Gyrosigma xiamenense Bing Liu, F.A.S. Sterrenburg et Bangqin Huang, 2015

Liu, Bing, Sterrenburg, F. A. S. & Huang, Bangqin, 2015, Gyrosigma xiamenense sp. nov. (Bacillariophyta) from the middle intertidal zone, Xiamen Bay, southern China, Phytotaxa 222 (4), pp. 259-266 : 261-264

Gyrosigma xiamenense Bing Liu, F.A.S. Sterrenburg et Bangqin Huang , sp. nov. ( Figs 2–30 )

Valve linear-sigmoid, 219–302 μm long, 18.9–22.7 μm wide. Striae: transverse 15–16 in 10 μm, longitudinal 18–19 in 10 μm. Central external raphe fissure deflection of isomorphic pattern. Differs (SEM) from all other species in the G. balticum complex in having T-shaped internal central raphe endings crossed by a tiny transverse silica bridge and a vermiform bar in the middle of the central raphe nodule.

Type: — CHINA. Fujian: Xiamen Bay, Dadeng Island, the middle intertidal zone of Yangtang Village, 24°31′54″N, 118°20′16″E, 0 m a.s.l., Bing Liu, 7 July 2013 (holotype JIU! G201502, individual located 10.9 mm south by 17.2 mm east from the benchmark cross on the slide, illustrated here as Fig. 3 . Isotype CL201302, individual located 9.8 mm south by 17.7 mm east from the benchmark cross on the slide, illustrated here as Fig. 5 ).

Description: —LM: Cells solitary, usually lying in valve view. Valves sigmoid with parallel sides over approximately the median three-fifths of the whole valve, tapering on one side towards the apex to narrowly rounded apices ( Figs 2–9 ). Valve face vaulting ( Figs 2–7 , 11 ), 219–302 μm long, 18.9–22.7 μm broad. Raphe sternum sigmoid with single curvature, displaced to its concavity in the apical portion, raphe angle + 5° ( Figs 2–9 ). The central area roughly forms a parallelogram, its major axis rotated to the convexity of the raphe sternum, the shadows of the central bars and internal proximal raphe endings are distinctive whilst the external raphe fissures are not clearly visible ( Figs 2–7 , 11 ). Terminal areas unilaterally dilated on the concavity of the raphe sternum ( Figs 2–7 ). At the apex, a long crescent of minute pores indicated by a crescent-shaped dark line both external and internal ( Figs 10, 12, 13, 15 ). Internally, the valve appearing wider than external views ( Figs 8–9 ) and a notable rib longitudinally crossing the nodule ( Figs 8–9 , 14 ). Striae transverse and longitudinal, parallel throughout valve. Transapical striae 15–16 in 10 μm, longitudinal striae 18–19 in 10 μm.

SEM: The LM findings are confirmed in SEM. Raphe sternum sigmoid with single curvature ( Figs 16 , 19 ). A long, isolated crescent of apical pores present ( Fig. 17 , three black arrowheads). Terminal raphe fissures curving in opposite directions and running over the mantle ( Fig. 17 ). Round pores located on the mantle and on one side continuing up to the longitudinal middle line of the valve ( Figs 17–18 , nine arrows). Proximal raphe fissures turning first in opposite directions, then both final parts of the fissures run nearly parallel to the longitudinal striae ( Figs 18 , 23–26 ). Peripheral stria condensation present ( Fig. 18 , three double-headed arrows). Internally, a long isolated crescent of apical pores present ( Fig. 20 , three curved arrows). A row of areolae closest to the raphe sternum is capped by small projections ( Fig. 20 , three arrows) on the primary side whilst these are absent on the secondary side. Central bars thickened with undulating outer edges. Proximal raphe endings club-shaped and wide, then the final parts of the two raphe endings form two T-shaped slits, each slit being separated from the club-shaped part by a tiny bridge ( Fig. 21 , two arrows). There is a vermiform bar in the middle of the raphe nodule ( Fig. 21 , curved arrow). In the central area a row of areolae closest to the raphe sternum capped by small projections ( Fig. 22 , four arrows), which continue up to the central bar on the primary side whilst on the secondary side these are absent ( Fig. 22 ). The shape of both the external central raphe fissures and the internal central raphe endings appears to be stable ( Figs 23–30 ).

Etymology: —Named after Xiamen, the area where the species was found.

Ecology and distribution: —In the sampling site of Yangtang Village, the sediment type (Wentworth scale) is classed as medium silt, salinity of water is 27 ± 1 ppt, pH of the sediment is 7.98 ± 0.01, and water temperature is 28.1 ± 0.3 °C. It is clear that G. xiamenense is epipelic and lives in marine habitats. From our observation data, G. xiamenense also was found from other four sampling sites: Caidian, Houtian, Jiageng and Yuemeichi ( Fig. 1 ), thus it appears common in the intertidal mud-flats of Xiamen Bay, China.

Observations: —The structure of both the external and internal proximal raphe fissure endings in G. xiamenense is unique ( Figs 18 , 21 ). No other species possessing the same structure has been found. Taking into account the valve outline, G. xiamenense resembles G. balticum and allied species. However, G. xiamenense has single curvature of raphe sternum and its raphe sternum is displaced to the concave side of the valve at the apex ( Figs 2–9 , 16 , 19 ). These characters separate it from G. balticum , G. sterrenburgii , G. pensacolae , and G. murphyi , which all have double curvature of raphe sternum and a raphe sternum displaced to the convex side of valve (see Stidolph 1992, Sterrenburg 1995, Reid & Williams 2003). G. xiamenense and G. waitangiana have nearly the same stria density (transverse 15–16 vs. 15, longitudinal 18–19 vs. 18), however, they have differently shaped external proximal raphe fissures (final part of raphe fissures parallel to the longitudinal striae vs. running into an inflated furrow) and differently shaped internal central area (thickened central bars with undulating outer edges vs. thickened central bars with several heavily silicified interstriae merged into) (see Stidolph 1993). Gyrosigma xiamenense and G. cali have also different external proximal raphe fissures (final part of raphe fissures parallel to the longitudinal striae vs. entering into the striae) and different internal central area (thickened central bars with undulating outer edges vs large and flattened central bars accompanied by a large hyaline area) (see Reid & Williams 2003). The external proximal raphe fissures both deflect first to the convexity of the raphe sternum in G. xiamensense ( Figs 16, 18 ), thus it is an isomorphic species according to Sterrenburg (1993).

A row of round pores located on the mantle has been rarely reported in Gyrosigma / Pleurosigma W. Smith (1852: 2) . In G. xiamenense , the row of round pores on the mantle is regular: each pore is coupled with an outermost slit and on one side continues up to the longitudinal middle line of the valve ( Figs 17–18 ). In G. rostratum , there is also a row of round pores located on the mantle ( Liu et al. 2015: 258, Figs 14 , 16 ). It is difficult to observe the row of round pores because they are located on the mantle. The valves should be suitably oriented so that the row of round pores can be seen clearly. This structure deserves more attention.

The structure of a row of small projections ( Figs 20, 22 ) is also found in some other Gyrosigma species. The same structure was referred to as “tooth-like projections (P)” in G. rautenbachiae Cholnoky (1957: 65) by Schoeman & Archibald (1986: 133, fig. 34). Stidolph (1992) called this structure “cavus areolae” in G. sterrenburgii . However, Stidolph (1993) used “tooth-like projections” for the same structure in G. waitangiana . Sterrenburg (1995) also found this structure in G. acuminatum (Kützing) Rabenhorst (1853: 47 ; basionym: Frustulia acuminata Kützing 1833: 555 ), a freshwater species. Therefore, the row of areolae closest to the raphe sternum capped by small projections occurs in marine as well as freshwater species.

The vermiform bar in the middle of the raphe nodule in G. xiamenense ( Figs 21–22 ) is also rarely found in Gyrosigma . However, Pleurosigma siberica (Grunow) A.Cardinal, M.Poulin & L.Bérard-Therriault (1989: 25; basionym: P. clevei var. siberica Grunow in Cleve 1883: 475) has also a similar vermiform bar (see Cardinal et al. 1989: 22, fig. 33). Not only possessing the three uncommon structures mentioned above, but also having the unique external and internal proximal raphe fissure endings, G. xiamenense should rightly be considered a new species.

Figures

FIGURE 1. Map of the study site showing the middle intertidal zone of sampling: DDI = Dadeng Island, YTV = Yangtang Village, CD = Caidian, HT = Houtian, JG = Jiageng, and YMC = Yuemeichi.

FIGURES 2–9. Gyrosigma xiamenense sp. nov., 2–7. External views. 8–9. Internal views, LM. 2–4. Three valves belonging to one type of valves, either epivalves or hypovalves. 3. Photograph of holotype specimen. 5–7. Three valves belonging to the other type of valves, either epivalves or hypovalves. 5. Photograph of isotype. 8–9. Internal views of two types of valves. Scale bars = 20 μm for all figures.

FIGURES 10–15. Gyrosigma xiamenense sp. nov., 10–12. External views. 13–15. Internal views, LM. 10, 12, 13, 15. Apex, note the apical pores are distinctive. 11. Central area, note the shadows of central bars. Scale bars = 10 μm for all figures.

FIGURES 16–18. Gyrosigma xiamenense sp. nov., external views, SEM. 16. A whole valve, note that the raphe sternum has single curvature only. 17.Apex showing an isolated crescent of apical pores (three black arrowheads), terminal raphe fissure, round pores located on the mantle (three black arrows) and on the other side continuing up to the longitudinal middle line of the valve (three white arrows), and elongated narrow slits. 18. Central area, showing proximal raphe fissures turning first in opposite directions, then both final parts of the fissures run nearly parallel to the longitudinal striae. Peripheral stria condensation (three double-headed arrows), and round pores located on the mantle (three arrows). Scale bars = 20 μm (Fig. 16), 1 μm (Figs 17–18).

FIGURES 19–22. Gyrosigma xiamenense sp. nov., internal views, SEM. 19. A whole valve, note that the raphe has single curvature only. 20. Apex, showing an isolated crescent of apical pores (three curved arrows), a row of areolae closest to the raphe sternum is capped by small projections (three arrows) on the primary side whilst these are absent on the secondary side. 21. Central area, showing thickened central bars with undulating outer edges. Proximal raphe endings club-shaped and wide, then the final parts of the two raphe endings form two T-shaped slits, each slit being separated from the club-shaped part by a tiny bridge (two arrows). There is a vermiform bar (curved arrow) in the middle of the nodule. 22. Central area, showing a row of areolae closest to the raphe sternum capped by small projections (four arrows), which continue up to the central bar on the primary side whilst on the secondary side these are absent. Scale bars = 20 μm (in Fig. 19), 1 μm (in Figs 20–22).

FIGURES 23–30. Gyrosigma xiamenense sp. nov., SEM. 23–26. External views of four specimens, showing variation in central raphe fissure morphology. 27–30. Internal views of four specimens, showing variation in central raphe ending morphology. Scale bars = 2 μm (Figs 23–24), 1 μm (in Figs 25–30).