Clade D Symbiodinium in Scleractinian Corals: A “Nugget” of Hope, a Selfish Opportunist, an Ominous Sign, or All of the Above?

Clade D Symbiodinium are thermally tolerant coral endosymbionts that confer resistance to elevated sea surface temperature and bleaching to the host. The union between corals and clade D is thus important to management and coral conservation. Here, we review the diversity and biogeography of clade D Symbiodinium, factors linked to increasing abundances of clade D, and the benefits and drawbacks of associating with clade D for corals. We identify clade D Symbiodinium as uncommon pandemically distributed generalists found in higher abundances on reefs exposed to challenging sea surface temperatures and local stressors or with a history of bleaching. This distribution suggests that clade D Symbiodinium are mostly opportunistic endosymbionts, whereby they outcompete and replace optimal symbionts in health-compromised corals. We conclude by identifying research gaps that limit our understanding of the adaptive role clade D Symbiodinium play in corals and discuss the utility of monitoring clade D Symbiodinium as indicators of habitat degradation in coral reef ecosystems.