Platichthys Lange-Bertalot, Kulikovskiy, Witkowski, Seddon & Kociolek, 2015

Lange-Bertalot, Horst, Witkowski, Andrzej, Seddon, Maxim S. Kulikovskiy Alistair W. R. & Kociolek, John P., 2015, Taxonomy, frustular morphology and systematics of Platichthys, a new genus of canal raphe bearing diatoms within the Entomoneidaceae, Phytotaxa 236 (2), pp. 135-149 : 137

Platichthys Lange-Bertalot, Kulikovskiy, Witkowski, Seddon & Kociolek , gen. nov.

LM ( Figs 1–18 , 39–49 ): cells solitary. Plastids currently unknown. Frustules lying exclusively in girdle view, linear-elliptical to linear with broadly rounded ends. Between both valves the girdle consists of several barely recognizable copulae. Prominent fibulae, approximately developed as circular structures are apically arranged at some distance below the frustule margins. In straight view the fibulae lie on the same level in both valves. Consequently a “nitzschioid” or “hantzschioid” symmetry classification (i. e. raphe on the same or opposite sides, respectively) becomes obsolete. Valve ends appear slightly rostrate in an appropriately oblique position. Transapical striae, areolae, raphe canals or raphe slit are not discernible, sometimes phase contrast optics allows observation of the central nodule and the apical raphe endings. EM, valve exterior ( Figs 19–29 , 50–59 ): the shape of the frustules resembles a flat tongue when the valves are separated from each other, or a cigar when the valves are connected through the girdle bands. The girdle is composed of numerous open, porous copulae. The valve face is steep and compressed laterally to form a narrow keel. Below the keel, around the mid-width, the valve becomes broader. The valve mantle is developed as a very narrow, structureless rim. The raphe runs within a well-separated, structureless axial area with two raphe branches separated by a narrow central nodule. External central raphe endings are relatively simple, slightly expanded, whereas the apical external endings are bent in the same direction. In a hypothetical cross-section of a frustule, the position of both valves is roughly like two hairpins lying opposed with the open ends connected, and the raphe canals are situated at the closed ends. Transapical striae uniseriate, very densely spaced, far beyond the resolution power of LM. The areolae are simple, sometimes, slightly transapically elongate pores. SEM, valve interior ( Figs 33–38 , 59, 60 ): internally the fibulae are column-shaped, short and arched. They separate the raphe canal from the major part of the valve lumen (cavity). The fibulae are positioned at some distance from the raphe. As the fibulae are large, usually a few striae merge into one fibula. Wave crests begin at the basis of fibulae while wave troughs continue into the interfibula spaces (portulae). The portulae are very simple and are formed as a result of a slight broadening of the base of the fibulae. Internally, the raphe slit is straight, the two raphe branches are separated by a narrow central nodule, whereas at the apices they terminate in a small helictoglossa. Internally, the areolae are positioned in shallow depressions formed by the virgae.

Type:— Platichthys krammeri Lange-Bertalot & Kulikovskiy , sp. nov. (see below).

Etymology:—the Greek name of the genus is similar to the scientific name of the flat-fish Platichthys flesus (flounder) and refers to the roughly comparable shape and symmetry of the valve and frustules respectively.

Comments: —One salient feature of Platichthys gen. nov. is the stabilization of very thin cell walls by, as seen as alternating wave crests and wave troughs in the SEM, whereas the crests can easily be mistaken for prolongations of the fibulae with LM microscopy. This is known from the “light construction” of the valves of Surirella ( Krammer & Lange-Bertalot 2004) and is also observed in Platichthys gen. nov. Striae- and areolae- densities are uncommonly high in comparison with other canal raphe diatom genera, except Entomoneis .

Figures

FIGURES 1–21. Platichthys darwiniana sp. nov. 1–18. LM. Diminution size series of P. darwiniana sp. nov. frustules and valves, all in valve view taken from the holotype slide. 6. Holotype specimen. 19–21. SEM. 19–20. External view of the complete frustules, note the valvocopula perforated with multiple rows of fine puncta (arrow in Fig. 19). 21. External view of the valve with slight constriction in the middle showing the striae composed of very fine areolae (arrow) and particularly well developed hyaline valve mantle.

FIGURES 39–53. Platichthys krammeri sp. nov. 39–49. LM. Valve view, frustules in various position with different focus. Note in particular diffuse prolongation of fibulae into the valve transapically. 50–53. SEM. 50. External valve view. 51, 52. External view of valve end. Holotype specimen in straight and marginal view. Note copulae of epitheca and shallow valves lacking distinction between faces and mantles. 53. External valve view of specimen with slightly lacerated surface.

FIGURES 22–26. Platichthys darwiniana sp. nov. 22–24 SEM. 25, 26.TEM. 22.Valve view from the keel side of the specimen illustrated in Fig.21 (strongly tilted).Notethe position of the raphe bearing keel andthe central nodule (arrow).23.Specimen showing the compression of the valve and the raphe branches with distinct central nodule (arrow). 24. General view of the valve positioned on the raphe bearing canal. 25, 26. Valve of a specimen showing a position of the fibulae below the raphe bearing canal (arrowheads) and the transapical striae composed of oblong areolae (white arrow). Note the presence of narrow hyaline valve mantle (black arrow in Fig. 26) and of the hyaline raphe bearing canal (white arrow in Fig. 26). Fig. 26 is a close up of a specimen illustrated in Fig. 25.

FIGURES 27–32. Platichthys darwiniana sp. nov. SEM. 27. Valve apex showing strongly bent raphe apical ending (arrow). 28. View of an apical valve part with small groove formed by the strongly bent raphe end (arrowhead) which is not visible. 29. Apical part of the frustule showing strongly bent raphe end (arrowhead). 30–32. Internal view of a broken valve. 30. Internal view showing the compression of the valve (marked by the small arrowheads) and narrow hyaline mantle (arrow). 31, 32. Close up of the broken valve, note the striae forming areolae positioned in shallow grooves (arrowhead in Fig. 32).

FIGURES 33–38. SEM. Platichthys darwiniana sp. nov. 33, 34. Internal valve view of the same specimen at different tilting degree, note the position of fibulae (black arrow) and the internal central raphe endings (white arrow). 35. Broken valve showing the position of the fibulae (arrow), note the transapical striae merging into the particular fibulae. 36. Close up of the specimen illustrated in Fig. 34 with an internal view of the central valve part, note the distant position of the two central fibulae (arrowheads) and the central nodule (arrow). 37. Central part of the valve interior with close up of the central nodule with double helictoglossa (arrowhead) and the presence of portulae (white arrow). 38. Apical part of the valve interior, note the presence of simple helictoglossa (arrowhead) and the portulae (white arrow).

FIGURES 59–62. Platichthys krammeri sp. nov. compared to Nitzschia sp. SEM. 59. External/internal valve view, the two broken valves of a frustules displaying valve cavity with range of fibulae (above); raphe slit outside (below) running parallel to the marginal crest; note transapical waves and the external central raphe endings—central nodule (arrow). 60. Internal view of areolae structure and fibulae. 61, 62. Nitzschia sp. from section Lineares for comparison, specimens associated in the same preparation. Main distinguishing feature vs. Platichthys gen. nov. is differentiation between valve faces and mantles with a consequently more or less rectangular cross-section.