Rivularia [Roth] C. Agardh ex Bornet et Flahault, 1886
Key # 15
1.) Classification: Order Nostocales, Family Rivulariaceae. There are 20 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 Rivularia, based on
analysis of its genetic material. For these reasons, some studies suggest reclassification
of some members of Rivularia into a novel genus (Berrendero et. al. 2008).
2.) Form: Filamentous.
3.) Groups: Filaments are arranged in dense, parallel colonies up to several centimeters wide.
Young colonies are hemispherical to spherical; older colonies are irregularly flat, bullate
stratified, gelatinous, leathery, and often encrusted with calcium carbonate (CaCO3).
4.) Sheaths: Individual or colonial; copious, firm, viscous, lamellate, colorless to yellowish.
5.) Filaments: Simple, mostly straight (some bends), cylindrical and the apical end is tapered.
Mature filaments are heteropolar.
Basal: an anchored heterocyst.
Apical: a hair-like cell at colony surface.
6.) Cells: Midfilament cells are cylindrical/rectangular with variable crosswall constrictions.
Heterocysts: Spherical to obovate at basal terminus. Some have intercalary heterocysts,
which divide and separate but remain in parallel arrangement in the original sheath.
Akinetes: Absent.
Aerotopes: Absent.
Apical cells: Hair-like structures; elongate, hyaline cells oriented to colony surface.
7.) Color: Varies from gray green to pinkish.
8.) Replication: Filament disintegration, heterocysts and hormogonia. Young filaments
undergo a short phase of hormogonium formation. Later hormogonia formation occurs
after loss of apical hair cells via necridic cells, which aids hormogonia in leaving the
9.) Habitat: Periphytic and epilithic or epiphytic, mostly in clear, unpolluted, streams;
some in marine littoral. Substrates are often calcareous, which aids calcium carbonate
(CaCO3) precipitation. They are important members of travertine-forming biocenoses.
The epiphytic species are not encrusted.
10.) Similar genera: Calothrix, Gloeotrichia. All have terminal, basal heterocysts, tapered
filaments, and obvious sheaths.
Rivularia and Calothrix live attached to benthic surfaces, but Gloeotrichia is plantonic.
Rivularia forms dense mats a few centimeters wide.
Calothrix filaments are solitary or in small, sparse, bristle-like clusters.
Gloeotrichia colonies are floating radial spheres.
11.) Toxins: None are reported.
12.) Distinguishing features: Rivularia filaments have a round, basal heterocyst and an
apical end that tapers down to a hair-like (hyaline) structure. They occur in densely
populated mats attached to a substrate. MAN
Rivularia 01
Rivularia is a group of filamentous cyanobacteria where the filaments are
arranged in dense, parallel colonies up to several centimeters wide. The filaments
have a darker, basal heterocyst that anchors the filament into the colony or to a
substrate. The filament gradually tapers apically to a hair-like, hyaline end with
a viscous sheath extending beyond the end. The cells are cylindrical to
rectangular with variable crosswall constrictions. No akinetes. They grow on
plants and other surfaces. This is from a prepared, stained slide.
100X Magnification.
Rivularia 02
This is also from a prepared, stained slide. Notice the dark-staining heterocysts
clustered in the interior of the colony. 400X Magnification.
Rivularia 03
This specimen is shallow water sample from Camp Ernst Lake. The arrows show
the basal heterocysts. 200X Magnification.