Morphology and molecular phylogeny of two marine diatom species, Falcula cf. media and F. rectangularis sp. nov. (Bacillariophyta) from Japan

ABSTRACT

Two members of the marine diatom genus Falcula, which has been little known for a long time, were found from the surface of some Japanese seaweeds; one was assigned to F. cf. media and the other was described as a new species, F. rectangularis sp. nov. Aside from the arcuate valves and slit-like apical fields, typical of the genus, detailed morphological observations using light and scanning electron microscopy revealed resemblance between F. cf. media and F. rectangularis in areolae, stria density and lack of costae. However, the two species can be distinguished based on several morphological features, including the valve length, plastid arrangement, and frustules in girdle view. A three-gene molecular phylogeny recovered these two species as monophyletic with a high interspecific rbcL difference, which in turn were sister to the clade containing taxa that belong to the orders Cyclophorales and Protoraphidales with low support. The members in this poorly supported assemblage share apical fields composed of slits. The two species of Falcula were phylogenetically distinct from Pseudofalcula hyalina, a member previously assigned to Falcula.

KEYWORDS:

• Epiphytic diatoms
• Marine diatoms
• Phylogenetic position
• Red algae
• Taxonomy

ACKNOWLEDGEMENTS

We thank Ms. Haruna Mutagamihigashi and Mr. Mizuki Kitano, Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Japan, and Ms. Yurika Mikame, Saitama Museum of Rivers, Japan, for providing us with the samples treated herein. We are grateful to Professor Tamotsu Nagumo at Echigo Natural History Laboratory, Japan, for his helpful suggestions.

DISCLOSURE STATEMENT

No potential conflicts of interest are reported by the authors(s).

Supplementary Information

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00318884.2024.2312574

Additional information

Funding

This work was financially supported by JSPS KAKENHI Grant Numbers JP22K05800 and a grants-in-aid of the Mikimoto Fund for Marine Ecology, Japan.