Gyrosigma tenuissimum var. gundulae Hargraves & Sterrenburg, 2014

Sterrenburg, Frithjof A. S., Haan, Myriam De, Herwig, Wulf E. & Hargraves, Paul E., 2014, Typification and taxonomy of Gyrosigma tenuissimum (W. Sm.) J. W. Griffith & Henfr., comparison with Gyrosigma coelophilum N. Okamoto & Nagumo and description of two new taxa: Gyrosigma tenuissimum var. gundulae var. nov. and Gyrosigma baculum sp. nov. (Pleurosigmataceae, Bacillariophyta), Phytotaxa 172 (2), pp. 71-80 : 76-78

Gyrosigma tenuissimum var. gundulae Hargraves & Sterrenburg var. nov. ( Figs 28–36 )

As the species, but valves larger and more robust.

Type:— USA. Indian River Lagoon, Florida ( BRM ZU 8 /74! holotype = fig. 28, coordinates 6.7 East, 10.0 South; collection Sterrenburg #721, isotype; collection Hargraves, isotype) .

As G. tenuissimum , with comparable striation ( Fig. 28, 30 ) but much more robust and larger: 159–190 µm long and 9.5–11 µm wide. With respect to G. tenuissimum var. tenuissimum , valve much broader at apices ( Figs 32, 34 ), with presence of small accessory apical pore ( Fig. 32 , arrow). Externally, central raphe fissures ( Figs 33 ) identical to G. tenuissimum var. tenuissimum . Some external areolar fissures of adjacent rows show tendency to fuse, forming the letter V ( Fig. 33 ), as in G. tenuissimum . Internal raphe fissures show sideways twisting ( Fig. 35 ).

Central bars around internal central raphe node markedly variable in their degree of silification but are often vestigial, reduced to 4 small “knobs” of silica ( Figs 35 and 36 ). The extreme resolution of the special LM technique used reveals that one central bar (the lower in Fig. 30 ) may also consist of a continuous sharp ridge, the other (upper in Fig. 30 ) being reduced to two separate “knobs”, which matches the aspect seen in Fig. 7 of G. tenuissimum var. tenuissimum .

Etymology:— The epithet refers to the long-term support of the first author’s diatom studies by his spouse.

Habitat:—Found at temperatures of 21.9–30.5° C and salinities of 26.2–38.9 PSU; typical spring and summer conditions. The Indian River Lagoon is quite shallow ( Hargraves & Hanisak, 2011), so that the presence of this taxon in plankton samples is not by itself indicative of a planktonic habitat.

Distribution:—Subtropical Indian River Lagoon, East coast of Florida: widespread (but rare).

Observations:— The vestigial central bars (4 “knobs”) match the situation in a specimen from the Adriatic illustrated in Höbel & Sterrenburg (2011: Fig. 12) that conforms to G. tenuissimum . The central bars are highly variable in shape in both G. tenuissimum and the variety described here. Deposition of silica is a quantitative process and marked variation in the solidity of the central bars is common in the genus. Probably the basic shape in the taxa discussed is as in Fig. 7 and 30 , reduction leading to the aspect as in Figs 35 and 36 .

Figures

FIGURES 28–36. Gyrosigma tenuissimum var. gundulae, type material. Figures 28, 30, 31. LM images. Fig. 28. Holotype, general view. Stitch of 5 stacks of 15–20 images each. Bar = 30 µm. Fig. 30. Enlargement showing centre, note resolution of vestigial (upper) and continuous (lower) central bars. Bar = 1 µm. Fig. 31. Apex showing irregular striation. Bar = 3 µm. Figures 29, 32–36. SEM images. Fig.29. General view. Bar = 30 µm. Fig. 32. Apex, external view. Arrow indicates small accessory pore. Bar = 3 µm. Fig. 33. Centre, external view. Bar = 3 µm. Inset: 1.5x enlargement of Fig. 33 showing overlapping central raphe fissures. Fig. 34. Apex, internal view. Bar = 3 µm. Fig 35. Centre, internal view. Note sideways twisting of raphe fissures. Bar = 3 µm. Inset: 1.5x enlargement showing vestigial central bars. Fig. 36. Vestigial central bars of another specimen. Bar = 1.5 µm.