Valves with apices that are sharply rounded, at less than 90°
Transversely linear chambers in the valve that have small openings externally and large openings internally
Annulae are comprised of one to four transapical striae that interrupt the typical striae at the poles. These structures are restricted to the genus Geissleria. The annulae may be distinctive or barely discernible.
The long axis of a diatom is considered the apical axis.
Valve ends that are abruptly tapered to a fine point
The araphid diatoms are pennate forms that lack a raphe system, hence lack motility. Examples of araphid genera include Diatoma, Fragilaria and Synedra.
Curved like a bow or bent along the apical axis
Perforation, or pore, in the diatom valve. The plural form is areolae.
Becoming thin or slender
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.
The axial plate is a structure within some species of Gomphoneis. The term refers to the internal ‘plate’ of silica that occludes the areolae internally. The margin of the axial plate results in the longitudinal line, visible in light microscopy.
A structure that is divided into two branches is said to bifurcate.
Pennate diatoms with a raphe system on both valves of the frustule are biraphid. For example, Amphora, Epithemia, Gomphonema, Navicula and Mastogloia are all biraphid genera. In contrast, some pennate diatoms lack a raphe completely (i.e., Diatoma), while others have a raphe system on only one valve (monoraphid), such as Planothidium.
Striae that are composed of two rows of areolae are termed biseriate.
Valve ends that are expanded into a rounded knob are termed capitate. Note that the shape terms subcapitate and subrostrate are quite close, and that many taxa can be described as possessing subcapitate to subrostrate ends. In general, capitate (and subcapitate) ends are more expanded than the most narrow point, while rostrate (and subrostate) ends are not more expanded than the most narrow point.
A type of central process unique to the genus Orthoseira. The internal openings are simple, while the external openings are composed of well-defined collars.
In raphid diatoms, the thickly silicified area located between the proximal raphe slits. The central nodule is the site where the first silica is deposited in by the silica deposition vesicle in valve formation. The central nodule is often thickened, both in valve and girdle views.
Cingulum is a term that refers to all of the collective elements of the diatom girdle. The cingulum is made up of delicate silica bands (= girdle bands, copulae) that join the two valves of a frustule. Most diatoms possess a cingulum, although some may not. The silica structures of the cingulum may be secreted after vegetative cell division that produces the diatom valve; that is, while the deposition of the silica valves are tied to cell division, the deposition of the cingulum may not be tied to division.
Club shaped; usually applied to describe the asymmetry of Gomphoneis and Gomphonema.
A valve surface covered with small bumps or elevated bits of silica is termed ‘colliculate’.
A structure found in the genus Aulacoseira. The collum is the narrow, hyaline area on the valve mantle. A small furrow, the sulcus, separates the collum from the portion of the valve mantle with areolae.
A thin siliceous covering over the striae on the external valve surface. The conopeum may be slightly to distinctly elevated from the valve face. The width of conopea is variable, but may extending to the valve margin. This structure is visible with scanning electron microscope. Conopea have been found to contain symbiotic nitrogen-fixing cyanobacteria and are found in a number of genera, including Fallacia, Lyrella, Microcostatus and Sellaphora.
Striae angled or curved away from the central nodule are termed convergent striae. Striae may be convergent in a specific part of the diatom valve. For example, the striae may be convergent throughout the valve, convergent only at the center valve, convergent only at the ends, etc. Convergent striae are opposite of radiate (divergent) striae.
Plural, costae. A general term for a unornamented thickening of silica on the valve face.
Craticula is a term applied to internal valves that are formed by species in the genus Craticula. Frustules that are normal in morphology produce the craticular stage under conditions of high solute concentration. The craticular valves consist of a raphe-sternum and robust transverse bars. These internal valves are formed within normal valves and, unlike normal valves, their formation is not linked to mitosis.
A thin covering of silica that possesses fine pores. The cribrum is located on the internal opening of an areola. This structure is visible only with scanning electron microscopy.
Cross-shaped, usually applied to describe the valve outline of some diatoms.
The portion of the raphe system that is near the valve end, or valve apices.
In diatoms that are asymmetrical to the apical axis, the dorsal side is considered to be the side that is more convex.
Having notably more length than width; being long and slender
Native or limited to a specific geographic area
Algae which grows within the subsurface of sediments, clays, and silt
An organism that lives inside another, benefitting both individuals, is considered to be an endosymbiont.
Algae which grows on the surface of rock or stone
Algae which grows on the surface of sediments, clays, and silt (i.e., residing at the water/sediment interface).
Algae which grows on the surface of plants or other Algae
Attached to (or moving through) sand particles
The larger valve of a frustule, in comparison to the hypotheca
A fascia is an area of thickened silica extending from the central area of pennate diatoms. A transverse fascia is formed by secondary deposition of silica into former depressions (Cox 2012). Also see definition for ‘stauros’. A number of genera possess a transverse fascia, including Luticola and Staurophora.
Series, or bundles, of rows of areolae oriented radially in some centric diatoms are termed fascicles.
Internal struts that provide structural support to the canal that contains the raphe. The fibulae extend transapically from the raphe canal to the valve face and are found in a number of genera including Denticula, Nitzschia and Surirella.
Footpole is a term used to describe the narrower end, or basal end, of the valve in genera (Didymosphenia, Gomphoneis, Gomphonema) that are club-shaped.
A small opening, perforation, or orifice; a fenestra.
Siliceous portion of diatom cell, a consisting of two valves, an epivalve and hypovalve and their associated cingulum elements
A tubular process of some centric diatoms, usually associated with the secretion of §-chitin to maintain buoyancy in the plankton. The fultoportula consists of a central tubular process surrounded by two or more satellite pores. Externally, the fultoportula appears as either a tube or a simple pore in the valve wall. The plural form is ‘fultoportulae’.
Broadest at the middle and tapering at both apices; spindle shaped.
Faint striae, composed of areolae that may not penetrate the valve wall.
Convex, rounded, protuberant; exceeding a semi-circle but less than a circle
The diatom cell is composed of two valves. When the valve is viewed from the side, this view is called the girdle view.
Headpole is a term used to describe the broader end the valve in genera (Didymosphenia, Gomphoneis, Gomphonema) that are club-shaped.
The internal, distal termination of the raphe is termed the helictoglossa. The structure is developed in many raphid diatoms, but not all. The helictoglossa is a lip-like structure. In many species, helictoglossae are distinguishable in the light microscope in valve view by optical dissection (focusing through many narrow, optical planes). The structure may also be visible in girdle view, as a thickened lip of silica.
Taxa that are asymmetrical to the transapical plane have apices that are dissimilar in shape, and are termed heteropolar.
Frustules in which one valve differs in morphology from the other. Many of the diatoms are heterovalvar based on valve ornamentation or the presence or absence of a raphe system. The term is often used to refer to the monoraphid diatoms, in which one valve possesses a normal raphe valve and the other possesses a rapheless valve (that has been secondarily filled with silica).
A holotype of a name of a species or infraspecific taxon is the one specimen or illustration used by the author, or designated by the author as the nomenclatural type. As long as a holotype is extant, it fixes the application of the name concerned
The hood is an internal silica extension that extends over the rimmed depression, found in species within the genus Planothidium.
‘Hyaline’ is a general term that is applied to any unornamented area (area lacking pores or other structures) of a diatom valve.
Very delicate silica membrane that occlude the pores
The smaller valve of the two valves that make up the frustrule, the larger valve is the epitheca
Usually applied to centric diatoms, the interfasciular area is the unornamented, thicked rib of silica located between fascicles.
Fissure that joins the proximal raphe ends in some cymbelloid taxa. Instead of distinct termination of the proximal raphe ends, they are connected, internally by a fissure.
Valves that are symmetrical to the transapical axis are termed ‘isopolar’. The poles of these valves are similar in shape and size.
An isotype is any duplicate of the holotype; it is always a specimen.
In the genus Entomoneis, the keel joins with the valve forming a line, termed the junction line. The junction line forms straight or irregular contours and may be variously ornamented.
A raised or elevated ridge that contains the raphe, formed from a folding of the valve wall
Lanceolate refers to a “leaf shape” and may range from wide to narrow or fusiform.
Silica extension of the cingulum; in some taxa, projects to intersect with neighboring girdle bands.
Lineolae are areolae that are transapically elongated and are characteristic of Navicula sensu stricto, or Navicula in the Lineolate group.
In the genus Aulacoseira, linking spines join frustules together in a chain, or filament.
Light microscopy is abbreviated by ‘LM’. The study of diatoms relies on a compound microscope in which a beam of light passes through optical lenses to view an image of the specimen. Contributors to the Diatoms of the United States project use research grade microscopes with the minimum specifications of 100x, 1.3 numerical aperature (NA) oil immersion objective lens and 1.3 NA condenser lens.
Valve walls that have more than one layer of silica may have a complex, chambered structure. Areolae may also be considered to be chambered, or loculate (the chamber is termed the loculus)
A canal positioned between the primary and secondary layer of silica. Longitudinal canals may be central or marginal and are found in some genera, including Neidium and Muelleria.
The longitudinal rib is a solid silica structure that runs along the apical axis.
The term, lunate, refers to a moon, or crescent, shaped structure. The term is used to describe a thickened silica shape in the genus Caloneis.
The part of a valve that extends from the valve face, forming the edge of the valve. When a frustule is viewed in girdle view, the mantle is visible.
Taxa that possess a raphe on only one valve are considered within the monoraphid group. Freshwater monoraphid genera include Cocconeis, Achananthes, Achnanthidium, Eucocconeis, Karayevia, Lemnicola, Planothidium, Platessa, Psammothidium and Rossithidium. The genus Nupela includes some monoraphid species and some biraphid species. On the Diatoms of the US website, Nupela is included in the biraphid group.
A monotypic genus is one for which a single binomial is validly published. An example of a monotypic genus is Diprora Main. The genus contains only one species, Diprora haenaensis Main.
Mucopolysaccharide secreted by diatoms
Naviculoid means ‘navicula-like’, or shaped like a boat. The genus Navicula was described in 1822 by Bory for its boat-like valve margin.
Obovate, with a concavity in each side, like the body of a violin
The genus Mastogloia possesses a distinct valvocopula (first girdle band). The valvocopula is marked by one to several internal chambers, called partecta, and tubes that open from the partecta to the outside of the diatom valve. The partecta function to secrete strands of mucilage from the cell.
A bilaterally symmetric diatom. A heterogeneous group that includes araphid, monoraphid, and biraphid taxa.
The axis that crosses the center of the apical and transapical axes.
A membrane-bound organelle and the site of photosynthesis. Most diatom plastids appear golden brown due to carotenoid pigments (beta-carotene, diatoxanthin, diadinoxanthin, fucoxanthin), which mask the color of the chlorophyll molecules (chlorophylls a, c2, and c1 or c3)
A polar bar is a hyaline thickening of silica, present at the apices in some pennate species. The genus Sellaphora possesses variable development of polar bars.
A term applied to a particular structure in the genus Surirella. Porcae are transapical undulations, or elevated ridges, of the external valve face.
An area of fine pores, or perforations through the diatom valve. Porefields function in the extrusion of mucopolysaccaride stalks, or pads, from the cell.
Small, closely packed perforations through the valve. The small perforations in ocelli, pseudocelli and apical pore fields are often referred to as porelli.
The primary side of a valve is the side that is formed from the initial branch of the sternum of raphid diatoms.
Many algae live attached firmly to a surface, such as rocks, macrophytes or other substrates. Species that ‘lie flat’ and are affixed to a surface are considered to have a prostrate habit of growth.
The terminus of the two raphe branches near the center of the valve. The proximal raphe ends border the central nodule.
A plate, or lamina, of silica projecting internally from the apical portion of the valve. Pseudoseptae are found in some species of Gomphonema, Gomphoneis, Stauroneis, and Navicula. Pseudoseptae are part of the valve, while septae are part of the copulae, or girdle bands.
Distinct, proteinaceous structure (often spherical), embedded in or associated with chloroplasts; in some, contains the enzyme RuBisCO; associated with starch in some diatoms
Striae angled or curved toward the central nodule are called ‘radiate’ striae. Striae may be radiate in a specific part of the diatom valve. For example, the striae may be radiate throughout the valve, radiate only at the center valve, radiate only at the ends, etc.
A slit through the valve face of monoraphid and biraphid diatoms, usually positioned along the apical axis. This is the structure which enables a diatom cell to move over substrates.
In the cell development of monoraphid pennate diatoms, a raphe is initially formed on both valves. The raphe of one valve is secondarily filled with silica, leaving a thickened axial rib. The other valve lacks a raphe and is termed the “rapheless valve”.
The proximal internal terminations of the raphe in a rectangular, elevated, elongate structure. This structure is found in Muelleria, Scoliopleura and Neidium.
This term describes a structure that is ‘bent back’ on itself.
Many Planothidium species, but not all, possess an asymmetrical central area, which bears internally either a rimmed depression or a hood. The difference between these two structures was not recognized before SEM studies, and both of them were termed ‘hufeisenförmige Fleck” in German or “hoof-mark” or “horseshoe-shaped area” in English. The rimmed depression is a structure found only in the genus Planothidium. It is a hyaline structure on one side of the central area of the rapheless valve.