Examining a jar of horsehair worms collected from a creek in Ventura County, California, during the Spring. The jar contains hundreds of adults, seen congregating as a mass, probably for mating. Photo by Regina Wetzer
J. Kirk Fitzhugh
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Leslie H. Harris
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Research & Collections
Natural History Museum of Los Angeles County
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In addition to sea worms, residents of Southern California occasionally encounter two types of worms that are not segmented, that is, not members of the phylum Annelida. One is an enigmatic native, known as the horsehair worm (phylum Nematomorpha), present in some freshwater streams and at least one species of introduced flatworm, or land planarian can sometimes be found in urban terrestrial habitats.
Photo by Dean Pentcheff Photo by Irene Kightley
Horsehair worms, or nematomorphs (from the Greek nematos, 'thread,' and morphe, 'form'), comprise a group that are similar in appearance to round worms (phylum Nematoda). Both groups differ from the flatworms (Platyhelminthes) and segmented worms (Annelida) in that they have a unique form of body cavity, sometimes referred to as a pseudocoel, and external cuticle. Flatworms have a solid body construction (except for the digestive system), and segmented worms have a body cavity called a coelom that surrounds the organs. Horsehair worms are unusual because the adults are aquatic (most species are freshwater, a few marine) and free living, but parasitic in arthropods during growth from larval to juvenile stages.
Adult horsehair worms can range from less than one inch to three feet in length, but are no more than one-tenth of an inch wide. Hence the common name of the group. Adults are free living, but nonfeeding as the gut is degenerate. Their principle activity as adults is reproduction.
The life cycle of freshwater species is unusual because hatched larvae are parasitic, penetrating the bodies of terrestrial beetles, crickets, grasshoppers, and cockroaches that come close to water. With maturation to a near-adult stage, the presence of the worm alters the host's behavior such that they seek water, at which point the worm breaks through the host's body wall (see photo below), and begins the free-living, aquatic adult phase. A dramatic video of this event can be seen here.
Australian Society for Parasitology
Photo by J. Hogue Photo by Martin LaBar
The land planarian, Bipalium kewense, is a member of the phylum Platyhelminthes (from the Greek platy, 'flat,' and helminth, 'worm'), which includes the free-living planaria, and parasitic flukes and tapeworms. While the native habitats of B. kewense extend from Vietnam to Malaysia, the species has been dispersed around the world and thrives in mild climates with adequate moisture. Indeed, the species was originally described from a greenhouse at Kew Botanical Gardens near London, England in 1878. In the U.S., B. kewense have been found in natural habitats in Florida, Louisiana, Georgia, Texas, North and South Carolina, and Southern California, and reported in greenhouses in some more northern states. The wide disperal of the species has likely been due to the transport of tropical plants and soil.
Bipalium kewense are about the diameter of an earthworm but can be up to 18 inches long. In Southern California, they can be found in moist conditions under rocks and in flower beds, and have been seen gliding across the ground during early morning. The species is easily recognized by its light brown color, dark lines along the body, and expanded, lunate head.
Bipalium kewense mainly reproduces asexually, by fragmentation of the posterior end of the body. New individuals readily regenerate from the fragments.
Land planarians are active carnivores, feeding on earthworms, small arthropods, and even other planarians. Unfortunately, some introduced species in England and Ireland have had drastic consequences, where they have decimated native earthworm populations. The potential for similar problems have been reported for B. kewense in the U.S., where they are regarded as a nuisance in earthworm farms.
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