Guide to the Freshwater Invertebrates of the Midwest



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Keys to the major groups

of freshwater invertebrates
(modified for the Southeastern United States in part from Guide to the Freshwater Invertebrates of the Midwest and from Ecology and Classification of North American Freshwater Invertebrates, Thorp, J. H. and A. P. Covich)

Identification of freshwater invertebrates is divided here into three keys based on lifestyle and size:


Key A, Freshwater Sessile (attached) Invertebrates, includes sessile organisms that are attached to substrate. Sessile organisms do not fall easily into micro- and macroscopic categories because colonies may be large but composed of small inconspicuous individuals, so all sizes of sessile organisms are included in this key.
Key B, Freshwater zooplankton, meiofauna, and typically microscopic epifauna,

includes motile invertebrates that are microscopic and near microscopic organisms, generally <2 mm in length (i.e. organisms too small to easily be collected by hand picking in the field).


Key C, Freshwater Benthic macro-Invertebrates, includes larger macroscopic, taxa (generally >2 mm in length) that typically are found on bottom surfaces or living within the sediment.
There will obviously be overlap among these keys. Some taxa are found in both keys. In addition note that:


  • The keys exclude some marine and brackish water groups that have a few representatives capable of inhabiting freshwaters only within coastal rivers and coastal lakes.

  • The keys exclude taxa that are exclusively parasitic.

  • Drawings cannot possibly include all variations within a given taxa; drawings are most useful in comparing the two descriptions within a couplet.

Start from the beginning until you are very familiar with each key. The characteristics that describe a taxon in its final couplet are not a complete description. The complete description includes all the preceding couplet descriptions that have directly led to that final couplet.

Key A: Freshwater Sessile (attached) Invertebrates

1a. Without bivalve shell ………… ………………………………………………………..….. 2
1b. Bivalve shell attached to substrate with byssal threads; shell with black zigzag markings …

…………….……Phylum Mollusca, Class Bivalvia, Family Dreissenacea




2a. Amorphous (without symmetry) and externally simple; microscopic spicules (siliceous skeleton structures) observable when tissue is digested with acid ...Phylum Porifera

Note: All species of freshwater sponges are classified within a single family Spongillidae. Freshwater sponges are inconspicuous and variable in size, shape, and color (even within a single species)


2b. With tentacles (though may be retracted especially in preserved specimens); radial or bilateral symmetry at level of individual……………………………………………………3


3a. Few tentacles (typically <8) surrounding mouth and solitary, or tentacles scattered along body and colonial forming branches… …………………………………………Phylum Cnidaria

3b. Numerous tentacles arranged in a well-organized (tight) circular or U-shaped crown and extending from a protective tube or case (zooecium) that may be thin to massive and gelatinous to tough; tubes often connected in a branched twiglike manner or some form thick encrusting colonies ……………………………….. 4

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