**4. Conclusions**

The newly proposed toxic species *C. cf. verruculosa* contains a new species-specific pigment, moraxanthin (**1**), whose structure was established as (3S,5 *R*,7 *R*,3'*S*,5' *R*,6'*S*)- 3-acetoxy-5',6'-epoxy-19'-(hexanoyloxy)-6,7-didehydro-5,6,5',6'-tetrahydro-Ά,Άcarotene-5,3'-diol, *i.e.*, 3- *O*-acetyl-19'- *O*-hexanoylvaucheriaxanthin. Two esterified forms of vaucheriaxanthin have been described, namely the 3- *O*-acetyl-19'- *O*octanoate and the 3- *O*-acetyl-19'- *O*-decanoate derivatives [47]. However, none of them contains the hexanoyl residue present in moraxanthin, which therefore can be easily distinguished from these known compounds on the basis of the HPLC retention time. 

New harmful species have been identified and the taxonomy of other species has been revised [48]. It is usually accepted that the routine identification of phytoplankton for monitoring studies in estuaries and coastal waters requires additional methods other than traditional microscopy, which can underestimate some taxonomic groups containing fragile or poorly differentiated small cells. In conjunction with microscopy, pigment separation using HPLC has become a more widely applied method for estimating and characterizing phytoplankton biomass and community structure [6–8]. 

However, algal pigments usually show complex overlapping patterns with different taxa, offering only a few unambiguous markers. In our case, *C. cf. verruculosa* may be readily identified in natural samples by means of HPLC chromatograms due to the distinct peak corresponding to the species-specific pigment moraxanthin described here. 
