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NATURAL HISTORY

GIANT PALOUSE EARTHWORM } Driloleirus americanus
FAMILY: Megascolecidae

DESCRIPTION: The giant Palouse earthworm can reach three feet or more in length, has light-pink skin, and emits a unique, sweet fragrance.

HABITAT: This species inhabits permanent or semipermanent vertical burrows up to 15 feet deep. The area in which the species is found has a temperate climate and is characterized by plains, hills, undulating plateaus, and some river breaklands; soils are generally deep and loamy to silty, although soils in mountainous areas are shallower and contain rock fragments. Grasslands and meadow-steppe vegetation are the prototypical natural vegetation of the region, but most of the area has been converted to crop lands.

RANGE: The giant Palouse earthworm inhabits the Palouse bioregion in southeastern Washington, west central Idaho, and northeastern Oregon. Covering about 16,000 square kilometers, the region lies between the western Rocky Mountains and the Columbia River basin.

MIGRATION: This species is nonmigratory.

BREEDING: Little is known about the reproduction of this species in particular. Earthworms are monoecious, meaning that both female and male organs are present within the same worm. In most earthworms, copulating adults overlap front ends and exchange sperm. Long after the worms have separated, the egg case is secreted and forms a ring around the worm. The worm then backs out of the ring and injects its own eggs and the other worm’s sperm into it. The ends of the egg case seal to form an oval-shaped incubator in which embryonic worms develop. Some earthworms are parthenogenetic, meaning that females can reproduce asexually.

LIFE CYCLE: Little is known about the life cycle of this species. In general, juvenile earthworms emerge from their egg case as small but fully formed earthworms, with sexual structures to be developed later.

FEEDING: This earthworm emerges from its burrows at night to feed on relatively fresh plant debris at the earth’s surface.

THREATS: This species is threatened by habitat loss due to agriculture and development and the invasion of exotic species.

POPULATION TREND: The giant Palouse earthworm was described as “very abundant” in 1897, but today sightings are extremely rare. The first person in nearly two decades to report a sighting of the species was a University of Idaho graduate student conducting soil samples in 2005. Prior to this sighting, two specimens were collected in 1988 and one specimen was collected 10 years earlier. An indication of the species’ rarity was documented by Fauci and Bezdicek in 2002, when they surveyed earthworms at 46 sites in the Palouse bioregion without a single collection of the giant Palouse earthworm.

Photo courtesy of University of Idaho