Arcellinid testate amoebae or Arcellinida, Arcellacean or lobose testate amoebae are single-celled protists partially enclosed in a simple test (shell). Arcellinid testate amoebae are commonly found in soils, leaf litter, peat bogs and near/in fresh water. They use their pseudopodia, a temporary cell extension, for moving and taking in food. Like most amoebae, they are generally believed to reproduce asexually via binary fission. However a recent review suggests that sexual recombination may be the rule rather than the exception in amoeboid protists in general, including the Arcellinid testate amoebae.

Morphology

[[File:2023 Arcellinid.svg|thumb|left|upright=2|Representation of an arcellinid 1. Test, cemented sand, diatom frustules 2. Contractile vacuole, regulates the quantity of water inside a cell 3. Mitochondrion; creates ATP (energy) for the cell (flat cristae) 4. Nucleolus 5. Golgi apparatus, packages proteins 6. Endoplasmic reticulum, the transport network for molecules going to specific parts of the cell 7. Digestive Vacuole 8. Lysosome, holds enzymes 9. Phagosome, vesicle formed around a particle (prey not to scale) 10. Xenosome, bacterium living inside cytoplasm 11. Vesicles 12. Lobopodia, a pseudopod or arm-like projection made of cytoplasm 13. Prey (not to scale)]] Arcellinida always have a shell or testa. The tests lie outside the cell membrane and consist of organic or mineral materials that are either secreted or incorporate external particles. The testa has a single main opening. Simple tests are made by secretion (autogenous tests), agglutination of foreign material (xenogenous tests), or sometimes a combination of both. Past environmental changes can be determined by analysing the composition of fossil tests, including the reconstruction of past climate change. Testate amoebae species have been used to reconstruct hydrological changes over the late Holocene, as a result of individual species possessing a narrow tolerance for ecohydrological conditions such as water-table depth or pH.

Evolutionary history

Fossils of arcellinid testate amoebae date back to the Tonian stage of the Proterozoic, around 789-759 million years ago. The fossils indicate that by 730 million years ago, arcellinids had already diversified into major lineages. Testate amoebae are theorized to be mostly polyphyletic (coming from more than one ancestral type), but testaceafilosea, one group of testate amoebae, are theorized to be monophyletic. Ancient tests of terrestrial fauna are commonly found in fossilized amber, although mid-Cretaceous testate amoeba (i.e., Diffligia, Cucurbitella) have been found in ancient lake sediments. It is likely that the group has evolved minimally over the course of the Phanerozoic.

Classification

The classification of Arcellinida, as of 2019: Arcellinida incertae sedis: