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

Clade DSymbiodiniumare 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 DSymbiodinium, factors linked to increasing abundances of clade D, and the benefits and drawbacks of associating with clade D for corals. We identify clade DSymbiodiniumas 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 DSymbiodiniumare 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 DSymbiodiniumplay in corals and discuss the utility of monitoring clade DSymbiodiniumas indicators of habitat degradation in coral reef ecosystems.