Frustulia rexii Graeff et Kociolek, 2012

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 : 70-73

Frustulia rexii Graeff et Kociolek , sp. nov. ( Figs 41–55 ; figure 47 = holotype)

Valvae lineares-lanceolatae ad rhomboides , constriction dilute prope rotundatis polos. Longitudo 68.0–111.0 µm. Latitudo 17.5–21.0 µm. Striae 26–28/ 10 µm ad centro, 28/ 10 µm ad apices. Striae parallelae pro parte maxima, sed dilute convergentes et interdum undulatae ad apices, areolis lanceolatis ad apices et aream centralem, rotudatis alibi. Areolae extensae circa apices. Striae longitudinales parallelae trans valvam pro parte maxima, sed dilute undulatae ad valvam centrum. Interne areolae rotundatis et occlusis. Externe area centralis areolis parvis irregulariter aggregatis variabilibus numeri. Areolae serials in ambo latere raphis terminates ante extremis distalibus raphis.

Type:— USA. Bryant’s Bog in Michigan (Holotype ANSP GC58973 , Fig. 47 ; Isotype COLO 5807 ) .

Valves rhomboid with a very slight constriction near rounded poles; 68.0–111.0 µm long, 17.5–21.0 µm wide. Striae with density of 26–28 in 10 µm at valve centre, 28 in 10 µm at valve ends. Striae mostly parallel but slightly convergent and sometimes wavy at apices ( Figs 42 and 44 ). Externally, areolae lanceolate or round ( Figs 51 and 52 ) with the most rounded areolae being located near valve centre ( Fig. 52 ). Adjacent to external central area, areolae can be slightly expanded ( Figs. 45, 46 , and 52). Internally, areolar openings round ( Figs 54 and 55 ). Areolae extend around apices, but striae greatly reduced ( Fig. 51 ). Longitudinal striae parallel over majority of valve but become slightly wavy at valve centre ( Figs 42 and 45 ). External central area possessing an irregular grouping of small poroids of varying number ( Figs 44, 47 , and 52). Rows of areolae adjacent to either side of raphe terminate before distal raphe ending ( Fig. 51 ). Internal longitudinal ribs robust, widen significantly but smoothly at central area before constricting at central nodule ( Figs 53 and 55 ). Longitudinal ribs widen very abruptly near apices, then terminate bluntly at helictoglossa ( Figs 42, 45 , and 53). Longitudinal ribs fuse with the linear helictoglossa to form a wide and prominent porte-crayon structure ( Fig. 54 ). External proximal raphe ends expanded into shallow crescent-moon shapes ( Figs 51 and 52 ).

Etymology:— Species epithet is named for Dr. Rex Lowe, faculty member of Bowling Green State University and University of Michigan Biological Station, who made the two collections from Bryant’s Bog in 1969 and 2008 and whose support is hereby acknowledged and who has been an inspiration to both CLG and JPK.

Distribution:— Frustulia rexii is present in Bryant’s Bog in Michigan, USA.

Observations:— The morphotype of F. krammeri from Jones Lake in North Carolina illustrated by Siver & Hamilton (2011: pl. 140) is highly morphologically similar to F. rexii . The Jones Lake specimens share a number of interesting morphological features with F. rexii not seen in most other species of Frustulia . Both have strongly rhomboid valves with a central area bordered on either side by a row of slightly transapically expanded areolae and a collection of unorganized poroids at the centre ( Siver & Hamilton 2011: pl. 140, fig. 2). At the external apices, the areolae of both groups of specimens form a uniquely shaped unornamented area, similar to a four-leaf clover, because the rows of relatively small areolae nearest the raphe end abruptly near the end of the raphe. The longitudinal ribs of both groups are robust and strongly constricted at the central nodule ( Siver & Hamilton 2011: pl. 140, fig. 1). The relationship between the helictoglossa and longitudinal ribs is also similar: where the longitudinal ribs attach angularly to the helictoglossa, but in the F. krammeri morphotype, the longitudinal ribs appear to widen much more noticeably before tapering into the helictoglossa ( Siver & Hamilton 2011: pl. 140, figs 5 and 6). Close examination of a low magnification SEM suggests that the F. krammeri morphotype ( Siver & Hamilton 2011: pl. 140, fig. 1) might also possess the abruptly widening longitudinal ribs seen in F. rexii near the apices.

The Michigan and North Carolina specimens share many morphological similarities. Regardless of the degree of morphological similarity between the Michigan and North Carolina specimens, distinguishing both populations from F. krammeri is most important. Indeed, LM and SEM illustrations of specimens identified as F. krammeri and its morphotype by Siver & Hamilton (2011) share morphological similarities in their ribs, helictoglossa, valve shape, and areolae with our specimens of F. rexii . However, LM images of specimens of F. krammeri from the type material from Finland showing noticeably curved longitudinal ribs and an offset central nodule as well as a lack of abruptly widening longitudinal ribs near the apices, combined with apices that lack circumradiating striae ( Lange-Bertalot, 2001, pl. 128), suggest the Atlantic Coastal Plain specimens are not F. krammeri . Further examination of these specimens could be used to determine whether the Michigan and North Carolina specimens belong to the same species or two distinct but similar species.

Frustulia rexii is a species that has been commonly collected by researchers in northern Michigan over several decades but not immediately recognized as a unique species. Patrick & Reimer (1966) included an illustration of a specimen of F. rhomboides from England that mirrors the strongly rhomboid valve shape of F. rexii . Specimens of F. rexii from Michigan were likely given the name F. rhomboides due to the frequent use of Patrick & Reimer (1966) for the identification of North American diatom species.

Figures

FIGURES 41–49. LM. F. rexii. Valve views showing the size diminution series for the species. Figure 47 is the holotype. Figs 41–45. Specimens collected in Summer 1969. Figs. 46–49. Specimens collected in Summer 2008. Scale bars: 10 µm.

FIGURES 50–55. SEM. F. rexii. External and internal valve views of four specimens collected in the summer of 2008. Fig. 50, Full external valve view showing the strongly rhomboid valve shape. Fig. 51, Crescent moon shaped external distal raphe end. Fig. 52, External central area showing a grouping of small poroids and the crescent moon shaped raphe endings. Fig. 53. Full internal valve view showing the robust longitudinal rib system. Fig. 54. Internal valve apex with distinct and prominent porte-crayon structure and abruptly widened longitudinal ribs. Fig. 55. Internal central area with greatly constricted longitudinal ribs. Scale bars: Figs 50, 53: 10 µm; Figs 51–52, 55: 2 µm; Fig. 54: 1 µm.