Frustulia cf. saxonica

Graeff, Carrie, Kociolek, John Patrick & Burliga, Ana Luiza, 2012, Valve morphology of four species of Frustulia (Bacillariophyta), including two described as new, Phytotaxa 42, pp. 62-76 : 68-69

Frustulia cf. saxonica ( Figs 28–40 )

Valves rhomboid ( Figs 28–30 ) to linear-lanceolate ( Figs 33, 34 , and 38), smaller specimens generally less rhomboid in shape than larger specimens. Apices moderately protracted and rounded, valve margins slightly triundulate. Valve dimensions range in length from 34.0 to 63.0 µ m, in breadth from 9.0 to 12.5 µm. Longitudinal striae present, parallel across much of the valve but appear slightly wavy or disorganized at valve centre ( Figs 30, 34 , and 35). Externally, areolae openings lanceolate ( Figs 36 and 37 ), internally round ( Fig. 40 ) and occluded ( Fig. 38 ). Striae density 33–34 in 10 µm (at valve centre); 34–36 in 10 µm (at valve end). Striae extend around apices. Internal longitudinal ribs widen slightly approaching central area; constrict at central nodule ( Figs 40 ). Longitudinal ribs fuse with helictoglossa but because the helictoglossa is relatively short and round and the interface between the ribs and the helictoglossa is extensive, the porte-crayon structure is not prominent ( Fig. 39 ). External raphe ends expanded into shallow crescent-moon or wide Tshapes ( Figs 36 and 37 ).

Distribution:— F. cf. saxonica is from small waterbodies of freshwater in the northern portion of the lower Peninsula of Michigan, USA, including Bryant’s Bog and the nearby Smith’s Fen (personal observations; 45.553258 N, 84.647925 W).

Observations:— Valves of Frustulia saxonica and F. cf. saxonica become less rhomboid at the lower limits of their size range and have T-shape external raphe endings, weak porte-crayon structures, minimally arched raphe branches and longitudinal ribs, and moderately protracted apices ( Lange-Bertalot 2001: pls. 126 and 127). Admittedly, the differences between the two taxa are minor. Most notably, the striae density of F. saxonica is given as 29–32 in 10 µm ( Lange-Bertalot 2001: 172) and 32–33 in 10 µm ( Lange-Bertalot & Jahn 2000), while the striae density of F. cf. saxonica is from 33–36 in 10 µm, with the highest densities at the valve apices. However, the recorded striae densities given by Lange-Bertalot (2001) for F. saxonica cannot be verified due to the low-quality light micrographs provided; they appear more rather than less dense as compared to those of specimens F. cf. saxonica from Michigan.

The description of two morphotypes of F. saxonica from the type material by Lange-Bertalot & Jahn (2000) complicates matters further; they note that both have also been found coexisting in peat bogs in Germany and Finland. Although we did not find two separate morphotypes in our sample from Michigan, there is a significant but predictable change in valve size and shape, central nodule breadth, and rib thickness across the size diminution series. Our specimens of F. cf. saxonica most closely fit with the morphometrics of what has been referred to as morphotype II (in Lange-Bertalot 2001); we did not observe any specimens in the material from Michigan with a valve breadth rivaling that recorded for what has been referred to as morphotype I (in Lange-Bertalot 2001). Further examination of the type material of F. saxonica should reveal whether the two morphotypes present are separate species. If they are, then providing unique descriptions for each will be necessary in determining the identities of specimens from North America that are similar to F. saxonica .

Patrick & Reimer (1966) provide a short description of F. rhomboides var. saxonica (Rabenh.) De Toni (1891: 277) and a drawing of a specimen collected in Powell County, Kentucky, U.S.A. The striae density listed for the species is 36 in 10 µm, which overlaps with that of F. cf. saxonica . However, while the length range included in the description (40–70 µm) matches that of our specimens, the breadth range of 12–20 µm is notably different from that of F. cf. saxonica ( Patrick & Reimer 1966: 308, pl. 21).

Siver & Baskette (2004) also provide a description and LM’s of F. saxonica specimens from North America. They provided a wide range of striae densities (28–38 in 10 µm) for the species and a breadth range of 8–18 µm. However, the widest specimen they illustrated is c. 12 µm in breadth and 68 µm long, which is near the upper length range limit of 75 µ m given by Siver & Baskette (2004). Without access to the micrographs of the 18 µm wide specimens, recognizing morphometric links between F. saxonica from Siver & Baskette (2004), F. saxonica from Lange-Bertalot (2001), and F. cf. saxonica is problematic. Ultimately, the dissimilarities between the recorded size ranges and illustrated size ranges of F. saxonica in North America raise some doubts that North American specimens are the same species as those found in the type material.

Figures

FIGURES 28–34. LM F. cf. saxonica. Valve views showing the size diminution series for specimens from Michigan. Figure 29 is the holotype. Note loss of the rhomboid valve shape as specimens decrease in size. Scale bar: 10 µm.

FIGURES 35–40. SEM. F. cf. saxonica. External and internal valve views of four specimens. Fig. 35. Full external valve view of a linear-lanceolate-rhomboid specimen. Fig. 36 External valve apex with expanded raphe end. Fig. 37. Central area with expanded raphe ends. Fig. 38. Full internal valve view of a linear-lanceolate specimen. Fig. 39 A weakly shaped porte-crayon structure. Fig. 40, Internal central area with constricted longitudinal ribs at the central nodule. Scale bars: Figs 35, 38: 5 µm; Figs 36–37, 39–40: 2 µm.