Conceptual origins The earliest record of an amoeboid organism was produced in 1755 by
August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus" ("the Little Proteus"). Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as
Amoeba proteus. The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid. In 1822, the genus
Amiba (from the
Greek ἀμοιβή
amoibe, meaning "change") was erected by the French naturalist
Bory de Saint-Vincent. Bory's contemporary,
C. G. Ehrenberg, adopted the genus in his own classification of microscopic creatures, but changed the spelling to
Amoeba. In 1841,
Félix Dujardin coined the term "
sarcode" (from Greek σάρξ
sarx, "flesh", and εἶδος
eidos, "form") for the "thick, glutinous, homogeneous substance" which fills protozoan cell bodies. Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells. Later workers, including the influential
taxonomist Otto Bütschli, amended this group to create the class Sarcodina, a taxon that remained in wide use throughout most of the 20th century. The Rhizopods were further subdivided into lobose, filose, plasmodial and reticulose, according to the morphology of their pseudopods. During the 1980s, taxonomists reached the following classification, based exclusively on morphological comparisons: •
Sarcodina : all amoebae. ::*
Eumycetozoea : plasmodial amoebae with filiform subpseudopodia that produce fruiting bodies. ::*
Plasmodiophorea : endoparasitic plasmodial amoebae with minute pseudopodia. This group is now an order within
Rhizaria, closely related to the endoparasites
Phagomyxida. ::*
Filosea : amoebae with filose pseudopodia. :::*Aconchulinia : filose naked amoebae, sometimes covered in scales. This group included two unrelated taxa: the
nucleariid amoebae, closely related to
fungi; and most of the
Vampyrellida, found in Rhizaria. :::*Testaceafilosia : filose testate amoebae. This group included taxa now found throughout Rhizaria, such as
Gromiida and
Euglyphida. ::*Granuloreticulosea : amoebae with delicate granular pseudopodia. This group included both the
Foraminifera (now in Rhizaria) and some members of Vampyrellida. ::*
Xenophyophorea : plasmodial amoebae enclosed in a branched-tube system composed of a transparent organic substance. This group is now fully integrated into the Foraminifera. :*
Actinopoda : spherical amoebae that float in the water column. This group included those organisms that have a
heliozoan-type appearance, with radially positioned filopodia, reticulopodia or axopodia surrounding the cell body. These were the
Radiolaria,
Phaeodaria,
Proteomyxidea (all three now in Rhizaria),
Centroplasthelida (now in
Haptista), and
Actinophryida (now in
Stramenopiles).
Transitional period }} In the final decades of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a
monophyletic group, and that amoebae evolved from flagellate ancestors. Shortly after, phylogenetic analyses disproved this hypothesis, as non-amoeboid
zooflagellates and amoeboflagellates were found to be completely intermingled with amoebae. With the addition of many flagellates to Rhizopoda and the removal of some amoebae, the name was rejected in favour of a new name
Cercozoa. As such, both names Rhizopoda and Sarcodina were finally abandoned as formal taxa, but they remained useful as descriptive terms for amoebae. The phylum Amoebozoa was conserved, as it still primarily included amoeboid organisms, and now included the Mycetozoa. •
Amoebozoa : includes all naked and testate lobose amoebae (traditional Lobosea) as well as the
pelobionts and
eumycetozoans, and a few flagellates. •
Rhizaria : includes amoebae bearing reticulose or filose pseudopodia, the majority of which were traditionally classified as Filosea, Granuloreticulosea and Actinopoda, such as
Euglyphida,
Gromiida,
Radiolaria,
Proteomyxidea,
Phaeodarea and
Foraminifera (including
Xenophyophorea). It also houses a large diversity of free-living flagellates, amoeboflagellates and parasites like the
Plasmodiophorida. •
Heterolobosea : amoebae with lobose pseudopodia but eruptive flow of cytoplasm. Currently it includes the aggregative Acrasida, as well as several other amoeboflagellates. They are a class of
excavates closely related to
Euglenozoa, with whom they share their characteristic discoidal
mitochondrial cristae. •
Stramenopiles : although primarily composed by flagellates, it contains a few groups of amoebae. For example: the
Actinophryida, an order with typical heliozoan morphology; the amoeboid
Rhizochromulina, a genus of
chrysophytes; or
Synchroma, a genus of amoeboid algae with reticulate axopodia. •
Rotosphaerida : also known as
nucleariids, includes a few filose amoebae traditionally classified within the Filosea, positioned as the sister group of
Fungi. •
Centroplasthelida : heliozoans with a centroplast from which axopodia arise. They are closely related to the
haptophyte algae inside the supergroup
Haptista. •
Rigifilida : a small order of filose amoebae previously interpreted as nucleariids. Together with the flagellate orders
Mantamonadida and
Diphylleida, it composes the
CRuMs clade, positioned closest to
Amorphea. •
Breviatea : includes enigmatic free-living
amoeboflagellates related to
opisthokonts. The following cladogram shows the sparse positions of amoeboid groups (in bold), based on molecular phylogenetic analyses: ==Amoeboid cells in other organisms==