Platichthys darwiniana Seddon & Witkowski, 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 : 137-141

Platichthys darwiniana Seddon & Witkowski , sp. nov. ( Figs 1–38 )

Frustules in girdle view 23–33 μm long, 7–8 μm broad ( Figs 1–13 ). Valves 4.0–5.5 μm broad, not including copulae. The dorsal valve margins are slightly concave in the middle, becoming convex towards the apices. The ventral valve margin is convex and tapers towards the apices ( Figs 14–18 ). The valve face is compressed laterally to form a narrow keel, the valve surface is steep. The valve mantle is very narrow in the form of structureless band (black arrow in Fig. 26 , arrow in Fig. 30 ). The fibulae are very large, 5.5–7.5 in 10 μm, the two median ones are somewhat wider spaced than the others implying the presence of a central nodule. The striae are not visible in LM ( Figs 1–18 ). The raphe in LM is barely observable, although in LM phase contrast optics both central external and apical endings are detectable. EM: the girdle is composed of a few, open bands perforated with 50–60 pores in 10 μm. The number of rows of pores amounts to ca. 10 for a single copula ( Figs 19, 20 , 29 ). The raphe is positioned in a very narrow axial area ( Figs 22, 23, 25 and white arrow in Fig. 26 ), the external central endings are close to each other (arrow in Figs 22 and 23 ), the apical endings strongly bent in one direction (arrow in Fig. 23 , 27, 28 ). Internally, the raphe is slit-like with internal central raphe endings close to each other and very slightly expanded (arrow in Fig. 36 ) terminating in a small, double helictoglossa (arrowhead in Fig. 37 ). Likewise the internal apical raphe endings terminate in a small helictoglossa ( Fig. 38 ). The transapical striae are only measurable on EM images and number 48–61 in 10 μm, whereas the number of areolae within a stria range from 56 to 72 in 10 μm. The striae are composed of straight, uniseriate rows of areolae (arrowheads in Fig. 25 ). The areolae are in the form of simple poroids ( Figs 30–32 ). Areolar occlusions have not been observed thus far. Internally, the fibulae are massive, column-shaped, short and arched (arrows in Figs 33–35 , arrowheads in Fig. 36 ). They are positioned in a single row at the same height within the valve interior and separate the raphe canal from the valve lumen. The fibulae are positioned at some distance from the raphe (arrowheads in Fig. 26 , Figs 33–38 ). As the fibulae are so large usually a few striae merge into one fibula (cf. Fig. 37 ). Wave crests begin at the basis of fibulae while wave troughs continue into the portulae (white arrows in Figs 37 and 38 ). The portulae are very simple and are formed as a result of a slight broadening of the base of the fibulae ( Figs 34–38 ).

Type: —GALAPAGOS ISLANDS. Island Isabela: Diablas Lagoon, littoral, a sample from the Late Holocene sediment core, a sample from the Late Holocne sediment core collected in 2005, subsampled by A Seddon, 2007 (holotype: individual in slide no. MCCDRS 7862 at the Charles Darwin Research Station, Puerto Ayora, Galápagos, illustrated in Fig. 4 ).

Etymology: —this species is named in honor of Charles Darwin.

Ecology: —so far only found at the type habitat from a lagoon that is changing in salinity from brackish-water to marine close to the inlet of the sea water. The new species occurred abundantly and was very well preserved and associated with brackish-water to marine representatives of Navicula Bory (1822: 128) , Amphora Ehrenberg ex Kützing (1844: 107) , Fragilaria Lyngbye (1819: 182) sensu lato and Nitzschia . All were benthic, halophilouseuryhaline species, containing genera typifying isolation basins in the littoral zone ( Seddon et al. 2011 a, 2014a).

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 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).