Pseudo-cryptic speciation in Braarudosphaera bigelowii (Gran and Braarud) Deflandre

Partial SSU rDNA sequences were obtained from 13 naturally collected cells of Braarudosphaera bigelowii obtained from various parts of seas surrounding Japan. Together with the two previously reported sequences, 15 specimens were classified into five SSU rDNA Genotypes I–V. Based on the side length of the pentaliths forming the coccosphere, these specimens were also classified into three size-morphotypes; Intermediate form-A, Intermediate form-B, and Large form. Genotypes of B. bigelowii were well correlated with size-morphotypes but not with sampling area. This result indicates that size differences in B. bigelowii are the results of speciation and not of intra-population variety. Therefore, Genotypes I and II (Intermediate form-A) and Genotypes IV and V (Large form) are regarded as pseudo-cryptic sibling species of typical B. bigelowii (Genotype III, Intermediate form-B), which corresponds to the original description of the species. From the SSU rDNA sequences, it is evident that Genotype V arose from Genotype IV, and Genotype IV originated from Genotype III. The specimens of Genotypes III–V showed size increase of pentaliths in accordance with their branching order. The consistency in the relationship between genotypes and size-morphotypes of living B. bigelowii-complex observed in this study suggests that inconsistency of size range of ‘B. bigelowii’ pentaliths among different geological ages reported by palaeontological studies stems from additional pseudo-cryptic speciation in the lineage of ‘B. bigelowii’ in the geological past.