Coscinodiscus variabilis Frenguelli

Lameiro, Rubén A., Cefarelli, Adrián O. & Vouilloud, Amelia A., 2023, Taxonomic revision of Coscinodiscus variabilis (Bacillariophyta): analysis of type material, lectotypification and transfer to the genus Actinocyclus, Phytotaxa 626 (4), pp. 269-278 : 270-272

Coscinodiscus variabilis Frenguelli Figs 3–30

Original description (in Spanish): “ Valva pequeña, de 0,039 –0,057 mm de diámetro; superficie llana hasta cerca del borde convexo. Estrías: en el centro 8 y en el borde 12 en 0,01 mm; formadas de perlas bien distintas y robustas; reunidas en 6-7 fascículos radiantes separados, cerca del borde valvar, por un apículo bien evidente, interfascicular. En los ejemplares menores, cada serie fasciculada está separada de la contigua por una hilera de perlas que llega hasta cerca del centro, donde se encuentra un pequeño espacio hialino de forma irregular. En los ejemplares mayores este espacio central es más grande y se prolonga en forma de estrella con rayos más o menos largos en relación con el diámetro de la valva, substituyendo, en parte o totalmente (hasta el apículo correspondiente), la hilera de perlas interfascicular. Por esta disposición, mientras los ejemplares más pequeños tienen aspecto actinocicloide*, los más grandes, con espacios hialinos interfasciculares prolongados hasta cerca del borde y bien definidos, asumen estructura cosmiodiscoide, tanto más que, el espacio hialino central, con el progresivo desarrollo de los rayos interfasciculares es invadido por perlas irregularmente distríbuidas.”

Original description: (our translation) “ valve small, about 39–57 µm of diameter; flat surface until near the convex margin. Striae: 8 in the center and 12 in the edge in 10 µm; formed by very distinct and robust pearls; reunited in 6–7 radiate fascicles separated, near the valve margin, by an evident interfascicular apiculus; in the small specimens, each fasciculated series is separated from the contiguous by a row of pearls that almost reaches the center, where a small irregular hyaline space is observed. In the larger specimens, this central space is bigger and prolongs in the shape of a star with rays more or less long in relation to the valve diameter, substituting, partially or completely (up to the correspondent apiculus) the row of interfascicular pearls. By this disposition, while the smaller specimens have an actinocycloid aspect, the larger ones, with well-defined interfascicular hyaline spaces that extend near the valvar edge, assume a cosmiodiscoid structure, so much more that, the central hyaline space is invaded by irregularly distributed pearls due to the progressive development of the interfascicular rays’’ ( Fig. 2 ).

Expanded description (n= 24): Valves circular with a flat valve face or with a slightly depressed valve center, gently curving into the mantle; valve diameter 29–59µm. Radial areolation with areolae arranged in fascicles, 6–10 areolae in 10µm. Central area hyaline and small, variable in shape, clearly defined or irregular with sparse areolae. Fascicles limited by 6–14 hyaline rays of variable development, with large radial areolae rows parallel to central row, 8–10 striae in 10µm at the center of the valve and 10–12 in 10µm at valve margins. Hyaline rays either conspicuous or not clearly defined, variable in number, length and thickness, connected with valve center or margins, or between them. One marginal pseudonodulus ( Figs 6, 8, 17, 22 , 25 ) not always clearly defined, located at junction between valve face and mantle and a marginal ring of rimoportulae distinctively separated from each other, 4.8–8.2 RP in 100µm, aligned with hyaline rays ( Figs 7, 20 , 24, 27 ). Mantle with 15–17 striae in 10µm, areolae smaller and reduced to half of valvar areolae size.

There was high morphological variability among the specimens of the type population that allowed the recognition of specimens without well-defined hyaline rays whose fascicles are delimited by conspicuous rows of areolae ( Figs 16–17, 22–25 ) and specimens with clearly defined nearly or complete hyaline rays that may connect with, or be near, the valve margins exhibiting a star-shaped valve face ornamentation with an irregular central area (3–6, 12–15, 29). Intermediate forms were also noted with variable length and degree of development of the hyaline rays recognizing specimens with hyaline rays that may be inserted from an irregular central area and do not reach the valve margins ( Figs 11, 18–21 , 30 ), and specimens with hyaline rays located in the middle of the valve face that may reach the valve margins but never connect with the central area ( Figs 7–10 , 27–28 ). The thickness of this structure is another variable character. In some specimens they have a clock-hand shape becoming thinner towards the margins ( Figs 12–13 ), some specimens have thickened rays that finish abruptly ( Figs 11 , 30 ) or have rounded ends ( Figs 7 , 28 ). Some of the thicker rays are interrupted by areolae or may include them in their structure ( Fig. 29 ). All these differences regarding the length and thickness of the hyaline rays even vary within a same specimen.

Figures

FIGURES 3–23. Coscinodiscus variabilis Frenguelli. LM. Specimens of type population, size diminution series; note the variability in hyaline rays, marginal rimoportulae, changes in the striation pattern between valve face and mantle, and pseudonodulus (arrowheads). Scale bar: 10 µm.

FIGURES 24–30. Coscinodiscus variabilis Frenguelli. LM. Specimens of type population. 24–26. Valve view showing the variation on the same individual from three different focuses. 24. Note the central area, fasciculation, mantle areolae and external marginal ring of rimoportulae which are coincident with the fascicles. 25. Detail of areolae and pseudonodule on valve face. 26. Note the marginal ring of rimoportulae. 27. Valve tilted showing the detail of the rimoportulae and their orientation on the valve mantle and the hyaline rays aligned with them. 28–30. Valve views showing the variation of the central area and hyaline rays. 28. Specimen with incomplete hyaline rays closer to the valve margins and with rounded ends. 29. Specimens with complete or nearly complete clock-hand shape hyaline rays that reach the valve margins and may not connect the valve center. 30. Specimens with incomplete hyaline rays with abrupt ends that insert from the center of the valve face and never reach the valve margins. Scale bar: 10 µm.

FIGURES 1–2. Materials (studied from the Frenguelli Collection). 1. Slides from the Series 167. 2. Original illustrations of Coscinodiscus variabilis Frenguelli (the pasted pictures reproduced in the original publication). Lectotype: Specimen of pl. 14, fig. 8 of the original publication.