<i>Azadinium caudatum</i> var. <i>margalefii</i>, a poorly known member of the toxigenic genus <i>Azadinium</i> (Dinophyceae)

<i>Azadinium caudatum</i> is a poorly known planktonic dinoflagellate, which mainly attracts attention due to the occurrence of many toxigenic species in the genus <i>Azadinium</i>. The availability of two Scottish isolates of <i>A. caudatum</i> var. <i>margalefii</i> allowed for the first time a detailed analysis of biological and physiological traits of the species, specifically on the potential presence of azaspiracid toxins (AZAs). With a mean overall swimming speed of 85 µm s⁻¹ and regularly interspersed high-speed jumps, <i>A. caudatum</i> var. <i>margalefii</i> exhibited a similar swimming behaviour to other species of the genus. Cells had a single, large, highly reticulated chloroplast with a typical pigment composition for peridinin-containing dinoflagellates. No pyrenoids were visible under light microscopy. A round to ovoid nucleus was centrally located. Cell division was by desmoschisis, i.e. the parent theca was shared by the daughter cells. Growth rate <i>µ</i> ranged from 0.07 to 0.32 day⁻¹ and was reduced at lower temperatures. With growth rate becoming light saturated at intensities of about 40 µE m⁻² s⁻¹ and a half-saturation light intensity of about 13 µE m⁻² s⁻¹, <i>A. caudatum</i> var. <i>margalefii</i> appears to be relatively low-light adapted. Neither strain produced any known AZAs in measureable amounts.