Progonoia diatreta Lobban, 2015

Lobban, Christopher S., 2015, External ultrastructure of Progonoia diatreta sp. nov. (Bacillariophyta, Scoliotropidaceae) differs from P. musca and P. intercedens comb. nov., Phytotaxa 234 (3), pp. 246-254 : 249-252

Progonoia diatreta Lobban , sp. nov. ( Figs 6 , 18–29 )

Synonyms: Navicula musca sensu Peragallo & Peragallo (1897 –1908: 79–80, pl. 14, figs 14–16), Oestrupia musca sensu Hendey (1964: 228 , pl. 29, fig. 17)

Differing from P. musca in having rings of small pores around the external edges of the areolae rather than simple large, uniseriate openings, and in having externally a wide, sculpted space between the two alveolae.

Type:— GUAM. Apra Harbor Guam: Outhouse Beach, 13° 27’ 50” N, 144° 39’ 27” E, calcareous sand sample GU52O-4, ca. 12 m deep, C. Lobban & M. Schefter, 13 December 2010 (holotype: CAS! frustule located 17.6 mm E and 8.5 mm S of reference mark on slide 1696, accession # [627432], slide # [223050], illustrated in Fig. 18 . Isotypes: GUAM! slides 1695 and 1697 in Guam Diatom Herbarium. Isotypes: GUAM! slides 821 (GU44P-5) and 1115 (GU52Q-2) in Guam Diatom Herbarium.

Etymology:— Diatreta L., n. pl. noun in apposition = filigree; from diatetrum = artisan, with reference to the elegant pattern of external pores around the areolae. [A more appropriate English analogy would be a doily, but I could find no Latin or Greek translation that conveys the sense of the elaborate pattern of holes.]

Description:— LM ( Figs 6 , 18–20 ): Valves elliptical-lanceolate, length 51–86 μm, width 19–25 μm, somewhat constricted in the middle to 15–19 μm, but axial area continuous across the central part of the valve ( Figs 18–20 ). Axial area occupies only about one quarter of the width of the valve; it is slightly depressed except for two narrow strips alongside the raphe slits from the central area halfway to the poles. Raphe is straight, central endings slightly expanded. Striae 7–8 in 10 μm, uniseriate, alveolate, with two chambers apparently separated by a sculpted, hyaline space rather than just a longitudinal rib ( Fig. 6 vs. Fig. 5 ). SEM ( Figs 21–29 ): The external valve face is elaborately perforated and sculpted ( Figs 21–26 ). The perforations include a series of large pores along the edge of the mantle, two series of small circular and C-shaped pores around the edges of the two alveoli ( Figs 22, 26 ), often with additional pores in the middle of the alveolus (especially in the longer cells: Figs 23–26 ), and a series of medium-sized, curved pores along the axial ends of the alveoli. The sculpted areas include the majority of the axial area and small patches offset between the two series of alveoli ( Figs 24 , arrow, and 26). Internally ( Figs 27–29 ), alveoli extend between fingers of the basal siliceous layer that are connected by an internal partition that separated the alveoli in adjacent striae. The alveoli are apparently divided by cross walls that formed a continuous longitudinal “rib.” In this species there also appear to be short transapical partial walls in both directions from the cross wall (inferred from the markings on the membrane) ( Figs 27 and 29 ). In spite of the appearance of the outer surface, the chambers are separated only by a thin wall. The junction between the basal siliceous layer and the alveolar membrane is perforated by a line of tiny pores ( Figs 28 and 29 ). The middle part of the alveoli is thicker, such that there was a ridge on both sides ( Figs 27–29 ). The external raphe central endings are expanded and pore-like ( Figs 23–25 ); internally they end in a beak-like double helictoglossa ( Fig. 28 ). External terminal endings deflect to the same side and end on the valve mantle ( Fig. 26 ); internal endings are straight, with a very small helicoglossa ( Fig. 29 ). Girdle bands ( Figs 24–26 ) are striate, the striae 21–25 in 10 μm.

Additional records:— The following records from the literature are probably this species, as indicated by the record of small pores along the sides of the alveoli, and cited above under synonymy: Peragallo & Peragallo (1897 – 1908, pl. 14, figs 14–16, as Navicula musca ) and Hendey (1964, pl. 29, fig. 17, as Oestrupia musca ). Hustedt’s (1955) drawing does not show pores at all but the large space separating the alveoli suggests that this, too, may be P. diatreta ; I was unable to find any specimens in subsamples of his material and he gave no description in either Hustedt (1955) or Hustedt (1935) to indicate his concept of the species. The following records cannot be identified to species because the photographs do not show the surface pores: López Fuerte et al. (2010, pl. 18, figs 21, 22), and Al-Yamani & Saburova (2011, pl. 62, figs a–c, both as Oestrupia musca ).

Figures

FIGURES 4–6. Comparison of stria detail in differential interference contrast illumination, focus on the outer surfaces of the valves. 4. Authentic Navicula musca from BM 1557 (detail of 3); arrow indicates the large pores. 5. Progonoia intercedens from Guam; arrow indicates the large pores in the striae and arrowhead the rib separating the two alveoli. 6. Progonoia diatreta from Guam (detail of holotype); arrow indicates the rings of small pores around the edges of the alveoli, and arrowhead the wide, sculpted rib between the two alveoli. Scale bars = 2 μm.

FIGURES 18–26. Progonoia diatreta, sp. nov., LM (Figs 18–20) and external SEM (Figs 21–26). 18. Stacked image (DIC) of holotype frustule. 19, 20. Valve at two focal planes, brightfield. 21, 22. Two valves in SEM, showing size and shape variation. 23, 24. Details of a frustule in a whole-mount preparation. 23. Half of a frustule showing deflection of terminal raphe ending and porate copulae. 24. Detail of central portion of valve showing central raphe endings, excavated areas between the two alveoli (arrow) and adjacent to raphe branches, and external pore patterns of the alveolae, including a curved slit at the inner edge of the inner alveolus and a large round pore at the outer end of the marginal alveolus. 25, 26. Details of longer frustule in 22 showing valve and copula features, including row of puncta in the middle of the areolae as well as around the edges. Scale bars: Figs 18–22 = 10 μm, Figs 23, 25, 26 = 5 μm, Fig. 24 = 2 μm.

FIGURES 27–29. Progonoia diatreta, sp. nov., internal SEM. 27. General view of valve. 28. Detail of apex showing terminal raphe ending and internal aspect of alveoli, with partial transverse walls and a series of small pores around the inner edge of one, the outer edge of the other, and the apical end of the last alveolus (arrow). Some of the pores of the external surface can be seen where the inner membrane has broken away (arrowhead). 29. Detail of central area showing the double helictoglossa (arrow) and the partial transverse walls extending into each alveolus from the longitudinal wall (arrowhead). Scale bars: Fig. 27 = 10 μm, Figs 28, 29 = 2 μm.