Long-term species and morphological changes for Stephanodiscus in Lake Biwa for the last 400 ka

Abstract

In order to explore the origin and evolution of an endemic diatom species, Stephanodiscus suzukii Tuji & Kociolek, living in Lake Biwa, species and morphological changes of Stephanodiscus spp. for the last 400 ka were elucidated using samples from a 140 m-long core. The first appearance of S. suzukii like-species was approximately 390 ka, close to the timing of the onset of Lake Biwa basin formation. However, their dominance started at about 170 ka. Since their first appearance, changes in dominance from S. sp. D, to S. suzukii can be recognized. During periods of transition, transitional forms of the two species were present in the populations. Morphological changes are apparent in the location of interrupted areolae fascicles, the number of areolae per fascicle and number of fascicles per 10 lam near the margin. The existence of individuals with transitional forms and slow morphological changes during the transition period suggest that S. sp. D, and S. suzukii are identical species. The changes in morphology of S. suzukii are related to shifts in global climates and local crustal movements, suggesting they are survival strategies against severe climate and limnetic changes.