A distinctive stellate species of the genus Actinocyclus (Bacillariophyta) from Oligocene sediments in the central Pacific Ocean and its morphological significance

ABSTRACT

Actinocyclus asteriscus, a notable species with star-like hyaline rays on the valves discovered in Oligocene sediments from the central Pacific Ocean, has been examined in detail with light and scanning electron microscopy. Its unique appearance with complete hyaline rays helps the initial identification of diatom valves in the microscope. The taxonomic relationships between A. asteriscus and species belonging to six well-defined groups of Actinocyclus species are discussed with a special focus on the hyaline structure. On the basis of the geological time range and ecological habitat of selected Actinocyclus species, we hypothesize that A. asteriscus possesses the primary type of hyaline structure and that this feature subsequently degenerated in other species of the genus. In addition, quantitative data on the relationship between valve diameter and number of hyaline rays and the areolae density are discussed to address the divergence in intraspecific morphological features.

KEYWORDS:

• Actinocyclus
• Hyaline ray
• Late Oligocene
• Morphological analysis

ACKNOWLEDGEMENTS

We thank the China Ocean Sample Repository for providing sediment samples. We thank Dr. Hongfeng Lu and Peixin Lai (GMGS) for their help with SEM observation and imaging. We are also very grateful to Nadja Ognjanova-Rumenova and anonymous reviewers for their careful reading of our article and their insightful comments and suggestions.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0209) and the Project of China Geological Survey (No. 20190216).