Cladocopium infistulum sp. nov. (Dinophyceae), a thermally tolerant dinoflagellate symbiotic with giant clams from the western Pacific Ocean

Members of the genus Cladocopium are the most ecologically dominant and geographically widespread group of mutualistic dinoflagellates. The addition of species descriptions to this newly erected genus should stimulate scientific investigations and communication about the biology of these important symbionts. Here we combine morphology and genetic evidence with ecology to formally describe Cladocopium infistulum sp. nov. An emended description for C. goreaui is also provided. Both are among the few species of Cladocopium capable of growing in culture medium. The cells of C. infistulum are significantly smaller in size and differ in amphiesmal plate tabulation (morphology of the motile cell) from C. goreaui. Both appear to lack knob ornamentation typical of the elongate amphiesmal vesicle (EAV) in other Symbiodiniaceae. Phylogenetic analyses of nucleotide sequences from nuclear ribosomal, chloroplast, and mitochondrial genes differentiate C. infistulum from other Cladocopium spp. This new species is ecologically specialised, and known to occur only at high densities in digestive diverticula–a complex system of capillary-like hollow tubes that ramify the siphonal mantles of giant clams (Tridacnidae). Clams living in predominantly warm and shallow inshore reefal environments harbour this symbiont; and its biogeographic range appears restricted to the equatorial western Pacific Ocean. This new species description emphasises the need to formally recognise the high species diversity and corresponding physiological and ecological breadth that encompass Cladocopium.