Differential cell size reduction of two sympatric Epithemia (Bacillariophyta) taxa in Nakaikemi Wetland, Japan

We observed contrasting temporal changes in cell size between two Epithemia taxa. The life cycle of diatoms is closely linked to their cell size, with a gradual decrease during the vegetative stage and a recovery through sexual reproduction. Sexual reproduction is triggered when cells become smaller than a species-specific size threshold and receive species-specific environmental cues. Few studies have documented the life cycles and frequency of sexual reproduction in diatoms, combining field observations and laboratory culture. We collected two Epithemia taxa, Epithemia gibba var. ventricosa and Epithemia sp. from a pond in Nakaikemi Wetland, and measured their valve lengths monthly for almost 3 years. Additionally, we established cultures of both taxa to examine the rates of cell size reduction, which affect the duration of the vegetative stage. In the field, E. gibba var. ventricosa exhibited a wide range of cell sizes and signs of size recovery. Furthermore, the cultures of this taxon showed a clear decrease in cell size through division, suggesting that sexual reproduction is required for size recovery. On the other hand, Epithemia sp. showed no obvious change in cell size both in the field and in the laboratory. This suggests that either sexual reproduction is not necessary for this taxon to maintain its population or their life cycle extends over several years to several decades. Thus, our findings reveal congeneric taxa sharing a common habitat and substratum, yet exhibiting distinct sexual and asexual strategies. Furthermore, we question the validity of taxonomic classifications previously assigned to these diatoms based on the range of morphological characteristics observed in the present study.