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Morphology and taxonomy of the planktonic diatom genus Chaetoceros Ehrenberg (Bacillariophyta)from the Adriatic Sea

The genus Chaetoceros Ehrenberg is one of the largest genera among marine planktonic diatoms comprising more than 170 described species. The species identification is notoriously difficult due to the large morphological variability within species, frequent occurrence of intermediate forms, and owing to the fact that some of the characters used for delineation can only be observed using electron microscopy. Although ca. 50 species has been recorded in the Adriatic Sea of which few taxa are of great ecological importance often dominating in the phytoplankton, there have not been any previous taxonomic investigations dealing specifically with the members of this genus. Therefore, the aim of present study is to look into the Chaetoceros diversity by analysing the laboratory clonal cultures and the field samples collected along the eastern Adriatic coast using light, transmission and scanning electron microscope. The acquired data were used for revision of the existent species descriptions and taxonomic interrelationships, with particular emphasis on comparison of the ultrastructural characters e.g. setae and of the vegetative cells as well as resting spores. A total number of 38 distinct taxa were recognized in the field material including 28 taxa identified from 43 cultured strains. Most of the taxa studied are characteristic for the temperate regions, however we recorded and studied in details some little-known species otherwise recorded only in the tropical seas such as Chaetoceros bacteriastroides and C. pseudodichaeta which were for the first time identified in the Adriatic Sea samples. The morphological analysis of the dominant species, except providing distinct novel ultrastructural details such as the absence of the spines on setae for C. vixvisibilis and thin silica wall structure covering the aperture between sibling cells in C. affinis, helped to clear previously common misidentifications as in the case C. contortus/C. compressus, C. decipiens/C. lorenzianus and C. lauderi/C. teres. The analysis showed that in all analysed material only first species from the giver pair could be identified. The accurate and complete diatom taxa descriptions obtained in this study will facilitate future correct species identification and improve the taxonomic resolution of this important genus on the local as well as global level which will in long term add information to the global biogeographical species distribution as well as contribute to the improvement of quality of the data on phytoplankton ecology in the future studies.

marine phytoplankton; diatoms

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