Fallacia decussata Yu H.Li & Hidek.Suzuki, 2015

Li, Yuhang, Suzuki, Hidekazu, Nagumo, Tamotsu, Tanaka, Jiro, Sun, Zhongmin & Xu, Kuidong, 2015, Fallacia decussata, sp. nov.: a new marine benthic diatom (Bacillariophyceae) from Northeast Asia, Phytotaxa 224 (3), pp. 258-266 : 260

Fallacia decussata Yu H.Li & Hidek.Suzuki , sp. nov. ( Figs 2–6 , 17–30 )

= Fallacia pygmaea sensu Chihara & Murano (1997: 213 , fig. 125)

= Navicula (Fallacia) forcipata var. densestriata sensu Joh (2013: 382 , pl. 6, figs 10, 11)

Valves elliptical with broad round poles, 13.7–20.6 μm long and 6.1–8.4 μm wide. Raphe straight with distant central endings. Lateral sterna thin, crossing or strongly constricted centrally, forming an X-shaped central area. Striae between lateral sterna parallel; striae outside lateral area radiate, 25–28 in 10 μm. Areolae occluded by a hymen with perforations arranged in a hexangular array. Whole valve face covered by a finely porous conopeum. Pegs on surface of distal end of each virga around valve face. Two to four pores on both sides of raphe terminal fissures formed by thickened virgae and flat edge of conopeum.

Type:— JAPAN. Miura Peninsula: Kanagawa Prefecture, Ena Bay (35° 08’ 39.34” N, 139° 39’ 49.60” E), Yuhang Li , 7 August 2012 (holotype BM! 101800, here illustrated in Fig. 3 , The Natural History Museum, London, UK; isotypes MBM! 283041 and 283042, Marine Biological Museum, Chinese Academy of Sciences , Qingdao, China) .

Etymology:—The Latin adjective decussatus (decussate) refers to the centrally crossing or strongly constricted lateral sterna appearing a X-shaped like central area.

Observations:—Valves are elliptical with broad round poles, 13.7–20.6 μm (16.2 ± 2.1μm, mean ± s, n=30) long and 6.1–8.4 μm (7.2 ± 0.5 μm, n=30) wide ( Figs 2–6 ). The external valve surface is slightly domed, with a shallow valve mantle ( Fig. 17 ). Internally, the lateral sterna are arched on valve surface ( Fig 18 , arrow). Axial area is very narrow. Raphe branches are straight. Polar raphe terminal fissures are externally curved to the secondary side of valve ( Fig. 21 ), the internal distal raphe terminal fissures end with a helictoglossa ( Figs 22 ). The central raphe endings are distant ( Fig. 28 ), taking about 6.1%–10.5% (8.0% ±1.0%, n=30) of valve length. Externally, they are slightly expanded and deflected to the primary side ( Fig. 19 ), but internally they are straight fissures ( Figs 20 ). Striae are composed of uniseriate areolae, 25–28 (26.3 ± 1.2, n=30) in 10 μm, interrupted by lateral sterna into two parts. The distal parts of areolae are radiate, but the proximal parts are parallel. The proximal parts take a max of 1/2 to 1/3 valve width ( Figs 2–6 , 28, 29 ). Areolae are occluded by a hymen with perforations arranged in a hexagonal array ( Fig. 30 ). Striae are covered by a finely porous conopeum. Pores on the conopeum are multiseriate above each striae ( Fig. 25 , arrow). The conopeum fuses with valve mantle by several pegs ( Fig. 25 , arrowhead, Fig. 21 , double arrows) located on one side of virgae, except at the two terminals. There, the margin of the conopeum and the thickened virgae, which are not covered by conopeum, give raise to two to four pores on both sides of the raphe terminal fissures ( Figs 21, 23, 24 , arrowheads). A number of areolae occur outside these pores on the valve mantle ( Figs 21, 24 , double arrowheads). The thin lyre-shaped lateral sterna are convex and strongly constricted centrally, appearing a more or less X-shaped central area ( Figs 2–6 , Fig. 28 ). The cingulum consists of at least two non-perforated bands. One open valvocopula bears an undulate advalavar edge of the pars interior and a copula ( Figs 18 , 27 ).

Distribution and ecology:— Fallacia decussata has been found in Japan, Korea and China ( Fig. 1 ). This species is an epipsammic, inhabiting in marine and estuarine sandy sediments in the northwest Pacific Ocean.

Figures

FIGURE 1. Map of the sampling and reported sites of Fallacia decussata in Northeast Asia. I. Estuary of Kushirogawa River, Kushiro City, Hokkaido, Japan. II. Ena Bay, Miura Peninsula, Kanagawa Prefecture, Japan. III. Huokun’ao sand beach, Nanji Islands National Marine Natural Reserve, Wenzhou City, Zhejiang Province, China. IV. Estuary of Nakdong River, Korea.

FIGURES 2–13. LM micrographs of cleaned valves of Fallacia decussata, F. pygmaea and F. scaldensis. 2–6. Morphological variation of F. decussata. Note the convex lateral area (Fig. 2, arrow). 3. Holotype specimen showing constricted X-shaped central area (arrow). 7–9. Specimen from isotype slide of F. pygmaea (BM77887). 10–13. Specimen from holotype slide of F. scaldensis (BM100177). Scale bar = 10 μm.

FIGURES 17–24. Scanning electron micrographs of Fallacia decussata. 17. External valve face with a broken margin showing the conopeum covering the whole valve face (arrow). 18. Internal valve face with cingula showing the arched lateral sterna (arrow).19. Detail of the external central area of the valve showing the slightly expanded central raphe ending (arrow). 20. Detail of internal central area of valve showing the central raphe ending fissures (arrow). 21. Detail of valve external terminal surface showing the finely porous conopeum (arrow), the dentate inner edge of the valve mantle by pegs (double arrows), three terminal pores (arrowheads), and a number of areolae (double arrowheads) present on the terminal valve mantle uncovered by the conopeum. 22. Detail of the internal terminal valve showing the raphe branch fissures ending in a helictoglossa (arrow). 23–24. Variation of the number of pores (arrowheads) on each side of the terminal fissure, comparing the areolae (double arrowheads) on the valve mantle. All scale bars = 2 μm, except Figs 17, 18 (5 μm).

FIGURES 25–30. Scanning and transmission electron micrographs of Fallacia decussata. 25. Detail of the finely porous conopeum covering the external valve surface, fusing with the valve mantle with a number of pegs (arrowhead), showing the multiseriate pores on the conopeum (arrow). 26. Detail of the lateral sterna (internal view) showing the arched canal (white arrow) and the finely porous conopeum (white arrowhead). 27. Internal view of a valve with valvocopula, showing the undulate advalvar edge of the parts interior (white arrow). 28. TEM photograph of a valve showing an uniseriate line of areolae around terminals (arrowheads). These areolae are located on the valve mantle. 29. Detail of a valve terminal under TEM showing the slightly expanded central raphe ending (white arrowhead) and voigt discontinuity (white arrow). 30. Areolae are occluded by a hymen with perforations arranged in a hexagonal array. Scale bars: 2 μm (Figs 26, 27, 29), 1 μm (Fig. 25), 3 μm (Fig. 28) and 200 nm (Fig. 30).