Symbiodinium necroappetens sp nov (Dinophyceae): an opportunist 'zooxanthella' found in bleached and diseased tissues of Caribbean reef corals

We describe Symbiodinium necroappetens sp. nov. found predominantly in diseased or thermally damaged tissues in some reef corals of the Greater Caribbean. Small, albeit fixed, differences in the ribosomal DNA (ITS2 and LSU) and cytochrome b (cob) indicate that S. necroappetens is evolutionarily separate, but closely related to S. microadriaticum (members of Clade A). However, haplotype sequences of the non-coding region of the psbA minicircle are highly divergent, signifying that the degree of genetic divergence between these sibling lineages is far greater than indicated by changes in rDNA. Small morphological differences also support the delineation of this species. The Kofoidian plate formula for S. necroappetens (x-plate, EAV, 4, 5a, 8, 9-11s, 21c, 6, 2, PE) is generally the same as described for S. microadriaticum, except for the number of cingulum plates (21 vs 22-24), but plate shapes and configurations differ. Nuclear and mitochondrial volumes calculated from ultrastructural serial sections (published previously) also distinguish it from S. microadriaticum and S. pilosum. There are significant physiological differences in the response of S. necroappetens to high pCO(2) and thermal stress when compared with S. microadriaticum, indicating that large functional differences exist even among closely related species. This species appears to be necrotrophic rather than mutualistic. Before it was recognized as a distinct entity, reports on its ecology contributed to the supposition that members of Clade A Symbiodinium were opportunistic. Available evidence indicates that S. necroappetens exists at low environmental background levels, but may proliferate' selectively in artificial growth media, or emerge opportunistically in bleached coral colonies during early recovery from severe stress, or in diseased necrotic tissues, especially in colonies of the Orbicella (formerly Monstastraea) annularis complex. However, S. necroappetens fails to persist at detectable levels as populations of the typical symbiont recover. The description of this species raises awareness of the broad functional and ecological diversity exhibited by members of this large dinoflagellate genus.