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Diatoms of the United States is now known as Diatoms of North America.
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Glossary terms starting with "a"



The direction away from the valve face along the mantle.


Acutely rounded

Valve apex that is sharply rounded, at less than 90°.



The direction toward the valve face along the mantle.


A stria in the shape of an elongated chamber. The external wall of the alveolus is perforated by many areolae and the internal wall is perforated by one long opening. Plural form is alveoli. An example genus is Pinnularia.


Annulus (centric taxa)

In some centric diatoms, a hyaline ring separating the areolae in the valve center from the rest of the valve face. For example, the valve face of the genus Discostella has an annulus.

Annulus (Geissleria)

In the genus Geissleria, a circular or semi-circular hyaline area on the valve face which interrupts two or more distal striae. An annulus may be distinctive or barely discernible, depending on species. Plural form is annuli.

In Krammer et al. (2000) the definition is in the key for the Navicula group Annulatae. The species in this group have been elevated to the genus Geissleria.



A small projection opposite the ligula that covers gaps in abvalvar (younger) split rings.

Apical axis

The long axis of the valve face of a pennate diatom, passing through the apices. Follows the midline of the valve and may be curved, as in the genus Cymbella. See Pervalvar axis and Transapical axis.



Valve apex that is abruptly tapered to a fine point.


Pennate diatoms lacking a raphe on either valve. Examples of araphid genera include Diatoma, Fragilaria, and Synedra.



Curved like a bow or bent along the apical axis.

Areola - punctum

Perforation, or pore, in the diatom valve. Usually many are grouped to form a stria. The plural form is areolae.

The shape of the areola can be important in diatom taxonomy, and may be lineolate, punctate, loculate, or in the shape of a letter C.


Attenuated - drawn-out

Becoming thin or slender.

Aufwuchs - epiphyton , periphyton

A German term for “surface growth” or “overgrowth”. A community of small animals and plants that adhere to surfaces in aquatic environments. Green algae and diatoms make up the dominant component of an aufwuchs. Small crustaceans, rotifers, and protozoans are also commonly found in both fresh water and marine aufwuchs faunas, while insect larvae, oligochaetes and tardigrades are peculiar to freshwater aufwuchs faunas. An English pronunciation might be “OWF-vooks”.



A special cell that develops and expands before producing a new frustule. Maximum size of a population is restored through auxospore formation. Auxospores are usually associated with sexual reproduction.

Axial costa - longitudinal rib

A narrow ridge of silica along the axial area on the inside of the valve bordering the raphe.

Mastogloia smithii is an example with two internal axial costae. Together they form the sides of a groove, or gutter (Paddock and Kemp, 1990), widening slightly at the central nodule and stopping short of the distal raphe ends.

See also costa.

Axial plate

Internal ‘plate’ of silica that occludes the inner openings of the areolae. Found in some species of Gomphoneis where its margin appears as the longitudinal line that is visible in light microscopy.