Abstract Multiple members of the phylum Cnidaria (e.g., corals, octocorals, and sea anemones) and other organisms such as mollusks, foraminiferans, and sponges associate with unicellular dinoflagellates belonging to the family Symbiodiniaceae. These symbioses are often obligatory, and for shallow coral reefs, form the foundation of the ecosystem. This chapter presents the current knowledge of Symbiodiniaceae genetic distinction and the ramifications of different Symbiodiniaceae genotypes on the host- Symbiodiniaceae entity, the holobiont. Since the early 1990s when molecular techniques using polymerase chain reaction (PCR) enabled rapid resolution of Symbiodiniaceae, both within and between host species, knowledge on Symbiodiniaceae diversity has grown exponentially. Subsequently, application of multiple molecular techniques and genetic markers enabled the analysis of Symbiodiniaceae diversity from broad genera (formerly cladal) groupings to the individual genotype. Previous lack of a standard Symbiodiniaceae nomenclature, however, led to naming redundancies and utilization of the same terminology to discuss different levels of taxonomic resolution. This ambiguity has now been addressed. Knowledge of Symbiodiniaceae genetic diversity enables understanding and puts in context the host- Symbiodiniaceae genotypic combination. Deciphering the holobiont’s ecology, including the holobiont’s responses to environmental conditions brought about by global climate change, requires knowledge of Symbiodiniaceae identity, as does contemplating the applicability of the deep reef refugia hypothesis. This review accentuates the current meager knowledge of Symbiodiniaceae genetic diversity in mesophotic coral ecosystems in general, which is confined to scleractinian and antipatharian coral hosts, and the lack of data on Symbiodiniaceae genotypes in other symbioses.

Depth range
0- 200 m

Mesophotic “mentions”
53 x (total of 5024 words)

Classification
* Focused on 'mesophotic coral ecosystem'

Fields
Climate Change
Connectivity
Disturbances
Ecology
Management and Conservation
Molecular ecology
Physiology
Reproduction

Focusgroups
Symbiodinium (zooxanthellae)