Calothrix Agardh ex Bornet et Flahault, 1886

Key # 16

  1.) Classification: Order Nostocales, Family Rivulariaceae. There are approximately 60

        described species. The Rivulariaceae, which includes the genera Calothrix, Gloeotrichia,

        and Rivularia, is one of the more structurally complex families among the cyanobacteria.

        Furthermore, there is a great deal of heterogeneity within the genus Calothrix.

        Examples of this include the presence or absence of terminal hairs and akinetes, and the

        the analysis of its genetic material. For this, published studies suggest revision

        of some members of this genus into a novel genus (Berrendero et. al. 2008, 2011).

  2.) Form: Filamentous.

  3.) Groups: Either solitary filaments or small, bristle-like clusters/thin mats that creep

        along the substrate. They do not occur in a common colonial mucilage. They are

        oriented toward the origin of growth.

  4.) Sheath: Individual, firm, yellow brownish, and, at times, lamellated (structures composed

        of thin, homogeneous layers) and/or widened at the end into funnel-shaped collars.

  5.) Filaments: Heteropolar. Basal pole is wider, and attached with spherical to hemispherical

        heterocysts. Apical pole usually tapers to elongate, hairlike, hyaline end cells.  Hair

        formation depends on phosphorus metabolism. False branches are rare.

        Crosswall constriction is variable.

  6.) Cells: Cylindrical to barrel shaped, with no aerotopes in vegetative cells.

        Heterocysts: Basal end, ellipsoidal to spherical, seldom intercalary.

        Akinetes: Rare, ellipsoidal. When they occur, they occur above the basal heterocyst.

        Aerotopes: Absent in vegetative cells, but present in hormogonia.

  7.) Color: Ranges from green to black.

  8.) Replication: Filament disintegration at heterocysts (maturing into new filaments) or

        by motile hormogonia (separating from filament at necridic cell and released from the

        sheath after loss of terminal hair).  Aerotopes form only in hormogonia. Cell division

        is perpendicular to long filament axis. Some have meristematic zones.

  9.) Habitat: Strictly periphytic; growing on macrophytes, other algae, or substrates in

        nonpolluted aquatic habitats.  Several are from marine littoral or supralittoral zones.

10.) Similar genera: Rivularia and Gloeotrichia. All 3 have terminal, basal heterocysts,

        tapering filaments and obvious sheaths. 

        Gloeotrichia colonies have a characteristically conspicuous radial arrangement.

        Calothrix filaments tend to be solitary or in small clusters; they are not radially arranged.

        Gloeotrichia  is planktonic; Calothrix and Rivularia are generally not planktonic.

11.) Toxins: None are reported.

12.) Distinguishing features: Calothrix filaments have a round, basal heterocyst, and the

        apical end tapers down to a hairlike structure. They are solitary or in small groups

        attached to a substrate. They are not planktonic.  MAN

Calothrix_01

Calothrix filaments have a round, basal heterocyst (bh) and the apical end tapers

into smaller cells. The trichome is covered with a sheath (s) that broadens out

at the apical end. They are solitary or in small groups attached to a substrate

(seen in this image). This sample is from shallow water in El Junco, Ecuador

taken in 2014. 400X magnification.