Fairy Shrimp

authored by GVM resident Terry W. Campbell, BS, MS, DVM, PhD

Shrimp in pool in GVM
A Colorado Fairy Shrimp pool in Glacier View Meadows

Colorado Fairy Shrimp (Branchinecta coloradensis) are part of the cryptobiotic* life found in the ephemeral (temporary) waters of rock basins in Glacier View Meadows. These remarkable animals are living fossil as similar forms have been found dating back in the fossil records for over 140 million years.

These types of shrimp live quickly, because they must, for their aquatic world disappears rapidly. All stages of the life cycle of some fairy shrimp: egg hatching (3-4 days); maturation (numerous molts of immature instars, 6-7 days); adult reproduction; and egg laying can occur in as few as 16 days! However, Colorado Fairy shrimp generally take up to 41 days to reach maturity, after which they must reproduce before they die, either by a natural life expectancy or by desiccation when their pool dries out. The species survives in these miniature vernal (seasonal) aquatic ecosystems by the development of cryptobiotic (hidden life) cysts that resist desiccation and freezing. The eggs hatch quickly (24 hours) and develop to 4,000 cells (called cysts) when they cease developing and become dormant. They will not develop any further until they are completely dried out, chilled and rewetted. The re-wetting and thus continued development of the cysts into adult Fairy shrimp occurs during the late winter and early springtime thaws.

Colorado Fairy Shrimp pool
A Colorado Fairy Shrimp pool

Adult fairy shrimp swim around upside down on eleven sets of oar-like structures called swimming legs or swimmerets. Each swimmeret is rimmed with swimming (natatory) setae that increase surface area, which increases mobility and oxygen absorption. They can regulate their oxygen consumption and withstand low oxygen levels (dissolved oxygen). Adult fairy shrimp eat bacteria, algae, microzooplankton, and detritus using their legs to filter feed and scrape food from the rocky substrate. Their color varies depending upon what they have eaten.

Colorado Fairy Shrimp
Colorado Fairy Shrimp

Adult fairy shrimp of the genus Branchinecta vary in size from one-half to one and one-half inches (13-38mm). Female fairy shrimp are easy to identify when they are carrying eggs in their brood pouch. Male fairy shrimp can be identified by their enlarged second antennae, which hang down like long trunks or tusks from the head and are used to clasp females during copulation. The females lack this modification, instead their second antennae, located between the compound eyes, are short and stout. The first antennae of both genders are thread-like.

Colorado Fairy Shrimp
Colorado Fairy Shrimp

Fairy shrimp are preyed on by several organisms, including salamanders (at lower elevations), beetles, and true bugs (hemipterans), to name a few. To avoid the increasing number of predators that find the pool by summer, fairy shrimp hatch in late winter and early spring, and they can be seen swimming under the surface of ice-encrusted pools. Female fairy shrimp lay their eggs before pools dry up. They can produce two types of eggs; summer eggs hatch quickly (16 days), and the young develop in the same season, while winter eggs develop into the cryptobiotic cysts that fall to the bottom of the pool and remain there after the pool dries up. The hard-shelled cysts withstand freezing and drying, and as previously mentioned, require a period of dryness to continue their development. In laboratory settings, fairy shrimp cysts have been known to be viable up to 15 years.

It is likely fairy shrimp are either accidentally or purposefully ingested by birds and other animals that drink from the vernal pools. This activity facilitates the introduction of fairy shrimp into new pools. Desiccated cysts can also be transferred to other pools by floating in gusts of wind.

Close up
Close up of Colorado Fairy Shrimp

Because fairy shrimp hatch in large numbers in late winter and early spring, they quickly utilize the resources of the pool, such as food sources and oxygen. In doing so, they may contribute to the control of mosquitos that might otherwise utilize the pools for the development of their offspring.

Colorado Fairy shrimp are found in all states west of and including the Rocky Mountains states, as well as prairie pools in Oklahoma, Alberta, and Saskatchewan and desert pools in Wyoming. They occur in a variety of alkaline habitat types, from mountain pools to sage flats, alkali flats and large seasonal lakes up to 3,000 m (9,842 feet) elevation.

Memorial Day weekend is a good time to look for them in the pools created by snow melt and rain that fill depressions in the rocky landscape around Glacier View Meadows.

*A dormant state induced by unfavorable environmental conditions such as high or low temperature, reduced oxygen, or drought, in which an organism’s metabolic activity is reduced to an imperceptible level.

Suggested readings

Bohonak AJ, Whiteman HH. 1999. Dispersal of the fairy shrimp Branchinecta coloradensis (Anostraca): Effects of hydroperiod and salamanders. Limnol. Oceanogr. 44(3), 487–493.

Cheng, E. 2014. Size matters: How pool volume affects the survival of fairy shrimp on the Colorado Plateau. Ecology & Evolutionary Biology Graduate Theses & Dissertations. University of Colorado, Boulder.

Dararat W, Starkweather PL, Sanoamuang L-O. 2011. Life history of three fairy shrimp (Branciopoda: Anestraca) from Thailand. J Crustacean Biology 31(4), 623-629.