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

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.