shrimp alive today.  All of them are very small (3 to 15 mm in size), live in freshwater pools and have very short lifespans (a week to two months). Clam Shrimp are found in a variety of natural, and artificial, seasonally ponded habitat types including: vernal pools, swales, ephemeral drainages, stock ponds, reservoirs, ditches, backhoe pits, and ruts caused by vehicular activities. They can be exceedingly abundant in cattle stock ponds in the Reserve.
The fossil record of clam shrimp is extensive and deep goes back to the Devonian period (350 to 400 million years old).  While very common they are not ubiquitous in the fossil record. Rather clam shrimp fossils are found almost exclusively in rock that has formed terrestrially rather than in the ocean.  The plants and other animals found fossilized with these clam shrimp strongly point to these clam shrimp living in shallow freshwater pools rather than in the ocean or even large freshwater bodies.