MORPHOMETRIC ANALYSIS OF SHAPE OF SPECIMENS FROM THE NEOTYPE OF TABELLARIA FLOCCULOSA (BACILLARIOPHYCEAE)

Shape variation of 67 specimens from the neotype slide of Tabellariaflocculosa was analyzed by computer-assisted morphometric techniques. Coefficients of a Legendre polynomial expansion of specimen shapes are shape descriptors used in principal component analysis (PCA). The first PC axis (PC,) is highly correlated with valve length. Variation along PC, is a combination of increase in width with a reduction in the distance between central and terminal inflations. The second axis (PC2) is the major length-free component of shape variation. Variation in PC2 scores is related to the degree of constriction between the poles and the center. Shape variability increases as specimen length decreases. Three nonexclusive hypotheses can explain this: 1) different genetic entities are involved, 2) control of shape is strict in specimens longer than 35 ,um, but not for those less than 35 5,m, 3) the pattern of variation reflects environmental variation during the period of development of the population. MEMBERS OF THE diatom genus Tabellaria occur in abundance in a diversity of aquatic habitats. Its taxa are considered important indicators of lake acidity, a subject of topical concern (Flower and Battarbee, 1985). Although Tabellaria is not a particularly diverse diatom genus in terms of number of described taxa, its taxonomy is extremely difficult. There are as many different opinions on the diagnosis of taxa as there are investigators (Knudson, 1952, 1953a, b; Koppen, 1975; Flower and Battarbee, 1985). However, these authors agree that there are several basic taxonomic characters to be considered. Shape of the valve outline is perhaps foremost among these in both diagnostic importance and utility. Shape in Tabellaria is highly variable, within and among taxa. Shape is to some degree dependent on valve length; this relationship confounds identification of taxa (Knudson, 1952). Until recently, diatomists lacked the necessary tools to quantitatively analyze shape of pennate diatoms, because they have few outline landmarks. Our laboratory has developed a method of shape quantification that relies on a minimum of landmarks (Stoermer and Ladewski, 1982). Applying this technique to the neotype of Tabellariaflocculosa (Roth) KiutzI Received for publication 19 April 1985; revision accepted 19 August 1985. Neotype slide provided by Ms. P. A. Sims, British Museum (Natural History). We thank Dr. R. Kalinsky and an anonymous reviewer for constructive criticism of an earlier version of this article. Great Lakes Research Division Contribution no. 428 supported by N.S.F. Grant no. BSR-8218008. ing, we discuss the morphological relationships between T. flocculosa and similar taxa, and evaluate the technique itself. MATERIALS AND METHODS-Shape variation in neotype material of Tabellaria flocculosa (BM 22750) was analyzed by computer-assisted morphometric techniques. Outlines of 64 specimens deemed suitable for shape analysis were digitized (i.e., valves and free girdle bands with valvar planes lying parallel to the plane of focus). Example specimens are illustrated in Fig. 1-8. Three specimens were digitized twice to estimate measurement error in the digitization process, yielding a total of 67 shapes in the data set. Each specimen outline shape was described using a Legendre polynomial expansion (Stoermer and Ladewski, 1982). Summarizing the method, the trans-apical distance is scaled to a standard length for all specimens, and the "half-width" (one half of the distance between margins measured perpendicular to the transapical axis) is measured at equal intervals along the trans-apical axis. A Legendre polynomial expansion of the half-width is calculated as a function of distance along the axis. The coefficients in the polynomial expansion are the shape descriptors used in this study and are labeled C0, C1, C2, etc. These descriptors are interpretable in terms familiar to diatom taxonomists. For example, C0 measures average width-to-length ratio, C1 measures "clavateness," C2 measures "naviculoid-ness." Evennumbered descriptors (C0, C2, C4, etc.) measure symmetric aspects of shape, and odd-num-