Vernal pool tadpole shrimp (Lepidurus packardi)

GenusLepidurus (1)
SizeAdult length: 1.5 – 8.4 cm (2)

Classified as Endangered (EN) on the IUCN Red List (1).

As an inhabitant of temporary pools of water, also known as ‘vernal pools’, this tiny, tadpole-like creature has an intriguing life history, and is considered a ‘living fossil’, having maintained its well-adapted body characteristics for millions of years (3).

It has a large, shield-like shell (carapace) attached to the head region, which covers most of the 30 to 35 pairs of phyllopods (2) (4). The abdomen protrudes beyond the carapace and ends in two long, thin tails, between which is a short paddle-like structure (3) (4). A fused pair of eyes is located on the top of the carapace (3). Tiny ovisacs (egg-containing capsules) attached to the female’s 11thpair of phyllopods are the only external feature that can be used to distinguish between the sexes (4).

In highly turbid water the vernal pool tadpole shrimp appears nearly translucent, or a buff colour with brown mottling. In clear water it is light to dark green with brown mottling or brown with green mottling (5), providing camouflage against aquatic plants or when burrowed horizontally in muddy sediment (3).

The vernal pool tadpole shrimp is found only in the Central Valley of California, USA. Its range extends from Shasta County in the north to Merced County in the south (3).

This freshwater crustacean inhabits temporary pools, which fill during the rainy season, over winter and spring, and are typically dry for the remaining seven to eight months of the year (3). The pools contain clear to highly turbid water, ranging between temperatures of 10 to 29 degrees Celsius (2).  

The vernal pool tadpole shrimp beats its phyllopods in a wave-like motion to propel itself through the water. As it moves over the sediment or through aquatic vegetation (4), the phyllopods collect food which is funnelled to the mouth through a groove at the base of the phyllopod (3). Adult vernal pool tadpole shrimps are omnivores, feeding on detritus, vegetation and other aquatic invertebrates (3) (4). Few predators have adapted to the ephemeral pools inhabited by this shrimp, although if a connection should occur to a permanent water body, for example by flooding, the vernal pool tadpole shrimp is readily consumed by fish, against which it has no defences (3).

The vernal pool tadpole shrimp can reach maturity in just 25 days and first reproduces after an average of 54 days (6), enabling effective use of its short-lived environment. Reproduction continues until the pool dries out (7). Details of reproduction in the vernal pool tadpole shrimp are lacking, although it is thought that the species may be hermaphroditic, that is, individuals possess both male and female sex organs (6). An individual may lay up to 6 clutches of eggs during a wet season, with clutch sizes ranging between 32 and 61 eggs (2).

The eggs, also known as cysts, are drought-resistant and can withstand the high temperatures of the Californian summers while embedded in top layers of sediment. Some cysts hatch within the same wet season, although most will only hatch after the pool dries out and refills in the next rainy season. It is possible for an egg to remain dormant and viable for up to ten years (3). Once hatched, the vernal pool tadpole shrimp continues growing throughout its life, periodically moulting its shell to allow this growth (6).

The main threat to this species is the continued loss of its vernal pool habitat to agricultural and urban development, in addition to degrading of its habitat as a result of overgrazing, off-road vehicle use, invasive plant species and contamination from fertilizers and pesticides (3) (4). In 2003 it was estimated that only nine percent of the vernal pool habitat in California’s Central Valley remained (2).

In 1994, the U.S Fish and Wildlife Service (FWS) gave the vernal pool tadpole shrimp protection as an Endangered species (5). Any projects which could directly or indirectly affect the vernal pool tadpole shrimp must first obtain a permit from the FWS which will only be issued if the continued existence of the vernal pool tadpole shrimp will not be jeopardized (3).

Conservation measures that will benefit the tadpole shrimp are detailed in the 2005 Recovery Plan for Vernal Pool Ecosystems of California and Southern Oregon. The plan comprises five key elements: habitat protection; adaptive management, restoration and monitoring; status surveys; research; and public participation and outreach (2).

To find out more about the Vernal Pool Recovery Plan see:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:

  1. IUCN Red List (May, 2010)
  2. U.S. Fish and Wildlife Service (2007) Vernal Pool Tadpole Shrimp (Lepidurus packardi) 5-Year Review: Summary and Evaluation. Sacramento Fish and Wildlife Office, California.
  3. Goettle, B. (1997) A “living fossil” in the San Francisco Bay area? Tideline, 17(1): 1-3.
  4. NatureServe Explorer (May, 2010)
  5. Rogers, C. (2001) Revision of the Nearctic Lepidurus (Notostraca). Journal of Crustacean Biology, 21(4): 991-1006.
  6. Science Applications International Corporation (2007) Draft Ecological Baseline Report for the Butte Regional Habitat Conservation Plan / Natural Community Conservation Plan. Butte County Association of Governments, Chico, California. 
  7. Gallagher, S.P. (1996) Seasonal occurrence and habitat characteristics of some vernal pool branchiopoda in Northern California, U.S.A. Journal of Crustacean Biology, 16(2): 323-329.