Testate amoebae (formerly thecamoebians, Testacea or Thecamoeba) are a polyphyletic group of unicellular amoeboid protists, which differ from naked amoebae in the presence of a test that partially encloses the cell, with an aperture from which the pseudopodia emerge, that provides the amoeba with shelter from predators and environmental conditions.

The test of some species is produced entirely by the amoeba and may be organic, siliceous or calcareous depending on the species (autogenic tests), whereas in other cases the test is made up of particles of sediment collected by the amoeba which are then agglutinated together by secretions from within the cell (xenogenic tests). A few taxa (Hyalosphenidae) can build either type, depending on the circumstances and availability of foreign material.^[1]

The assemblage referred to as "testate amoebae" is actually composed of several, unrelated groups of organisms. However, some features they all share that have been used to group them together include the presence of a test (regardless of its composition) and pseudopodia that do not anastomose.^[2]

Testate amoebae can be found in most freshwater environments, including lakes, rivers, cenotes,^[3] as well as mires and soils.

The strong and resistant nature of the tests allows them to be preserved long after the amoeba has died. These characteristics, along with the sensitivity that some species display to changes in environmental conditions (such as temperature, pH, and conductivity), has sparked their use as bioindicators and paleoclimate proxies in recent years.^[4]

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  Naked amoeba (i. e. not testate) showing food vacuoles and ingested diatom
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  Shell or test of a testate amoeba, Arcella sp.
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  Xenogenic testate amoeba covered in diatoms (from Penard's Amoeba Collection)

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  Euglypha tuberculata, a species with a siliceous autogenic test
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  The autogenic test of Arcella discoides, made up of organic plates
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  Shell of Cyphoderia ampulla, composed of circular, siliceous plates produced by the amoeba

Taxonomy and classification

Testate amoebae are a polyphyletic assemblage. The main testate amoebae groups are the lobose Tubulinea, which include Arcellinida, Difflugina and Phryganellina (within the Amoebozoa),^[5] and the filose Euglyphida (within the SAR supergroup),^[6] although there are smaller groups that also include other testate amoebae.^[7]

Order Arcellinida

Order Euglyphida

Other Cercozoa

Order Stramenopila

Unclassified testate amoebae

The following table includes a few examples of testate amoebae genera, and reflects their position within the classification by Adl et al. (2012),^[7] where five supergroups (Amoebozoa, Opisthokonta, Excavata, SAR and Archaeplastida) were proposed to classify all eukaryotes. This classification purposefully avoids the use of Linnaean higher category names (phylum, class, order, family). While it has been noted that the names that Adl et al. provide for the clades may result confusing or uninformative regarding the relative degree of phenotypic distinctiveness amongst groups when used in isolation,^[8] this system avoids creating superfluous ranks where unnecessary and provides stable group names that can be retained even when a group is moved to a different lineage, as is often the case with protists, as their classification remains in constant review.^[7]

Traditionally, those species that form large networks of anastomosing pseudopodia, despite some of them having tests, are not counted amongst testate amoebae; this comprises genus Gromia and the Foraminifera (both in Rhizaria).^[2]