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| NAS - Nonindigenous Aquatic Species |
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Common Name: a flatworm
Synonyms and Other Names: D. lugubris, Planaria polychroa, and P. lugubris
Taxonomy: available through 
Identification: This uniform to irregularly pigmented brown flatworm exhibits a rounded to somewhat triangular head that narrows slightly at the neck. The two eyes are located close to the anterior end and spaced further apart than in the North American native D. tigrina. Posterior to the eyes there are two unpigmented auricular sensory organs near the lateral margins that barely protrude from the body and are not especially pointed. The single pharynx is completely white. Cocoons are brown, round, and occur on a stock. (Kenk 1972; Pennak 1989; Wienzierl et al. 1998) Average length in a population varies between 4.5 and 10 mm depending on the time of year but maximum size can reach 20 mm (Reynoldson and Davies 1970; Kenk 1972; Boddington and Mettrick 1974).
Size: can reach 20 mm
Native Range: D. polychroa is primarily found in western Europe. However, its Palearctic range encompasses all of Europe, northern Asia, and northern Africa within the Mediterranean basin (de Vries 1985; Mills et al. 1993).
Nonindigenous Occurrences: D. polychroa occurred for the first time within the Great Lakes basin in Lake Ontario in 1968 (Ball 1969; Mills et al. 1993).
Ecology: D. polychroa is hermaphroditic. Different lineages of: a) diploid sexual individuals; and b) polyploid, pseudogamous, parthenogenetic individuals exist. In the latter type of lineage, reproduction occurs when unfertilized eggs develop. Sperm is still required to trigger initial egg development, but genetic material from the sperm is not incorporated. Interbreeding between the two different types of lineages sometimes occurs when sperm produced by parthenogens fertilizes sexual individuals. When copulations take place, both individuals involved transfer and receive sperm. Self fertilization does not occur (Beukeboom et al. 1996; Peters et al. 1996; Weinzierl et al. 1998).
D. polychroa is iteroparous, reproducing many times in a lifetime. In the field, each adult in established populations produces around 0–8 young in a breeding season, but with unlimited food and little competition, each adult can produce around 30– 45 young. Populations in Toronto Harbour, Canada remain low over winter; expand rapidly in spring and summer; peak in August; and then decline. The breeding season in this locality is around 10 weeks long (Boddington and Mettrick 1974; Reynoldson 1977; Bladon and Calow 1987). Distribution of D. polychroa in Toronto Harbour, Canada is limited mostly by pollution. In cleaner areas densities can reach up to 2200 individuals per m2. In this harbor D. polychroa feeds predominantly on tubificid oligochaetes and supplements its diet with gastropods, isopods, and amphipods. In some British habitats this flatworm mainly consumes gastropods, supplemented by amphipods, asellids, and sometimes chironomids. Differences in relative contribution of food items between British populations and North American ones are probably related to prey availability and competition with other predators. In British habitats, competition with other triclads and leeches is very common (Reynoldson and Davies 1970; Boddington and Mettrick 1974; Reynoldson and Piearce 1979; Seaby et al. 1996).
D. polychroa typically occurs in littoral zones, especially in rocky regions of productive lakes. In The Netherlands it is only found in waters above pH of 6.0. It rarely occurs in waters of chlorinity above 2‰ and cannot survive permanently in salinities of 3.8‰ and more, although chlorinity up to 6.6‰ can be tolerated for short periods of time. D. polychroa is relatively thermophilic and requires temperatures of 10–23ºC to complete a full life cycle. Its ability to tolerate increased salt concentrations decreases as temperature increases. It requires water temperatures of 10ºC or more for egg case production and does not grow in water temperatures below 5ºC (Reynoldson et al. 1965; Reynoldson and Davies 1970; Boddington and Mettrick 1974; Van der Velde et al. 1986; Seaby et al. 1996).
Means of Introduction: D. polychroa was very likely introduced in ballast water to North America (Ball 1969; Mills et al. 1993).
Status: Established where recorded.
Impact of Introduction:
A) Realized: In Toronto Harbour, Canada D. polychroa coexists with D. tigrina but the latter is always found at lower densities than the former (Boddington and Mettrick 1974).
B) Potential: Unknown.
Remarks: In the Great Lakes, D. polychroa benefits from changes in substrate due to increased interstitial space and increased biodeposition provided by introduced mussels in the genus Dreissena (Ricciardi et al. 1997).
References
Ball, I. R. 1969. Dugesia lugubris (Tricladida: Paludicola), a European immigrant into North American fresh waters. Journal of the Fisheries Research Board of Canada 26:221-228.
Beukeboom, L. W., R. P. Weinzierl, K. M. Reed, and N. K. Michiels. 1996. Distribution and origin of chromosomal races in the freshwater planarian Dugesia polychroa (Turbellaria: Tricladida). Hereditas 124(1):7-15.
Bladon, T. and P. Calow. 1987. Effects of ration and temperature on reproduction of a semelparous and an iteroparous triclad. International Journal of Invertebrate Reproduction and Development 11(3):323-334.
Boddington, M. J. and D. F. Mettrick. 1974. The distribution, abundance, feeding habits, and population biology of the immigrant triclad Dugesia polychroa (Platyhelminthes: Turbellaria) in Toronto Harbour, Canada. The Journal of Animal Ecology 43(3):681-699.
Boddington, M. J. and D. F. Mettrick. 1977. A laboratory study of the population dynamics and productivity of Dugesia polychroa (Turbellaria: Tricladida). Ecology 58(1):109-118.
de Vries, E. J. 1985. The biogeography of the genus Dugesia (Turbellaria, Tricladida, Paludicola) in the Mediterranean region. Journal of Biogeography 12(6):509-518.
Kenk, R. 1972. Freshwater Planarians (Turbellaria) of North America. Biota of Freshwater Ecosystems Identification Manual No. 1, Water Pollution Control Research Series 18050 ELDO2/72, U. S. Environmental Protection Agency, Washington, D. C. 81 pp.
Mills, E. L., J. H. Leach, J. T. Carlton, and C. L. Secor. 1993. Exotic Species in the Great Lakes: A History of Biotic Crises and Anthropogenic Introductions. Journal of Great Lakes Research 19(1):1-54.
Pennak, R. 1989. Fresh-water Invertebrates of the Unites States, 3rd ed. Protozoa to Mollusca. John Wiley & Sons, Inc., New York, New York State. 628 pp.
Peters, A., A. Streng, and N. K. Michiels. 1996. Mating behaviour in a hermaphroditic flatworm with reciprocal insemination: do they assess their mates during copulation? Ethology 102(3):236-251.
Reynoldson, T. B. 1977. The population dynamics of Dugesia polychroa (Schmidt) (Turbellaria, Tricladida) in a recently-constructed Anglesey pond. The Journal of Animal Ecology 46(1):63-77.
Reynoldson, T. B. and B. Piearce. 1979. Predation on snails by 3 species of triclad and its bearing on the distribution of Planaria torva in Britain, UK. Journal of Zoology (London) 189(4):459-484.
Reynoldson, T. B. and R. W. Davies. 1970. Food niche and co-existence in lake-dwelling triclads. The Journal of Animal Ecology 39(3):599-617.
Reynoldson, T. B., J. O. Young, and M. C. Taylor. 1965. The effect of temperature on the life-cycle of four species of lake-dwelling triclads. Journal of Animal Ecology 34:23-43.
Ricciardi, A., F. G. Whoriskey, and J. B. Rasmussen. 1997. The role of the zebra mussel (Dreissena polymorpha) in structuring macroinvertebrate communities on hard substrata. Canadian Journal of Fisheries and Aquatic Sciences 54:2596-2608.
Seaby, R. M. H., A. J. Martin, and J. O. Young. 1996. Food partitioning by lake-dwelling triclads and glossiphoniid leeches: field and laboratory experiments. Oecologia (Berlin) 106(4):544-550.
Van der Velde, G., F. Husken, and L. Van Welie. 1986. Salinity-temperature tolerance of two closely related triclad species, Dugesia tigrina and D. polychroa (Turbellaria), in relation to their distribution in The Netherlands. Hydrobiologia 132:279-286.
Weinzierl, R. P., K. Berthold, L. W. Beukeboom, and N. K. Michiels. 1998. Reduced male allocation in the parthenogenetic hermaphrodite Dugesia polychroa. Evolution 52(1):109-115.
Author: Rebekah M. Kipp
Contributing Agencies: 
NOAA - GLERL
Revision Date: 4/10/2007 Citation for this information:
Rebekah M. Kipp. 2009. Dugesia polychroa. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
<http://nas.er.usgs.gov/queries/FactSheet.asp?speciesID=2359> Revision Date: 4/10/2007
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