The freshwater zooplankton are comprised of three major groups of invertebrate metazoans: the rotifers, copepods, and cladocerans.  The copepods and cladocerans are both groups of the large subphylum Crustacea, and are not included on this web site.  Photos and further information for these groups can be found at http://www.cnas.missouristate.edu/zooplankton/Default.htm.  Photos in the current web site show rotifers from large rivers.

 The phylum Rotifera includes over 2000 species.  The rotifers are found primarily in freshwater environments, although a few species are marine (Wallace and Snell 2001).  Rotifers are often associated with plants, but other species are adapted to pelagic (open water) environments.  About 25 species are colonial and float together in masses or attach to plants or mosses.  The reader can find detailed descriptions of the biology of rotifers in Edmondson (1959), Pennak (1978), and Wallace and Snell (2001).  These sources also provide information on sampling, culturing, and identification techniques.

 Identification of rotifers should proceed with the use of taxonomic keys.  A key to the families of freshwater Rotifera can be found in Wallace and Snell (2001) and genera can be identified using Edmondson (1959) or Stemberger (1979).  For identification of species, see Ruttner-Kolisko (1974) and Koste (1978).  Photos in the current web site can be used as an aid for students and researchers learning to recognize different rotifer genera.  Since these photos are from only a sample of representative species and don’t show all the taxonomically-important features, users should not rely solely on photos for identification.  Use the above keys to confirm the identifications. 

Because of their small size, rotifers must be examined under a compound microscope at 40-100X to locate taxonomic characteristics.  While taxonomic keys guide you through specific dichotomous features, you can develop an eye for certain characteristics useful for discriminating rotifers by examining your animal under the microscope and by referring to photos in this web site.   

Rotifers are pseudocoelomate invertebrates characterized by a corona at their anterior end and hardened jaws called trophi.  The shape of the trophi reflects the rotifer’s feeding habits and are important features for identification.  The corona is a band of cilia that functions in locomotion and feeding, and can be used to assist in the identification of some groups.  Some other important morphological characteristics include the lorica, foot, and anterior projections.  The lorica is a stiffened cuticle that may be organized into plates or in one continuous shell around the body.  Stiff cuticles are simple to recognize, and loricate genera can often be recognized based upon lorica shape alone.  Semi-loricate or non-loricate animals are more difficult, and require examination of additional features. 

The foot projects from the posterior end of the body, and is either annulated (ringed) or segmented (can retract into the lorica).  The opening from which the foot protrudes can be taxonomically important, either taking the form of a longitudinal cleft or a hole.  Anterior projections include spines or paddles.  Spines can be quite long or short, and can be quite easy to recognize.  Paddles are present in some swimming forms, and may appear feather-like.  Paddles can sometimes become folded under the body, so specimens should be examined carefully to locate these appendages.  When exterior features are insufficient for identification, the trophi must be examined.  Bleach is typically used to dissolve the body, allowing the trophi to be seen.  See Edmondson  for details on identification techniques.

 

Literature Cited

 Benke, A. C., and C. E. Cushing, editors. 2005. Rivers of North America. Elsevier, Oxford.

Downing, J. A., and F. H. Rigler, editors. 1984. A manual on methods for the assessment of secondary productivity in fresh waters, 2nd edition. Blackwell Scientific Publications, Oxford.

Edmondson, W. T. 1959. Rotifera. Pages 420-494 in W. T. Edmondson, editor. Freshwater Biology. John Wiley & Sons, Inc., New York.

Koste, W. 1978. Rotatoria in Die Rädertiere Mitteleuropas. Gebrüder Borntraeger, Berlin.

Pennak, R. W. 1978. Freshwater invertebrates of the United States, 2nd edition. John Wiley & Sons, New York.

Ruttner-Kolisko, A. 1974. Plankton rotifers: biology and taxonomy. Nägele u. Obermiller, Stuttgart.

Stemberger, R. A. 1979. Guide to the rotifers of the Lauretian Great Lakes. USEPA-600/4-79-021, US Environment and Protection Agency, Washington, D.C.

Wallace, R. L., and T. W. Snell. 2001. Rotifera. Pages 195-254 in J. H. Thorp and A. P. Covich, editors. Ecology and classification of North American freshwater invertebrates. Academic Press, San Diego.