Eubranchipus oregonus Creaser 1930 (Oregon Fairy Shrimp) in British Columbia
by Thalia Grant
Species Description
Eubranchipus oregonus is a fresh-water Branchiopod crustacean. It can grow to over 35 mm in length (pers. obs, Thalia Grant) but more typically attains lengths of 20-25 mm (Coopey, 1950; pers. obs, Thalia Grant). It exhibits a variety of colours from reddish orange to white to green and blue.
Eubranchipus oregonus exhibits the typical fairy shrimp behavior of swimming “upside down” while beating its 11 pairs of abdominal thoracopods to propel itself, for respiration, and to transport organic particles to its mouth. Like all fairy shrimps it has compound eyes and lacks a carapace. It exhibits highly sexually dimorphic features, most noticeable in the second antennaes of the males, which are modified claspers for amplexing females during copulation. The sickle shape and ornamentation of these antennaes, and the male genitalia are key to distinguishing the species (Brendonck et al., 2007). The female’s ovary is long for this genus, and extends in both directions from the brood pouch.
Distribution
In the US, Eubranchipus oregonus is found in Washington, Oregon, and northern California with a few disjunct populations in Oklahoma (Hill et al., 1997). In Canada, it is found only in British Columbia. It has been recorded from Vancouver Island, Salt Spring Island, Texada Island, Galiano Island and on the lower mainland between the US border and Whistler, with one historical outlying record from Kamloops (RBCM collection). It is now rare on the mainland of British Columbia. The species was first described in Oregon, in 1830. It was first collected in Hope, BC, in 1918 but at the time was misidentified as Eubranchipus vernalis (Johansen, 1921; Ferguson, 1935).
Habitat
This species occupes small, freshwater ephemeral ponds from sea level to 1500 meters elevation. It is restricted to small water bodies that are neutral to mildly acidic, low in dissolved solids and turbidity, have little or no flow, and are inundated for several months (Eng et. al, 1990). Habitat ranges from deciduous or mixed forest, to wet meadow and Typha-dominated pools, with some shade and proximity to coniferous forest being frequent habitat descriptors (Hill et. al, 1997). As it lacks any defense against vertebrate predation, Eubranchipus oregonus is never found in water bodies with fish.
Diet
This species is an omnivorous filter feeder, acquiring food directly from the water column, and by scraping periphyton from the bottom of the pond using specialized setae on its thoracic legs (Dodson, Caceres and Rogers, 2010; Thorpe and Rogers, 2011).
Life Cycle
The life cycle of Eubranchipus oregonus begins with the resting eggs, which experience a period of drought when the temporal ponds dry out during the summer. The eggs hatch when the wetland is recharged, typically with the first significant rains of the fall/winter season (Coopey, 1950; Thalia Grant, pers. obs). Not all eggs hatch at once, allowing for pond replenishment should the pond dry and flood multiple times during the year or over several years. This leads to large fluctuations in population numbers on both a spatial (between ponds) and temporal (between months/years) scale.
Growth from the larval (naupli) stage to egg-carrying adults is accomplished through a series of moults and instars (Coopey, 1950). The life span of an individual is unknown, but populations in the Pacific Northwest can persevere for over 25 weeks. The largest females can carry well over 100 eggs in their brood pouch at a time, with multiple clutches produced.
Predators
The primary predators of Eubranchipus oregonus are notonectids, dyticids and other aquatic insects (Thorpe and Rogers, 2011; pers. obs., Thalia Grant). Metamorphic salamanders, mallards and other water fowl also prey on Eubranchipus oregonus.
Threats
Habitat elimination or modification through urban and agricultural land development, wetland conversion, drainage practices and pollution are the species biggest threats (Dodson, Caceres and Rogers, 2010). These practices have already wiped out most Eubranchipus oregonus populations in the Fraser Valley and in the vicinity of Victoria on Vancouver Island.
