Edward Theriot

TAXONOMIC INTERPRETATION OF THE RIMOPORTULA OF FRESHWATER GENERA IN THE CENTRIC DIATOM FAMILY THALASSIOSIRACEAE

It is difficult to delineate Cyclotella, Cyclostephanos and Stephanodiscus—the three most taxonomically diverse, predominantly freshwater genera of the Thalassiosiraceae. Cyclostephanos was originally distinguished by a Stephanodiscus-like exterior of fasciculate striae and raised radiating costae, and a Cyclotella-like interior, with chambers near the margin. After the creation of Cyclostephanos, Cyclotella and Stephanodiscus were each diagnosed by absence of characters. However, this study suggests that most Cyclostephanos and Stephanodiscus species can be classified on the basis of their unusual rimoportula morphology. Over half of the Cyclostephanos species known, including the lectotype, have the rimoportula underneath a spine; we call this the ‘subspina’ species group. The rimoportula of all known Cyclostephanos species lacks a long external tube, regardless of its location, although some may have a small raised annulus around the external opening. In contrast, the lectotype of Stephanodiscus and al...

SOME ASPECTS OF MORPHOLOGICAL VARIATION IN STEPHANODISCUS NIAGARAE (BACILLARIOPHYCEAE)1

Valves of S. niagarae var. niagarae Ehr. and S. niagarae var. magnifica Fricke from geographically dispersed sediment and plankton collections were observed by light microscopy and scanning electron microscopy (LM and SEM). Measurements made by LM can be arranged so that means and ranges of diameter, areolar density, or strial density intergrade from one population into the next. Mean diameter is negatively related to increasing areolar and strial densities. No unique features observable by SEM distinguish the two described varieties. Thus, S. niagarae var. magnifica, having large diameter valves with low areolar and strial densities, may represent one end of a trend in overall variation in S. niagarae. At the opposite extreme are the populations with small valves and high areolar densities which are often erroneously referred to as S. astraea. Type material of S. niagarae lies intermediate to these forms. Three populations considered in this study have distinct morphological characteristics. Valves from Yellowstone Lake sediments have spine placements distinctly different from valves of all other populations. Specimens from Lake Superior have nearly flat central areas. Ribs of valves from Grand Traverse Bay (Lake Michigan) are covered with granules.

Principal component analysis and taxonomic interpretation of environmentally related variation in silicification in Stephanodiscus (Bacillariophyceae)

Principal component analysis (PCA) was performed on the correlation matrix of 16 characters measured on each of 129 specimens of the freshwater centric diatom Stephanodiscus niagarae from 14 lentic North American habitats. There were two major independent components to overall silicification. Firstly, larger valves were more heavily silicified than smaller ones, regardless of environment of development. Secondly, specimens from waters of higher ambient silicon: phosphorus ratios (Si : P) were more heavily silicified than specimens of similar size from low Si : P waters. Si concentrations alone were only weakly correlated with silicification. S. superiorensis and S. yellowstonensis are morphologically similar to S. niagarae. They are as heavily silicified as S. niagarae under similar Si : P conditions but allocate different amounts of silica to different valve features. S. yellowstonensis and S. niagarae were analysed with PCA in a previous study. The first two PC axes in both the present and previous stud...

Effects of low level salinity concentrations on the growth of Cyclotella meneghinianaKütz. (Bacillariophyta)

Summary Cyclotella meneghiniana was grown at salinity levels ranging from 18–4,000 mg Cl 1−1. Growth rates were significantly greater at the highest salinity treatments, averaging about 1.15 to 1.17 divisions per day. Four possibilities are proposed as an explanation for this. Additionally, morphological variability was examined for the different treatments. More robust valves predominated at the lower salinity treatments. These valves had well-developed costae and spines, as well as numerous siliceous granules along the valve mantle. Cells from higher salinities had thin valves with poorly developed costae and spines, and lacked siliceous granules along the valve mantle. The results of particulate silica analysis confirmed that valves from the higher salinities contained less silica.

Two new pliocene species of Cyclostephanos (Bacillariophyceae) with comments on the classification of the freshwater Thalassiosiraceae

Two new species of the diatom genus Cyclostephanos Round are described from Pliocene fossil deposits in western North America. Cyclostephanos undatus is distinguished from other Cyclostephanos species by its tangentially undulate valve face; Cyclostephanos fenestratus is distinguished by its extremely shallow alveoli. This paper records previously unreported morphological detail of Cyclostephanos and speculates that structure of the punctum, labiate process and strutted process may enhance diagnosis of the freshwater genera of the Thalassiosiraceae Lebour emend. Hasle. Cyclostephanos undatus is similar to several Cyclotella species, but its external costae are raised and its alveolar morphology is similar to that of Cyclostephanos dubius (Fricke) Round. Cyclostephanos fenestratus is similar in external view to Stephanodiscus Ehrenb. However, the two species described here have flat cribra covering the mantle puncta and the labiate processes appear to lack external tubes, whereas Stephanodiscus species have domed mantle cribra and external tubes.

Validation of the centric diatom genus name Cyclostephanos

The diatom genus Cyclostephanos Round is invalidly published because a legal type was not designated at the time of description. Round apparently intended for Stephanodiscus novaezeelandiae Cleve to be the type of the genus. That species is proposed here as the lectotype of the name Cyclostephanos and a lectotype is proposed for the name S. novaezeelandiae. Names of species placed in Cyclostephanos are here validated according to the rules of botanical nomenclature.

Cyclostephanos species newly reported from North America: C. tholiformis sp. nov. and C. costatilimbus comb. nov.

A new species, Cyclostephanos tholiformis Stoermer, Hakansson & Theriot, and a new combination Cyclostephanos costatilimbus (Kobayasi & Kobayashi) Stoermer, Hakansson & Theriot are proposed. These species are abundant in a eutrophic, hardwater, habitat in central North America. They resemble other small species of Stephanodiscus and Cyclostephanos commonly found in such habitats, and can be distinguished from them with certainty only on the basis of characters visible with electron microscopy.

An empirically based model of variation in rotational elements in centric diatoms with comments on ratios in phycology

Ratios are commonly used to relate taxonomic, physiological and ecological properties of algal cells to variation in cell shape and size. However, ratios can be more difficult to interpret than the original variables. A detailed example of a ratio commonly used in diatom taxonomy is presented. The abundance of a reported rotational element on a centric diatom valve is often expressed as density (e.g. number of marginal spines per μm of circumference), rather than simply as total number. It is empirically demonstrated that density if often curvilinearly related to diameter, whereas total number is linearly related (when related at all) to diameter. This linear relationship is the basis for an empirical model which accounts for variation in density in real examples. Under this model, densitywill be a poorer taxonomic character than total number except under rare occasions (e.g. when the slope of the total element versus diameter regression in zero and variance in the numerator is not dependent on the denominator). Other ratios are similarly difficult to interpret. Length/width (or width/length) is often curvilinearly related to width (or width/lenght) is often curvilinearly related to width (Length) because variation is compounded from several sources. Surface/volume has been employed to investigate the relative contributions of size and shape to phytoplankton sinking. However, the properties of this ratio have not been fully investigated, and the relationship between size, shape and sinking rate has not been fully elucidated for the data set in question.

THE RELATIONSHIP BETWEEN VALVE DIAMETER AND NUMBER OF CENTRAL FULTOPORTULAE IN THALASSIOSIRA WEISSFLOGII (BACILLARIOPHYCEAE)1

Cells well below the reported minimum size for Thalassiosira weisslogii (Grunow) Fryxell et Hasle were observed in two unialgal cultures. The relationship between diameter and number of central fultoportulae was found to be very regular. Regresion analysis of three different populations showed equivalent slopes and r2 values above 80%. The y‐intercepts were significantly different and support observations in the litrature that nonmarine valeve of T. Weissflogii have fewer central fultoportulae than marine forms.