Taxonomic relationship between two small-sized Chaetoceros species (Bacillariophyta): C. tenuissimus and C. salsugineus, and comparison with C. olympicus sp. nov. from Catalan coastal waters (NW Mediterranean)

ABSTRACT

The majority of species of the highly diverse genus Chaetoceros are described as chain-forming, although several species are described as strictly solitary (such as C. tenuissimus) or having an alternate solitary and a chain-forming phase during their life history (such as C. salsugineus). In this study, the diversity of small forms of Chaetoceros from the NW Mediterranean coastal waters was explored through the morphological and molecular characterization of four different strains belonging to two distinct species. Based on their morphology, three of the strains were identified as C. salsugineus (Takano, 1983). The SSU and LSU rDNA sequences represented the first available for well-characterized C. salsugineus strains and were 96.6–100% similar to publicly available C. tenuissimus (Meunier, 1913) sequences. Both species share the same morphological features, such as setae and ultrastructure of the valves, as well as the rimoportula characteristics. In addition, the morphology of the solitary form of C. salsugineus matched with that of C. tenuissimus. Here, we propose the two species as synonyms (the name C. tenuissimus prevailing as it has priority for this taxon), emend the original description and designate an epitype. The fourth strain was identified as C. olympicus sp. nov., a new species, which alternates solitary and chain-forming forms within its life history. The main differential characteristics of this species are the absence of rimoportula both in terminal and intercalary valves, the setae ultrastructure, which is thin and circular in cross-section with a few, slightly twisted, rows of small rectangular poroids and some spirally arranged spines, and the morphology of the resting spores, with primary valve vaulted and covered with short to medium-sized spines, and secondary valve smaller, rounded and smooth. A comparison of C. tenuissimus and C. olympicus with other species as well as information on their life cycle and ecology is also provided.

HIGHLIGHTS

• Description of a new Chaetoceros species: C. olympicus.

• Proposal that C. tenuissimus and C. salsugineus are synonyms.

• Importance of morphological variability in life cycles for species identification.

KEYWORDS:

• Chaetoceros tenuissimus
• Chaetoceros salsugineus
• ecology
• life cycle
• molecular
• morphology
• small-sized Chaetoceros

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Acknowledgements

We are grateful to J.M. Fortuño (Head of the ‘Electron and Optical Service’ of the Institut de Ciències del Mar (ICM-CSIC) for SEM assistance and SEM microphotography. Inorganic nutrient analyses were performed by M.I. Abad in coordination with E. Berdalet (Head of the Service) at the ‘Nutrient Analysis Service’ of the ICM-CSIC. We also thank Ll. Cros for the helpful advice on algae culturing and taxonomy and the people involved in the Barcelona samplings. We are also grateful to two anonymous reviewers for helpful comments on the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Author contributions

L. Arin: original concept, culture establishment, morphological observations, taxonomy, cell counts, drafting and editing the manuscript; A. Reñé: analysis of molecular data, taxonomy, drafting and editing the manuscript; R. Gallisai: analysis of molecular data, review and editing the manuscript; D. Sarno: taxonomy, review and editing the manuscript; E. Garcés: resources, review and editing the manuscript; M. Estrada: resources, review and editing the manuscript.

Additional information

Funding

This work was supported by MINECO Project COPAS ‘Understanding top-down control in coastal bloom-forming protists’ (CTM2017-86121-R), the ANIMA Project (CTM2015-65720-R), and institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) from the Spanish government.