Adding more taxa to the Cocconeis placentula group (Bacillariophyta): two new species from streams in biodiversity hotspots

Cocconeis ectorii and C. azteca are described as new species based on unialgal strains, cultivated from samples collected in streams of Mexico, California, and Germany. For C. ectorii, strains from Mayotte Island and Spain were re -analysed. Although these two new species belong morphologically within the C. placentula group, genetic distances for the nuclear -encoded 18S -V4 and the plastid gene rbcL show that these species are distinct from other species with available sequencing data. Morphologically, these two taxa can be distinguished from similar species by a combination of characters such as length, width, striae and areolae count, size of hyaline rim, number of submarginal areolae, by closed or open valvocopulae of both valves and their additional features fimbriation and smoothness. These characters are difficult to assess in environmental samples containing several taxa from this group especially when only small specimens are identified with light microscopy. Despite the disjunct origin of the 13 analysed C. ectorii strains, encompassing different climatic zones, their diagnostic morphological features are broad but stable, and share identical sequences for the two markers, indicative of a cosmopolitan distribution. Lastly, we provide occurrence notes for freshwater species of the placentuloid group based on eDNA metabarcoding data from subpolar, temperate, and tropical zones, further supporting the global distribution of C. ectorii and the probable endemic occurrence of C. azteca in North America.