Entomoneis tenera Mejdandžić & Bosak, 2017

Mejdandžić, Maja, Bosak, Sunčica, Orlić, Sandi, Udovič, Marija Gligora, Štefanić, Petra Peharec, Špoljarić, Igor, Mršić, Gordan & Ljubešić, Zrinka, 2017, Entomoneis tenera sp. nov., a new marine planktonic diatom (Entomoneidaceae, Bacillariophyta) from the Adriatic Sea, Phytotaxa 292 (1), pp. 1-18 : 5-6

Entomoneis tenera Mejdandžić & Bosak sp. nov. ( Figs 1–35 )

LM morphology: Cells delicate with very lightly silicified frustules. One multi-lobbed chloroplast. Frustules panduriform in girdle view, constricted in half of the frustule length, often twisted around the apical axis with the different degrees of torsion ( Figs 1–7 ). Cells 16–21 μm long, 5–15 μm wide in constricted central part and 7–20 μm in widest part. Valves broad lanceolate ( Fig. 8 ), 11–22 μm long and 3–7 μm wide in central part. Scalpeliform valve apices ( Figs 9, 10 ). Sigmoid raphe-bearing keel distinct in valve view ( Fig. 9 ). Elevated keel separated from the valve body with straight to slightly arcuate junction line ( Figs 6, 9 ). In girdle view, junction lines are positioned at an angle of about 45° from the apical axis ( Fig. 10 ). Junction lines are sometimes hardly visible and valve striation is undiscernible in LM due to the light silification of frustules ( Fig. 11 ).

EM morphology: Valve striation becomes apparent in EM ( Figs 12–15 ). The transapical costae and striae are arranged parallel on the valve body, extending from the valve margin towards the junction line ( Figs 14–20 ). Costae are straight and simple, continuous from valve margin to keel, sometimes bifurcated at the valve margin or near the junction between valve body and wing ( Figs 14–17 ). Valve striae 30–55 per 10 μm. Strongly bilobate wing elevated from the valve body with wing costae and striae following contour of the keel, fusing along the junction line and further continuing parallel near the raphe to give a radial appearance ( Figs 14–16 ). Wing striae 18–42 per 10 μm. The striae are closed by a hymen with rectangular perforations arranged in two parallel lines along the stria edges ( Figs 19–22 ). Arrangement and density of the perforations similar in striae on both valve body and wing, 20–39 per 1 μm near the valve margin and 26–37 per 1 μm near keel margin ( Figs 19–21 ). Series of basal fibulae born on each wing costa form a junction line ( Figs 16, 18, 21, 22 ); 60–70 basal fibulae per 10 μm. Basal fibulae sometimes interconnected with adjacent fibulae with transverse connections in shape of H or W ( Fig. 21 ). Sigmoid raphe with simple linear central and terminal endings (Figs 23–28). The raphe slit is plicate, located at the apex of the keel. The raphe canal is separated from the valve by raphe fibulae, except in the central nodule which is three to four costae wide (Figs 23, 24). Raphe fibulae 29–42 per 10 μm. The central and terminal raphe endings are simple and very slightly curved downwards at valve apices (Figs 25–28). The cingulum is composed of one valvocopula and three to four copulae with smooth external surface ( Figs 29–30 ) and similar ultrastructure with two rows of distinct, elongated areolae ( Figs 31–33 ). In several observed valvocopulae, teardrop shaped areolae, with more elongated drop apex and larger radius in abvalvar than in advalvar ones ( Fig. 31 ). Between each two abvalvar teardrop shaped areolae, silica thickenings ( Fig. 31 ). Areola density in valvocopulae 40–50 per 10 μm. Oblong areolae in copulae are occluded by very lightly silicified hymenes perforated with round to rectangular poroids ( Figs 34, 35 ).

Type:— CROATIA: Adriatic Sea , south-eastern coast (N 42°29’ E 17°17’). Plankton net sample collected on March 2, 2015 by S. Bosak. Holotype slide of the strain PMFEN2 deposited in The Friedrich Hustedt Diatom Study Centre, Bremerhaven, Germany as BRM ZU10 /75 (holotype illustrated in Figs 9, 10, 11 ). PMFEN1 and PMFEN3 permanent slides deposited as isotypes at Macedonian diatom collection, Skopje, Macedonia under accession numbers MKNDC / Acc. No. 10517, MKNDC / Acc. No. 10518, respectively.

Etymology:—From Latin adjective tenera (soft, delicate, gentle, fragile) referring to the delicate valves and general appearance of the cells in light microscope.

Habitat:—Marine plankton.

Comments:—Summarized morphological features of E. tenera and comparison with five other Entomoneis species ( E. japonica , E. paludosa , E. punctulata , E. aequabilis and E. vertebralis ) are presented in Table 4. The new species, Entomoneis tenera sp. nov., is morphologically similar to other species in the genus, having a panduriform frustule with a well-developed winged keel elevated from the valve face and the sigmoid raphe positioned on the keel apex in the raphe canal, numerous girdle bands and junction line. In addition, the following features are considered to be characteristic for the newly proposed species: (1) general appearance is very delicate with lightly silicified frustules, with no valve striation visible in LM, (2) the cells are much smaller than that of other species, (3) one multi-lobed plastid, (4) broad lanceolate valves with scalpeliform apices, (5) junction line straight to slightly arcuate, positioned at an angle of about 45° from the apical axis.

Figures

FIGURES 1–11. Entomoneis tenera, LM. Live cells (Figs 1–8); cleaned material (Figs 9–11). Figs 9–11 taken from holotype permanent slide BRM ZU10/75. (1–4) Cells with various degree of torsion along the apical axis. (5, 6) Recently divided cells. Arrow in Fig. 6. shows the junction line. (8) Lanceolate valve. (9) Valve with sigmoid keel and scalpeliform apices. (10) Girdle view of the valve with straight to arcuate junction line (arrows in Figs 9, 10). (7, 11) Panduriform cell. Scale bars: Figs 8, 1, 2, 5, 6, 7=10 μm; Figs 3, 4, 9, 10, 11=5 μm.

FIGURES 12–15. Entomoneis tenera strain PMFEN2, SEM. Girdle view (Figs 12–14), valve view (Fig. 15). (12) Three cells attached with keels. (13) Cell twisted around the apical axis. (14) Girdle view of valve and cingulum with visible striation (costae bifurcation near the junction line indicated with an arrow). (15) Striation on the wing and valve body. Scale bars: Fig. 12=10 μm; Figs 13, 14, 15=2 μm.

FIGURES 16–22. Entomoneis tenera strain PMFEN2, SEM and TEM. Girdle views (Figs 18–20), valve views (Figs 16, 17, 21, 22), RF-raphe fibulae, BF-basal fibulae (junction line). (16) Valve with scalpeliform apices and junction line (arrowhead). (17) Valve with valvocopulae and sigmoid raphe-bearing keel (costae bifurcations are indicated by arrow). (18) Cell with complete girdle and indicated junction lines (arrowheads). (19, 20) Fine structure of the wing and valve body. (21) Adjacent basal fibulae fused with transverse connections (arrows). (22) Basal fibulae separating wing from valve body. Scale bars=1 μm.

FIGURES 29–35. Entomoneis tenera strain PMFEN2, SEM and TEM. VC-valvocopula; C-copula. Girdle views (Figs 29–35). (29) Frustule with the girdle. (30) Fine structure of the copulae. (31) Fine structure of valvocopulae with teardrop shaped areolae and interareolae thickenings (arrow). (32) Cingulum. (33) Valve with cingulum and decussate appearance of the costae on the valve between valvocopulae (arrowhead) and junction line. (34, 35) Fine structure of copulae. Scale bars: Figs 29, 30=2 μm: Figs 32, 33=1 μm; Figs 31, 34, 35=300 nm.