Planothidium lanceolatum and Planothidium frequentissimum reinvestigated with molecular methods and morphology: four new species and the taxonomic importance of the sinus and cavum

Abstract

Molecular data (rbcL and 18S) of 36 Planothidium strains were analysed. Twenty-seven strains were also studied morphologically by light microscopy and scanning electron microscopy: six strains from Berlin, two strains from the Faroe Islands, four strains from Lake Baikal, seven strains from Korea and eight strains from Mexico. The findings were compared to International Nucleotide Sequence Database Collaboration data of strains from New Zealand, Germany, France and the USA. The molecular and morphological data differentiated eight species, and the molecular trees underlined the clear differentiation between the two clades, with taxa possessing an asymmetrical central area on the sternum valve which is devoid of striae, either in the form of a sinus or of a cavum clustering together, versus the one taxon lacking a sinus or a cavum. In addition to Planothidium lanceolatum and P. cf. subantarcticum, the clade with a sinus contains two new species, P. cryptolanceolatum and P. taeansa, whereas the other clade with a cavum contains, in addition to P. frequentissimum and the recently described P. victori and P. caputium, the new species P. naradoense. The species without a sinus or cavum is also described as new, P. suncheonmanense. With respect to their distribution, P. victori is the most common, with 11 strains from the studied continents, Europe (Berlin), Asia (Lake Baikal), Americas (Mexico, USA), Australia/Oceania (New Zealand), whereas P. frequentissimum, represented by 5 strains, was restricted to Germany, France and New Zealand. A different geographical pattern seems to apply to the P. lanceolatum clade, with four strains occurring in Germany, the Faroe Islands, USA and Lake Baikal. Planothidium cryptolanceolatum (eight strains) only occurred in Korea, Mexico and USA. One strain from the Faroe Islands, morphologically very similar to P. subantarcticum and recently described as new from the Subantarctic, was also identified.

Keywords:

• Bacillariophyceae
• Planothidium
• molecular data
• morphology
• sinus or cavum
• new species

Acknowledgements

We gratefully acknowledge Eileen Cox for polishing our English, and Phil Novis, Bart Van de Vijver and Carlos Wetzel who provided comments and data on the taxonomy of Planothidium species which they had studied. The Mexican samples from 2014 were taken by DM under permit CONAPESCA PPF/DGOPA-149/14. Additionally, the Berlin Group acknowledges the help of Jana Bansemer for the retrieval of molecular data and of Monika Luchow at the SEM.

Supplemental data

Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/0269249X.2017.1312548.

Additional information

Funding

The work of MK and EG was partly supported by the Russian Science Foundation [14-14-00555]. The work of DM was supported by the Mexican Government through Consejo Nacional de Ciencia y Tecnologia (CONACYT) and Consejo de Ciencia y Tecnologia del Estado de Queretaro (CONCYTEQ). RJ, NA, DM, OS, WHK and JZ acknowledge funding by the Deutsche Forschungsgemeinschaft Grant INST 130/839-1 FUGG for SEM, by DFG Grant JA 874/8-1 for Names Registration, and by BMBF Grant 01LI1501E for GBOL-2 Diatoms.