tocIdentification key for Holocene Lacustrine Arcellacean (Thecamoebian) taxa

Arun Kumar and Andrew P. Dalby

Article number: 1.1.4A
Copyright Palaeontological Assocation, 28 January 1998

Plain-language and multi-lingual abstracts
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Submission: 10 November, 1997. Acceptance: 7 January, 1998



Arcellaceans (thecamoebians), predominantly freshwater protozoans, have been studied for over a century. However, in the last ten years their usefulness as paleoenvironmental indicators has become better understood because researchers have recognized that certain morphotypes (strains) prefer distinct microenvironments. Unfortunately, the monoclonal nature and simple morphorphology of the group has led to considerable taxonomic confusion, threatening their utility in paleolimnological research. This key was created to help new researchers studying the group, and to standardize the taxonomy.

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KEY WORDS: thecamoebian(s), arcellacean(s) Holocene, identification key

Final citation: Kumar, Arun and Dalby, Andrew P. 1998. Identification key for Holocene Lacustrine Arcellacean (Thecamoebian) taxa, Palaeontologia Electronica Vol. 1, Issue 1; 4A; 34p;



Arcellaceans (thecamoebians) are clonal, predominantly freshwater protozoans, although they can also occur in brackish water environments and moist soils (Medioli and Scott 1983). They can be found in a wide range of geographic settings, ranging from tropical to arctic latitudes (Nair and Mukherjee 1968; Green 1975; Dallimore et al. 1997). These organisms have an amoebid sarcodine cell with pseudopods and a simple sac-like test, either flattened or rounded with an aperture located on or near the tapered end, or a beret-shaped test with an invaginated aperture on the ventral side which is more or less flattened. A substantial amount of morphological variability has been observed among these two broad groups.

A few forms like Lagenodifflugia and Pontigulasia appear to have a second chamber consisting of an enlarged collar separated from the rest of the test by a distinct constriction (Medioli and Scott 1988; Medioli et al. 1990). Tests are either secreted (autogenous), or agglutinated (xenogenous) with sand particles or diatom frustules (xenosomes) in autogenous cement. Autogenous tests are usually smooth, proteinaceous, sometimes made of siliceous platelets (idiosomes), or rarely calcareous. (See Medioli et al. 1990, for a review of fossil arcellaceans).

Arcellacean tests do not demonstrate a high degree of morphological diversity, but important taxonomic characteristics include (Asioli et al. 1996): presence or absence of spines; nature and shape of xenosomes and idiosomes; shape and composition of tests; and morphological features associated with the apertures, such as diaphragms, collars, lobes, and teeth.

Test size can vary between 50 to 300 um or even greater, thus it is of no taxonomic importance as it is determined at the time of fission by the volume of cytoplasm available in the parent test (Medioli and Scott 1983). Cytoplasmic volume is in turn probably controlled by the availability of food in the period preceeding reproduction (Medioli et al. 1990).

It has long been known that various arcellacean species preferentially inhabit specific environments. Rcent research has determined that some infrasubspecific variants (strains) are particularly sensitive to environmental variations (Asioli et al. 1996; Patterson et al. 1996; Reinhardt et al. 1998). In particular, arcellaceans have proven to be useful tools in assessing remediation rates within industrially polluted settings (Asioli et al. 1996; Patterson et al. 1996; Reinhardt et al. 1998).

Unfortunately, the subtle criteria used to differentiate the rather simple arcellacean morphologies, and confusion over the proliferation of taxonomic names during the last hundred years has limited their utility (Medioli and Scott 1983). In this paper, we present a taxonomic key designed to:

  1. illustrate the various morphologies that we find useful in paleolimnological research and;
  2. provide an arcellacean identification guide for future researchers engaged in paleolimnological research.

Taxa illustrated in this key are from Crosswise and Peterson lakes, near Cobalt, Ontario; Swan Lake, north of Toronto, Ontario; Lake Erie, Ontario; and Lake Orta, northern Italy. Exact sample locations are provided with individual illustrations.

Key Position: 1
1a. Test circular, hemispherical to ovoid; made of proteinaceous matter and agglutinated grains.

Proceed to 2

1b. Test ovoid, pyriform, elongate to acuminate; made of agglutinated mineral grains or diatom frustules.

Proceed to 7



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