<i>Symbiodinium tridacnidorum</i> sp. nov., a dinoflagellate common to Indo-Pacific giant clams, and a revised morphological description of <i>Symbiodinium microadriaticum</i> Freudenthal, emended Trench & Blank

<div><p>ABSTRACT</p><p>The dinoflagellate genus <i>Symbiodinium</i> contains numerous genetically distinct lineages that appear ‘morphologically cryptic’. However, detailed morphological assessments of plate formulae visible in the motile phase (mastigote) of two distantly related <i>Symbiodinium</i> spp., representing Clades ‘A’ and ‘E’, were recently shown to be different. While there are several formally described species in Clade A, genetic evidence indicates that many more remain uncharacterized. We focused on closely related phylogenetic lineages within this group to examine whether differences in morphology can be used together with genetic and ecological evidence to describe new species. We found fixed differences in nuclear (internal transcribed spacer (ITS) and large subunit rDNA), chloroplast (cp23S) and mitochondrial (cob) gene sequences from cultured and field-collected samples of <i>Symbiodinium microadriaticum</i> (<i>sensu</i> Trench & Blank, 1987) and <i>Symbiodinium</i> sp. associated predominantly with giant clams and Pacific <i>Cassiopea</i> jellyfish (comprising members of the ITS2 <i>A3</i> lineage, <i>sensu</i> LaJeunesse 2001). Amphiesmal plate tabulations were formulated for the strain of <i>S. microadriaticum</i> (CCMP2464) used by Trench & Blank (1987) in their emended description, and two strains of type <i>A3</i> (CCMP832 and rt-272) cultured from Indo-Pacific giant clams in the subfamily Tridacninae. The Kofoidian plate formula for type <i>A3</i> consists of the small plate (x), the elongated amphiesmal vesicle (EAV), 5′, 6a, 8′′, 9–11s, a 2-row cingulum, 7′′′ and 3′′′′ (and occasionally 2′′′′), and is different from <i>S. microadriaticum</i>, which has a plate formula of x, EAV, 4′, 5a, 8′′, 9–13s, a 2-row cingulum, 6′′′ and 2′′′′. Based on morphological and genetic comparisons, we recognized <i>Symbiodinium tridacnidorum</i> sp. nov., a new Indo-Pacific species. The plate arrangement exhibited by <i>S. microadriaticum</i> appears to be more similar to the distantly related <i>S. natans</i> (also in Clade A). When the tabulations for all three Clade A species are compared with <i>S. voratum</i> (Clade E), the amount of morphological differentiation between species does not correspond to their degree of genetic divergence.</p></div>