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| NAS - Nonindigenous Aquatic Species |
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Common Name: opossum shrimp
Taxonomy: available through 
Identification: Superficially, Mysis relicta resembles a crayfish. Differences include a thin carapace not completely covering the thorax, with the last two segments exposed dorsally, and to a lesser degree laterally. Appendages are long, thin, setose, biramous, and many segmented. The first thoracic segment is fused with the head and bears a pair of maxillipeds. The remaining seven segments are distinct with the secondary maxillipeds on the second thoracic segment and a pair of swimming legs on each of the remaining five segments. Eyes are extremely large, compound, and stalked. The telson has a wide, bifurcated tip. In mature males the exopod of the third and fourth pleopods is six segmented and the exopod of the third pleopod is well developed (Pennak 1989).
Size: maximum length 30 mm
Native Range:
This species is circumpolar and occurs in the United States, Canada, the United Kingdom, northern Germany, northern Russia, and the Scandinavian countries. In the U.S. and Canada it is native to the Great Lakes; Green Lake, Trout Lake, and Lake Geneva in Wisconsin; the Finger Lakes in New York; Waterton Lake, Alberta; a few lakes in the northern states; and many deep lakes in Canada (Pennak 1989).
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Interactive maps: Continental US, Alaska, Hawaii, Caribbean
The first nonindigenous introduction was proposed by Clements et al. (1939) for the lakes of the Okanagan basin, British Columbia, to enhance the forage base for the previously introduced lake whitefish (Coregonus clupeaformis). In 1949 Kootenay Lake, British Columbia, was the first lake to be stocked with M. relicta (Sparrow 1964). Subsequent stockings were preformed in many montane lakes of the western U.S. and Canada. A specimen was collected in Lake Demopolis, Alabama (Jackson, unpublished data).
Means of Introduction: Stocked to provide additional forage for sport fish (Clements 1939).
Status: Established
Impact of Introduction: Mysis relicta is an opportunistic feeder with both predatorial and filter feeding habits. When zooplankton are abundant they serve as the primary food source; when scarce M. relicta will feed on suspended organic detritus or from the surface of benthic organic deposits (Pennak 1989). Within its native range it has been shown to be an important prey item for freshwater fishes (Nesler and Bergersen 1991). However, when introduced into what was considered to be an "empty" niche, its impact on the aquatic community was significant. Dramatic changes and species extinctions of native zooplankton communities have been attributed to this opportunistic lifestyle. Declines in the number and size of game fish have been documented since the introduction of M. relicta, provoking doubt regarding their utility as a forage base for game fishes.
Remarks:
Found in deep, cold oligotrophic lakes; has been reported from brackish and estuarine waters. Can live for 2 years; breed in the winter; adults carry young in a brood pouch until fully developed in the spring; sexually mature at 12-14 mm.
In 1981 a resolution calling for a moratorium on introductions of M. relicta was drafted by participants of a Mysid symposium held during a conference of the American Society of Limnology and Oceanography in Milwaukee, Wisconsin.
References
Clements, W.A., D.S. Rawson, and J.L. McHugh. 1939. A biological survey of Okanagan Lake, British Columbia. Fisheries Research Board of Canada Bulletin 56.
Nesler, T.P. and E.P. Bergersen. 1991. Mysids and their impacts on fisheries: an introduction to the 1988 Mysid-Fisheries symposium. Pages 1-4 In: T.P. Nesler and E.P. Bergersen, editors. Mysids in fisheries: hard lessons from headlong introductions. American Fisheries Society Symposium 9, Bethesda, Maryland.
Pennak, R.W. 1989. Fresh-water invertebrates of the United States, protozoa to mollusca. John Wiley & Sons, New York, 3rd ed.
Sparrow, R.A.H., P.A. Larkin, and R.A. Rutherglen. 1964. Successful introduction of Mysis relicta Lovén into Kootenay Lake, British Columbia. Journal of the Fisheries Research Board of Canada 21:1325-1327.
Author: Ann Foster and Amy Benson
Revision Date: 1/29/2007 Citation for this information:
Ann Foster and Amy Benson. 2010. Mysis relicta. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
<http://nas.er.usgs.gov/queries/FactSheet.asp?speciesID=1142> Revision Date: 1/29/2007
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