Mastogloia cf. pisciculus Cleve, 1893

Lobban, Christopher S. & Witkowski, Andrzej, 2023, Marine benthic diatoms of Guam: new records, Dictyoneis apapae sp. nov., and updates to the checklist, Micronesica 2023 (2), pp. 1-75 : 10-11

Mastogloia cf. pisciculus Cleve Figs 68–70

Ref. illus.: Cleve 1893; Hustedt 1931 –1959, p. 473, fig. 990; Stephens & Gibson 1980, p. 228, figs 31–36; Pennesi et al. 2012. p. 1254, figs 7A–H; Loir & Novarino 2013, p. 42, pl. 18c

Samples: GU52X-5

Dimensions: Length 30–35 µm, width 14 µm, striae radiate, 25 in 10 µm

Diagnostics: Broadly lanceolate, rostrate, depressed area not strongly differentiated but a longitudinal line visible in the middle of it ( Figs 68, 69 arrows). Chambers quadrate to elongate with convex inner edges, not reaching apices. In SEM external view ( Fig. 69 ) uniform areolae in the outer zone, transapically elongated areolae in the depressed zone, irregular rib between them; raphe thickened. Internally, outer zone with transapical costae, inner zone smooth. Areolae opening internally, by quadrate set of small pores; outer zone of striae with costate virgae, inner zone with areolae in irregular longitudinal line, sporadically two lines ( Fig. 70 arrow). Partecta 3 in 10 µm, in middle portion only, apically elongated with convex inner margins and scattered pores on the abvalvar surface.

Comments: Uncommon in this biofilm sample. Specimens were smaller than Cleve’s and at low end of range in Hustedt. Hustedt (1931 –1959), Stephens & Gibson (1980) and Loir & Novarino (2013) all show much variation. Moreover, there are considerable discrepancies between the various descriptions, leading us to qualify our identification with “cf.” First, Hustedt (1931 –1959: 558) said that although the forms he found (only near Seychelles) differed from Cleve’s (1893) description (specimens from Florida), e.g. in stria density, he decided not to separate them. Stephens & Gibson reported stria densities in line with type material in Cleve (1893) and Boyer (1927). Specimens in Pennesi et al. were from Suluwesi, Indonesia. The SEM studies by Stephens & Gibson (1980) and Pennesi et al. (2012) are not completely comparable because partecta found by the latter were all broken. The partecta are square to apically lengthened with convex inner margins. Stephens & Gibson showed them to have delicate silica plaques with perforations; partecta in our specimens seem longer than in these other reports and we saw pores but no plaques ( Fig. 70 ). Stephens & Gibson did not observe the edge of the mantle, where we and Pennesi et al. saw biseriate striae. Internally, there is a single line of quadripunctate foramina under the depressed area shown in both Stephens & Gibson and Pennesi et al. but in our specimens, there was a much looser row, sometimes double. Stephens & Gibson (1980; Florida) considered whether their specimens might match M. arabica Hendey , or if that might be synonymous with M. pisciculus . Establishing boundaries in these disparate populations of rare taxa will require more work; for now, we present our SEM observations to define what we found here.

Figures

Figures 65–70. Mastogloia. Figs 65, 66. Mastogloia ovata exterior and interior valve faces. Fig. 67. Mastogloia pseudolatecostata (Bikar Atoll, Marshall Is.), external valve face with part of valvocopula showing partecta. Figs. 68–70. Mastogloia cf. pisciculus. Fig.68. LM (DIC) with foci on valve face and partecta Arrows on this and next image point to longitudinal rib bisecting the inner zone. Fig. 69. External surface, SEM. Fig. 70. Internal valve face and partecta, also showing partly double row of areolae (arrow). Scale bars: Figs 67, 68 = 10 µm, Figs 65, 66, 69, 70 = 5 µm.