Image from An Image-Based Key To The Zooplankton Of North America.

More than 20,000 species of malacostracan crustaceans of the Superorder Peracarida are found in freshwater, marine, and terrestrial habitats worldwide. A total of seven orders are represented in in-land waters, including Mysida, Cumacea, Tanaidacea, Spelaegriphacea, and Thermosbaenacea. However, species richness in the taxon is dominated by the Isopoda and Amphipoda.

Isopods are perhaps best recognized from terrestrial environments where they are known as sowbugs, pillbugs, or rollie-pollies. There are more than 10,000 species of isopods globally, but only 940 (194 genera, 27 families) are currently known from fresh waters. It is estimated, however, that actual richness maybe 3x higher (Wilson, 2008). In North America, there are 130 species (18 genera, 4 families) of freshwater isopods. Amphipods (>8000 total species) have about 2x the number of freshwater species (and subspecies) as isopods: 1870. The Nearctic freshwater amphipod assemblage is comprised of 236 species (23 genera, 12 families) (Väinölä et al., 2008). Freshwater amphipods are often referred as scuds.

The other peracaridean orders are only minor components of North American freshwater crustacean diversity. There are eleven species of mysids (“opossum shrimp”), mostly in northern, cold-water lakes (Porter et al., 2008). There is a single freshwater species each of the Cumacea and Thermosbaenacea, in New England and Texas, respectively (Jaume & Boxshall, 2008; Jaume, 2008). Neither the Spelaegriphacea nor Tanaidacea occurs in North America.

The peracaridean body plan is based on 19 segments (and paired appendages) divided into three body tagmata. The first thoracic segment is fused to the head, rendering a cephalothorax of five crustacean head appendages (2 antennae, mandible, 2 maxillae) plus one (in the case of isopods and amphipods) maxilliped. The other seven thoracic segments are the pereopods (walking legs). Among amphipods, the six abdominal segments bear three pairs pleopods (for swimming) and three pairs of uropods (plus a telson). The isopod abdomen is modified as a pleotelson, with the pleopods adapted as gas exchange surfaces. In general, isopods are dorso-ventrally flattened while amphipods are laterally compressed. Both orders have sessile compound eyes. Most freshwater species are in the mm-cm size range, although interstitial species may be smaller.

Among both isopods and amphipods, reproduction is closely associated with ecdysis. Mating occurs immediately after the female has molted. The proximal segments of female's pereopods bear oostegites, lateral projects that create a ventral thoracic space for parental care of offspring. Development is direct.

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Bibliography

• Lowry, J.K. & A.A. Myers. 2012. A phylogeny and classification of the Senticaudata subord. nov. (Crustacea: Amphipoda). ZooTaxa 3610: 1-80. [Available online]
• Covich, A.P., J.H. Thorp & D.C. Rogers. 2010. Introduction to the Subphylum Crustacea. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 695-723.
• Wilson, G.D.F. 2008. Global diversity of isopod crustaceans (Crustacea; Isopoda) in freshwater. Hydrobiologia 595: 231-240. [Available online]
• Väinölä, R., J.D.S. Witt, M. Grabowski, J.H. Bradbury, K. Jazdzewski & B. Sket. 2008. Global diversity of amphipods (Amphipoda; Crustacea) in freshwater. Hydrobiologia 595: 241-255. [Available online]
• Porter, M.L., K. Meland & W. Price. 2008. Global diversity of mysids (Crustacea-Mysida) in freshwater. Hydrobiologia 595: 213-218. [Available online]
• Jaume, D. & G.A. Boxshall. 2008. Global diversity of cumaceans & tanaidaceans (Crustacea: Cumacea & Tanaidacea) in freshwater. Hydrobiologia 595: 225-230. [Available online]
• Jaume, D. 2008. Global diversity of spelaeogriphaceans & thermosbaenaceans (Crustacea; Spelaeogriphacea & Thermosbaenacea) in freshwater. Hydrobiologia 595: 219-224. [Available online]
• Smith, D.G. 2001. Minor Malacostraca. Pennak's Freshwater Invertebrates of the United States, 4th edition: 545-551.
• Smith, D.G. 2001. Isopoda (Aquatic sow bugs). Pennak's Freshwater Invertebrates of the United States, 4th edition: 553-567.
• Smith, D.G. 2001. Amphipoda (Scuds, sideswimmers). Pennak's Freshwater Invertebrates of the United States, 4th edition: 569-584.
• Underhill, J.C., W.D. Schmid & D.E. Gilbertson. 1982. Taxonomic Keys to the Common Animals of the North Central States, exclusive of the parasitic worms, terrestrial insects, and birds, 4th edition. 205 pp.
• Holsinger, J.R. 1976. The freshwater amphipod crustaceans (Gammaridae) of North America. U.S. Environmental Protection Agency. 89 pp. [Available online]
• Chace, F.A., jr., J.G. Mackin, L. Hubricht, A.H. Banner & H.H. Hobbs, jr. 1959. Malacostraca. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 869-901.
• Unwin, E.E. 1920. Notes upon the reproduction of Asellus aquaticus. Journal of the Linnean Society of London, Zoology 34: 335-343. [Available online]

Student Essays

• "Diversity of the Order Amphipoda (Suborder Gammaridea) in Wisconsin" by Chris Arrowood (BIOL/WATR 361, Fall 2012 )

Web Pages

• Integrated Taxonomic Information System
• Tree of Life
• WikiSpecies
• Wikipedia
• World Amphipoda Database
• World Isopoda Database
• World Register of Marine Species