<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

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.摘要 甲藻属虫黄藻（Symbiodinium）包含众多遗传上彼此独立、形态上呈隐存状态的谱系。然而近期研究显示，分别隶属于分支A与分支E的两个远缘虫黄藻未定种，其运动阶段（鞭毛体）中可见的板片式存在显著形态学差异。尽管分支A中已有多个正式描述的物种，但遗传学证据表明仍有大量类群尚未被表征。本研究聚焦该类群内亲缘关系较近的系统发育谱系，旨在探究形态差异能否与遗传、生态学证据相结合，用于新物种的描述。我们针对微扁虫黄藻（Symbiodinium microadriaticum，指Trench与Blank于1987年定义的类群）以及主要与砗磲、太平洋冠水母共生、隶属于ITS2 A3谱系（指LaJeunesse于2001年定义的类群）的虫黄藻未定种的培养样本与野外采集样本展开分析，在细胞核基因（内部转录间隔区（internal transcribed spacer，ITS）与大亚基核糖体DNA）、叶绿体基因（cp23S）及线粒体基因（cob）的序列中发现了稳定的遗传差异。我们为Trench与Blank（1987）修订描述中所用的微扁虫黄藻菌株CCMP2464，以及两株从印度-太平洋砗磲亚科砗磲中分离得到的A3型菌株CCMP832与rt-272，构建了表质板片排列模式。A3型的科福伊德板片式为：小 plate（x）、伸长表质膜囊泡（EAV）、5′、6a、8′′、9–11s、双行腰带、7′′′与3′′′′（偶见2′′′′）；该板片式与微扁虫黄藻不同，后者的板片式为x、EAV、4′、5a、8′′、9–13s、双行腰带、6′′′与2′′′′。基于形态学与遗传学比较结果，我们确立了新种砗磲共生虫黄藻（Symbiodinium tridacnidorum sp. nov.），该种为印度-太平洋特有物种。微扁虫黄藻的板片排列模式与远缘的分支A类群游动虫黄藻（S. natans）更为相似。将上述3种分支A物种的板片排列模式与分支E的 Voratum虫黄藻（S. voratum）进行比较后发现，物种间的形态分化程度与其遗传分化程度并不匹配。