Dicladia

Li, Yang, Boonprakob, Atchaneey, Gaonkar, Chetan C., Kooistra, Wiebe H. C. F., Lange, Carina B., Hernández-Becerril, David, Chen, Zuoyi, Moestrup, Øjvind & Lundholm, Nina, 2017, Diversity in the Globally Distributed Diatom Genus Chaetoceros (Bacillariophyceae): Three New Species from Warm-Temperate Waters, PLoS ONE (e 0168887) 12 (1), pp. 1-38 : 34-35

Additional morphological characters in section Dicladia ?

A feature of C. laevisporus which occurs also in C. decipiens and C. elegans ( Figs 1G and 4D,

arrowheads) is the presence of a non-silicified, V-shaped protrusion on the terminal valves

( Fig 8A, arrowhead). Chaetoceros cf. lorenzianus from the Gulf of Panama shows the same fea-

ture [ 35]. This, together with the reduced external tube of rimoportula on the terminal valve

(loc. cit. figs 77–80 in [ 35]) clearly places this material as belonging to C. laevisporus , The V-

structure was observed in C. decipiens by Rines & Hargraves [ 10] and Jensen & Moestrup [ 9],

and may be present in more species of the section. Because of its non-silicified nature, the

structure disappears in acid-cleaned material.

The ear-like structures at the marginal border of the apertures have been observed in all the

studied strains, often in different shapes on the intercalary valves and terminal valves. In C.

decipiens and C. mitra , the structures are very distinct, and they interconnect the edges of sib-

ling valves on each side of the aperture ( Figs 3B, 3C and 14F). In C. elegans , C. laevisporus and

C. mannaii they are smaller, sometimes with overlapping parts ( Figs 6A, 6B, 9A, 9B, 11C and

11D). On the terminal valves, these structures vary in shape, as fringes in C. decipiens , C. laevis-

porus and C. mitra ( Figs 2F, 9C and 14G) and as slice-shaped structures in C. elegans and C.

mannaii ( Figs 5D and 11E). Based on the morphological variation observed, and the potential

influence of environmental factors on the development of the structures, it is somewhat uncer-

tain whether they can be used as a species-specific character.

Although our study was based on established strains, with the risk of culture condition arte-

facts affecting the morphology, we regard the present species delineation sound, due to the fol-

lowing: 1) strains were fixed soon after establishment and morphology is thus not very likely

affected by artefacts, 2) several of the species are based on strains from separate locations show-

ing the same morphology, 3) observations of field samples made it possible for us to identify

the taxa based on the descriptions provided, 4) evaluations with previously published records

of species belonging to Dicladia , for which electron micrographs are available (S3 Table) sup-

port our observations and 5) the study was performed over a long period, and subsequently

established strains confirmed the species descriptions.