Cosmarium Corda ex Ralfs, 1848

Lectotype species: Cosmarium undulatum Corda ex Ralfs

Original publication: Ralfs, J. (1848). The British Desmidieae The drawings by Edward Jenner, A.L.S. pp. [i]-xxii, [i], [1]-226, 35 pls. London: Reeve, Benham & Reeve, King William Street, Strand.

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Type designated in: Silva, P.C. (1952). A review of nomenclatural conservation in the algae from the point of view of the type method. University of California Publications in Botany 25: 241-323.

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Description: Cells solitary, tiny to large with shallow to deep median constriction (isthmus). Semicells rounded, reniform, pyramidate, quadrate with entire or undulate margin; subcircular to elongate-oval (biradiate) in apical view. Triradiate forms known in collections and cultures. Cell wall smooth with scattered pores or ornamented with small or large granules, emarginate verrucae, round or triangular pits, or short spinules. Central and marginal ornamentation differing or identical. Mucilaginous sheath, secreted through cylindrical cell wall pores, often surrounds cell. Chloroplasts one to several per semicell, axial or parietal, each with one to several pyrenoids per chloroplast. Nucleus in isthmus.

Asexual reproduction by cell division and new semicell formation (used as model for so-called Cosmarium-type desmids). Mitosis followed by cytokinesis at isthmus. Transverse septum of primary wall material msplits top become primary wall layer of new semicells. New semicells expand, bounded by primary wall, eventually reaching size and shape of parental semicell. Nucleus in new semicell during expansion; later migrates back to isthmus. After expansion complete, secondary wall with pores and wall ornamentation produced beneath primary wall. Daughter cell primary walls usually joined at apices; shed as single unit or disintegrate.

Sexual reproduction by homothallic or heterothallic conugation. Conjugating cells come together within mucilaginous envelope; semicells split at isthmus. Amoeboid gametes fuse between gametangia. Mature to zygospore spherical with short, acute or truncate spines (sometimes with furcate tips). Chloroplasts of only one mating type survive in zygospore. Karyogamy at germination followed by meiosis. Two gones (rarely one). Mating type controlled by single pair of Mendelian alleles. Recessive gene causes zygote mortality in C. turpinii. Crosses of haploid and diploid triradiate cells and normal biradiate cells with normal biradiate cells produce viable diploid and nonviable triploid zygospores. Inheritance of triradiate form non-Mendelian. Cosmarium in acidic, oligotrophic, aquatic environments. Occasionally subaerial or in basic, eutrophic water.

Information contributed by: J.F. Gerrath & M.D. Guiry. The most recent alteration to this page was made on 2013-09-12 by M.D. Guiry.

Taxonomic status: This name is of an entity that is currently accepted taxonomically.

Most recent taxonomic treatment adopted: Guiry, M.D. (2013). Taxonomy and nomenclature of the Conjugatophyceae (=Zygnematophyceae). Algae. An International Journal of Algal Research 28: 1-29.

Comments: Cosmarium is found mainly in acidic, oligotrophic, aquatic environments. Occasionally subaerial or in basic, eutrophic waters. Several species are common with cosmopolitan distributions; others rarely collected, with very restricted distributions.

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Some of the descriptions included in AlgaeBase were originally from the unpublished Encyclopedia of Algal Genera, organised in the 1990s by Dr Bruce Parker on behalf of the Phycological Society of America (PSA) and intended to be published in CD format. These AlgaeBase descriptions are now being continually updated, and each current contributor is identified above. The PSA and AlgaeBase warmly acknowledge the generosity of all past and present contributors and particularly the work of Dr Parker.

Descriptions of chrysophyte genera were subsequently published in J. Kristiansen & H.R. Preisig (eds.). 2001. Encyclopedia of Chrysophyte Genera. Bibliotheca Phycologica 110: 1-260.

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M.D. Guiry in Guiry, M.D. & Guiry, G.M. 12 September 2013. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway.; searched on 24 June 2024

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