Coral Reefs: A Reef Resilience Toolkit Module

Zooxanthellae Genetics

Symbiodinium is a genetically diverse group of dinoflagellates, including nine phylogenetic types, distinguished as clades A-I.1 The genetically distinct clades possess unique environmental, ecological and geographic variations, which influence the resilience of corals to elevated temperatures and bleaching. Reef-building corals can form associations with members of six of the nine clades, and some of these associations seem to be more flexible than others.

The reason why corals show differences in the composition of their symbiotic communities among locations or with ontogeny of the host is not clearly understood but is attributed to some combination of local adaption of the host to abiotic conditions, delayed onset of the clade specificity in juveniles, and genetic variation by both the host and Symbiodinium. Studies have revealed that the different clades of zooxanthellae have different susceptibilities to thermal and light stress.

Clade D Symbiodinium

Clade D Symbiodinium are thermally tolerant and increase the resistance of corals that harbor them to elevated SSTs.,2,3,4,5 Clade D Symbiodinium are found in a diverse range of coral species that encompass a variety of characteristics. Coral hosts include fast growing branching corals (e.g., Acropora, Stylophora, and Pocillopora), slow-growing massive species (e.g., Montastrea), encrusting forms (e.g., Montipora), and solitary corals (e.g., Fungia). Depending on the coral host, clade D Symbiodinium can be acquired vertically (passed from parent to coral offspring) or horizontally (acquired from ocean environment).10

Clade D Symbiodinium are present in higher abundances on some reefs than others, and these are often reefs exposed to relatively high levels of SST or local stressors such as sedimentation or reefs with a history of coral bleaching. For example, clade D Symbiodinium are more abundant in Acroporid corals from back-reef lagoons in American Samoa, where the SSTs reach higher maximum temperatures than the fore-reef environments, where Acropora primarily hosts clade C.6,7

A close-up of the coral Galaxea in Guam. The dark areas are the Zooxanthellae. Photo © Stacey Kilarski, Guam

Additionally, several studies have shown increases in the abundance of clade D Symbiodinium in corals following bleaching events.8,5 Given the range of challenging environments where clade D Symbiodinium are found naturally, the repopulation of recovering bleached coral hosts with clade D Symbiodinium is thought to represent a survival mechanism.10

Clade D can be useful for management because they are often found in increased abundance on coral reefs that are exposed to environmental stressors, and therefore can be a biological indicator of negative changes in coral health. Monitoring abundance of Symbiodinium in corals provide information on dynamics of coral health states.