Psammodictyon pustulatum (Voigt ex Meister) Lobban, 2015

Lobban, Christopher S., 2015, Benthic marine diatom flora of Guam: new records, redescription of Psammodictyon pustulatum n. comb., n. stat., and three new species (Colliculoamphora gabgabensis, Lauderia excentrica, and Rhoiconeis pagoensis), Micronesica 2015 (2), pp. 1-49 : 12-13

Psammodictyon pustulatum (Voigt ex Meister) Lobban , n. comb., n. stat. Figs 118–128

Basionym: Nitzschia panduriformis var. pustulata Voigt ex Meister 1937 , Berichte der Schweizerischen Botanischen Gesellschaft 47: 270, pl. 11, fig. 9.

Samples: GU66A-1, GU66A-3, GU66F-2, GU66F-7A; GU52Q-10

Dimensions: Length 22–33 µm, width 13–17 µm; striae 20–24 in 10 µm, fibulae ca. 14 in 10 µm.

Diagnostics: Differs from all other species of Psammodictyon in the presence of a pair of rounded elevations on the distal side of the sternum.

Description: Valves panduriform, highly sculptured, with a pair of rounded elevations on the distal side of the sternum and a pair of shallow depressions on the proximal side. Valves apiculate and loculate; areolae in quincunx arrangement, the striae interrupted by a hyaline sternum near the mid line ( Figs 123, 124 ). (The part next to the raphe is considered proximal, and the side toward the mantle where the elevations occur is distal.) The raphe system is submarginal, dividing the valve into a wide ventral portion and a narrow dorsal band; pores on the dorsal portion consist of a longitudinal row of transapical slits ( Figs 127, 128 : arrows). The dorsal part of the valvocopula has a repand ab valvar edge ( Figs 124, 125 ). The distal part of the ventral valve face bears two rounded elevations; the proximal portion slopes down from the raphe-keel to the sternum in two shallow depressions ( Figs 125, 126 , 128 ). Loculae are evident on the elevations ( Figs 125, 126 ) but also can be seen on the proximal side of the sternum ( Fig. 125 ). Girdle bands lack pores, except for a single longitudinal row on the first pleura ( Fig. 125 : arrow). The valvocopula delimits the ventral margin of the valve with a repand rim and there is a single row of pores at its base on the valve side ( Figs 125, 126 ); however, along the dorsal portion of the valvocopula, the opposite edge is repand ( Figs 124, 125 ).

Comments: The loculate structure places this taxon in Psammodictyon rather than Nitzschia or Tryblionella ; the character combinations distinguishing these genera are described by Round et al. (1990). However, the valve face is sharply delimited by a vertical rim, which appears to be formed by the valvocopula, rather than merging imperceptibly into a shallow mantle on the ventral side, as described by Round et al. (1990) for Psammodictyon . The possibility that this rim is simply due to collapse of the frustule during preparation is unlikely because of the consistency of its appearance in the SEM images and the way it goes down under the dorsal margin ( Figs 127, 128 ).

I examined Meister’s slide 3509092, and found one specimen —apparently not the type he photographed, however, since it is partially overlain by another diatom ( Fig. 121 ).

Although Meister (1937) in the same paper also named Nitzschia pustulata , this is entirely different and certainly not a Psammodictyon , so I am confident there will be no future confusion from using pustulatum as the epithet in this new combination.

Specimens labeled as N. panduriforme var. pustulata occur on two slides in the Hustedt collection (ZT2/46 and W1/18), kindly photographed by Friedel Hinz (one shown in Fig. 122 ), are identical to my specimens; a similar specimen is illustrated in Stidolph et al. (2012, pl. 25, fig. 83). The known geographic range of P. pustulatum is thus Nagasaki ( Japan), Guam, Singapore, Galapagos, and Vera Cruz ( Mexico).

The size and outline of P. pustulatum are similar to P. constrictum in our flora, but the areolae, with single pores on both sides, are like those in P. panduriforme , unlike those in P. constrictum , where there are complex external openings and clusters of 3–4 interior pores. Further differences in P. constrictum are that the areolae on the dorsal side of the raphe-keel are also large, and the pleura has transapical striae all along it ( Lobban et al. 2012, pl. 59, figs 7, 8).

Figures

Figures 114–126. Figs 114–117. Protokeelia cholnokyi. Fig. 114. Valve in LM (GU44I-1) (DIC). Fig. 115. External view of valve (GU44J-2 / ECT3569), SEM; image courtesy of Elizabeth Ruck. Fig. 116. Internal view of valve (GU44Z-15) (SEM). Fig. 117. External, dorsal view of whole frustule showing girdle bands and dorsal faces of valves (SEM); courtesy of Elizabeth Ruck. Figs 118–126. Psammodictyon pustulatum. Figs 118, 119. Frustule at two focal planes (GU66F-7A) (DIC). Fig. 120. Valve from GU66A-3 (DIC). Fig. 121. Specimen from Nagasaki on Meister’s slide 3509092. Fig. 122. Valve from Hustedt collection, slide W1/18, specimen from Vera Cruz, Mexico, image courtesy of Friedel Hinz. Fig. 123. External view of valve face (GU52Q-10a) (SEM). Fig. 124. Internal view of valve and part of the valvocopula. Arrows indicate where the valvocopula is broken. Inset shows full-resolution detail of valve margin and valvocopula, with the row of pores on the latter. Fig. 125. Frustule in oblique view showing the valves and girdle components. The ventral part of the valvocopula (vc) of the epivalve (ev) is visible at the front, and at the back the distal part of the valvocopula for the hypovalve (hv). Arrow indicates row of pores on the first pleura (p). Fig. 126. External detail of valve, showing the loculate character; arrow indicates row of pores on valvocopula. Scale bars: Figs 114–117, 123, 124 = 5 µm, Figs 118–122 = 10 µm, Figs 125, 126 = 2 µm.

Figures 127–138. Figs 125–126. Psammodictyon pustulatum (GU66A-3) (SEM). Fig. 127. Oblique view of frustule showing linear pores on the dorsal valve face (arrow). In both Figs 127 and 128 the transition from the ventral to dorsal parts of the valvocopula is visible (arrowheads). Fig. 128. Apical view of frustule showing the distinctive topography of the ventral side of the raphe-keel, along with the linear pores on the dorsal side of the other valve (arrow) (two stacked images). Figs 129, 130. Pteroncola inane (SEM). Fig. 129. Frustule in girdle view (GU41D-1A). Fig. 130. Broken frustule showing internal and external valve faces, with the single row of internal pores (arrows) (Jordan collection, image by Misaki Ishizawa) (SEM). Figs. 131–138. Rhoiconeis pagoensis. Figs 131, 132. Same live cell in valve and girdle view showing plastids (GU7X-8) (DIC). Figs 133, 134. Holotype (valve view) and a girdle view from the same slide (GU7X-7) (DIC). Fig. 135. Frustule in valve view (GU7X-7) (SEM). Fig. 136. Frustule in girdle view; note the very short striae at the central area on the concave valve (GU7X-2) (SEM). Fig. 137. Detail of a pair of frustules showing difference in striae lengths at central area on convex and concave valves (GU7X-2) (SEM). Fig. 138. Apex of a frustule in girdle view showing the segmented valvocopula (S1 and S2) and two pleural bands (P1 and P2) (GU7X-5) (SEM). Scale bars: Fig. 136 = 10 µm, Figs 127–128, 131–135 = 5 µm, Figs 129, 130, 137, 138 = 2 µm.