Hyalospheniidae

Characteristics

Test rigid, either chitinoid, clear, completely organic, non areolar or agglutinate composed of shell plates of small euglyphids (e.g. Euglypha, Trinema, Tracheleuglypha), or plates of Quadrulella and diatom fragments. The shape of the tests varies between genera and species, but tests are often oval to flask-shaped, compressed, and the pseudostome is always terminal.

The assumption in the older literature that genera like Nebela, Apodera etc. can synthesize their own siliceous ideosomes has not been proven. There is some evidence from scanning electron microscopy that members of the genus Argynnia, which is traditionally placed in this group, can coat foreign building material. Usually genera  that use external material (e.g. Nebela, Apodera or Certesella) fix the building material in a sheet-like organic cement matrix, which is normally visible between the plates; only Argynnia and Physochila use distinct cement units. In some species, under building-material-limited conditions, the test can be completely organic (e.g. N. tincta, A. cockayni).

Classification

The concept of the family Hyalospheniidae has changed during the last century. Deflandre (1953) has included most of the genera that are here treated in the Heleoperidae, Hyalospheniidae or Lesqueusiidae in the family Nebelidae.

Meisterfeld (2002) has classified the genera we treat here in this group in two separate families:

1. Hyalospheniidae (Hyalosphenia, Leptochlamys)
   Genera with a rigid, chitinoid, completely organic and non-areolar test. 
2. Nebelidae (Nebela, Alocodera, Apodera, Argynnia, Certesella,  Physochila, Porosia, Schoenbornia?)
   Genera with tests composed of shell plates of small euglyphids (e.g. Euglypha, Trinema, Tracheleuglypha) or plates of Quadrulella and diatom fragments.

Lara et al. (2008) have shown with molecular data that the family is probably not monophyletic and that some genera are perhaps paraphyletic. But a new classification that reflects the phylogenetic relations between Hyalosphenia and the old families Nebelidae and Heleoperidae correctly is not yet available. 

Deflandre (1936) has proposed several groups within Nebela to accommodate the large morphological diversity in this genus. Jung (1942) split Nebela into 11 genera unfortunately without type designations. Only the monotypic genera are valid, all others are nomina nuda. With the exception of Physochila none of the genera have been recognised by subsequent workers. Loeblich & Tappan (1961) validated Apodera and Certesella. Here, genera with structured cement and distinct shell morphology (Argynnia, Physochila) and those taxa with sheet-like cement, sufficient morphological particularity and restricted geographic distributions are recognised as distinct genera, the remaining species being assigned to Nebela.  But the position of Argynnia, Physochila and Schoenbornia remains uncertain.

References

Deflandre, G. 1936. Etude monographique sur le genre Nebela Leidy (Rhizopoda - Testacea). Ann. Protistol., 5:201-286.

Jung, W. 1942 Südchilenische Thekamöben. (Aus dem südchilenischen Küstengebiet, Beitrag 10). Arch. Protistenk., 95:253-356.

Lara, E., Heger, T.J., Ekelund, F, Lamentowicz and Mitchell, E.A.D. 2008: Ribosomal RNA Genes challenge the monophyly of the Hyalospheniidae (Amoebozoa: Arcellinida). Protist 159: 165-176.

Loeblich, A. R. JR. & Tappan, H. 1961. Remarks on the systematics of the Sarkodina (Protozoa) renamed homonyms and new and validated genera. Proc. Biol. Soc. Wash., 74: 213-234.

Meisterfeld, R. 2002. Order Arcellinida Kent. In: An Illustrated Guide to the Protozoa. Lee, J.J.; Leedale, G.F. & Bradbury, P. (eds.). 2nd edition. Soc. of Protozoologists, Lawrence Kansas U.S.A., Allen Press Inc. Vol 2: 827 860.