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

ABSTRACT: The dinoflagellate genus Symbiodinium 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 Symbiodinium 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 Symbiodinium microadriaticum (sensu Trench & Blank, 1987) and Symbiodinium sp. associated predominantly with giant clams and Pacific Cassiopea jellyfish (comprising members of the ITS2 A3 lineage, sensu LaJeunesse 2001). Amphiesmal plate tabulations were formulated for the strain of S. microadriaticum (CCMP2464) used by Trench & Blank (1987) in their emended description, and two strains of type A3 (CCMP832 and rt-272) cultured from Indo-Pacific giant clams in the subfamily Tridacninae. The Kofoidian plate formula for type A3 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 S. microadriaticum, 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 Symbiodinium tridacnidorum sp. nov., a new Indo-Pacific species. The plate arrangement exhibited by S. microadriaticum appears to be more similar to the distantly related S. natans (also in Clade A). When the tabulations for all three Clade A species are compared with S. voratum (Clade E), the amount of morphological differentiation between species does not correspond to their degree of genetic divergence.