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Actinosphaerium eichhornii
Actinosphaerium eichhornii

Family Actinophryidae Claus 1874, emend. Hartmann 1926

Diagnosis: Axonemal pseudopodia emerging from amorphous centrosome near nuclei; axonemal microtubules in double interlocking coils; single central nucleus or several peripheral nuclei; tubular mitochondrial cristae; two types of extrusomes for prey-capture along axopodia; cysts covered with siliceous elements; autogamy reported within spores.

Description: Actinophryid heliozoa are sperical bodied unicellular organisms without cilia. They have a single central nucleus or many small nuclei located in the endoplasm, the central part of the cell. Numerous stiff axopodia, noticeably tapering from the base to the tip, radiate from the whole body surface. These axopodia are supported internally by microtubules arranged in a double hexagonal spiral and terminate in electron-dense material located on the nuclear envelope or near a nucleus. Mitochondrial cristae are tubular (bleb-like) and have an electron-dense matrix. There are two types of simple extrusomes – a larger osmiophilic type and a smaller granular type. The surface of trophic cells is naked.
Actinophryids feed mainly by predation, often accompanied by fusion of several cells. Cysts may form which have multiple walls, one of which is comprised of siliceous elements. Reproduction is mainly by binary fision. Sexuality is limited to autogamy and occurs in the cyst and is accompanied by the formation and subsequent fusion of amoeboid gametes.

Remarks: Species identities in this taxon are very unclear. Individuals exhibit considerable variation in form, especially as a result of recent feeding history. In the absence of type material for any of the species, many of the identities remain ambiguous. At this site I don’t describe those species which cannot be unambiguously identified.

Two taxa have been described:

1 Cells with one central nucleus – Actinophrys

Cells with several peripheral nuclei – Actinosphaerium

Actinosphaerium
Actinophrys sol, with large central nucleolus and ingested diatom
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