Description
''Staurastrum'' consists of singular cells with two identical halves, called semicells; in most species, the cell is deeply constricted at the middle, making the division between semicells clear. When viewed from the polar axis, they are biradiate (with two planes of symmetry) or pluriradiate (with more than two planes of symmetry). Biradiate individuals have long, arm-shaped extensions (termed "processes") attached to the main body of the semicells. Pluriradiate individuals may lack these processes. The cell wall is either smooth, or covered with granules or spines. In each semicell there is a chloroplast, usually located axially, with radiating lobes and/or ridges; some species have parietally located chloroplasts.Identification
''Staurastrum'' as a genus is usually easy to identify, due to its long processes and symmetry. Some smaller species, particularly those without long processes, can be confused with ''Cosmarium'', but that genus has biradiate (not pluriradiate) cells and never with long processes. The smaller species may also be confused with ''Tetraedron'' or various xanthophyte algae, but differ in shape, ornamentation, and chloroplast morphology. Although some species of ''Staurastrum'' are unmistakable, identification of species within the genus is often difficult. Important characters for identification include the degree of radiation (the number of planes of symmetry), overall shape and size of the cells, and the distribution and type of ornamentation on the cell wall. As with other genera of desmids, identification is complicated by the fact that these algae exhibit considerable phenotypic variability.Taxonomy
''Staurastrum'' is the second largest genus of desmids (after ''Cosmarium''), with about 800 species. It is the most heterogeneous desmid genus, and has been described as a wastebin taxon. Several attempts have been made to dismantle the genus and split it up into smaller, more homogeneous units, such as G. M. Palamar-Mordvintseva's segregate genera ''Cylindriastrum'', ''Cosmoastrum'', and ''Raphidiastrum''. These attempts have not been widely accepted, because they are also arbitrary. ''Staurastrum'' is not monophyletic, as molecular phylogenetic studies have shown that members of the genus are dispersed across many clades; however, a "core" ''Staurastrum'' clade was resolved. The genus is still in use, because there is no known way of classifying desmids in line with their evolutionary history.Habitat and distribution
''Staurastrum'', as well as other desmid genera, live primarily in acidic lakes, ponds, and swamps with low nutrient levels. They usually live on sediments or Periphyton, submerged surfaces. A number of species are characteristic of eutrophic waters and are planktonic. Many species have a cosmopolitan distribution. However, some species are restricted to a particular biogeographic realm.Species
* ''Staurastrum affine, S. affine'' * ''Staurastrum alternans, S. alternans'' * ''Staurastrum arctiscon, S. arctiscon'' * ''Staurastrum asteroideum, S. asteroideum'' * ''Staurastrum avicula, S. avicula'' * ''Staurastrum brachiatum, S. brachiatum'' * ''Staurastrum brachycerum, S. brachycerum'' * ''Staurastrum brebissonii, S. brebissonii'' * ''Staurastrum capitulum, S. capitulum'' * ''Staurastrum cerastes, S. cerastes'' * ''Staurastrum clepsydra, S. clepsydra'' * ''Staurastrum crenulatum, S. crenulatum'' * ''Staurastrum cristatum, S. cristatum'' * ''Staurastrum cyclacanthum, S. cyclacanthum'' * ''Staurastrum dejectum, S. dejectum'' * ''Staurastrum dickiei, S. dickiei'' * ''Staurastrum dilatatum, S. dilatatum'' * ''Staurastrum dispar, S. dispar'' * ''Staurastrum galeatum, S. galeatum'' * ''Staurastrum gemelliparum, S. gemelliparum'' * ''Staurastrum gracile, S. gracile'' * ''Staurastrum hantzschii, S. hantzschii'' * ''Staurastrum hexacerum, S. hexacerum'' * ''Staurastrum hirsutum, S. hirsutum'' * ''Staurastrum iotanum, S. iotanum'' * ''Staurastrum isthmosum, S. isthmosum'' * ''Staurastrum javanicum, S. javanicum'' * ''Staurastrum laeve, S. laeve'' * ''Staurastrum lapponicum, S. lapponicum'' * ''Staurastrum lunatum, S. lunatum'' * ''Staurastrum maamense, S. maamense'' * ''Staurastrum majusculum, S. majusculum'' * ''Staurastrum margaritaceum, S. margaritaceum'' * ''Staurastrum monticulosum, S. monticulosum'' * ''Staurastrum muticum, S. muticum'' * ''Staurastrum natator, S. natator'' * ''Staurastrum omaerii, S. omaerii'' * ''Staurastrum ophiura, S. ophiura'' * ''Staurastrum orbiculare, S. orbiculare'' * ''Staurastrum paradoxum, S. paradoxum'' * ''Staurastrum pingue, S. pingue'' * ''Staurastrum polymorphum, S. polymorphum'' * ''Staurastrum pseudotetracerum, S. pseudotetracerum'' * ''Staurastrum punctulatum, S. punctulatum'' * ''Staurastrum pyramidatum, S. pyramidatum'' * ''Staurastrum quadricornutum, S. quadricornutum'' * ''Staurastrum sebaldi, S. sebaldi'' * ''Staurastrum simonyi, S. simonyi'' * Staurastrum sp. BBM-2004, ''S.'' sp. BBM-2004 * Staurastrum sp. desmid10, ''S.'' sp. desmid10 * Staurastrum sp. M752, ''S.'' sp. M752 * Staurastrum sp. M753, ''S.'' sp. M753 * ''Staurastrum spongiosum, S. spongiosum'' * ''Staurastrum subavicula, S. subavicula'' * ''Staurastrum subgemmulatum, S. subgemmulatum'' * ''Staurastrum thunmarkii, S. thunmarkii'' * ''Staurastrum tumidum, S. tumidum''References
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