Why would coral and coralline algae become free living? 

Coral and coralline algae adopt a free-living lifestyle for various ecological and evolutionary reasons, as evidenced by research across different marine environments. For corals, the phenomenon of becoming free-living, such as in the case of coralliths, allows them to create stable habitats in otherwise uninhabitable environments with poor substrates. This adaptation enables coral species to settle and survive on mobile substrates until they reach a critical mass that prevents further movement, facilitating secondary reef colonization by other species . Similarly, the free-living coral Fungia fungites demonstrates a capacity for asexual reproduction through budding, which contributes to its abundance and distribution in reef zones . Coralline algae, on the other hand, form free-living structures like rhodolith beds, which are crucial for marine biodiversity and carbonate production. These beds are built by free-living coralline algae, with their morphology playing a significant role in their physiology and potentially affecting their susceptibility to ocean acidification . The discovery of unique free-living geniculate coralline algae (GCA) beds in Brazil highlights the ecological significance of these formations in providing habitat, refuge, and grazing areas for juvenile fishes and invertebrates, thereby increasing carbon fixation . Moreover, the free-living stages of symbiotic algae, such as Symbiodinium, play a critical role in the resilience of coral reefs to environmental stressors. These free-living populations are essential for establishing symbiosis with many corals and for the recombination of host-symbiont associations recovering from stress . The dynamics between free-living Symbiodinium cells, hosts, and their habitat underscore the importance of these free-living stages for the health and recovery of coral reefs . In summary, the transition to a free-living state in both corals and coralline algae is driven by the need to adapt to environmental challenges, reproduce, and contribute to the structural complexity and biodiversity of marine ecosystems .

The free-living Symbiodinium reservoir and scleractinian coral symbiont acquisition

Not addressed in the paper.

Subterraniphyllum and free-living Neogoniolithon (coralline algae)from the Oligocene reef facies of Costa d’Ovada (Tertiary Piedmont Basin, Alessandria, NW Italy)

Coralline algae like Neogoniolithon raripunctatum and Subterraniphyllum thomasii may become free-living due to moderately exposed conditions in warm tropical or subtropical, clear, and very shallow waters near coastlines.

Reduced physiological performance in a free-living coralline alga induced by salmon faeces deposition

Coral and coralline algae become free-living to form maerl beds, accumulating and thriving independently, supporting diverse flora and fauna, with significant ecological importance in marine environments.

Free-living and symbiotic lifestyles of a thermotolerant coral endosymbiont display profoundly distinct transcriptomes under both stable and heat stress conditions.

Coral and coralline algae may become free-living due to distinct transcriptomic patterns tailored for processes like cell signaling, environment response, and motility, as observed in D. trenchii.

Calcification in free-living coralline algae is strongly influenced by morphology: Implications for susceptibility to ocean acidification

Free-living coralline algae and coral can become free-living to enhance biodiversity and carbonate production in marine ecosystems, as highlighted in the research paper.

Late detachment conceals serial budding by the free-living coral Fungia fungites in the Inner Gulf of Thailand

Coral and coralline algae become free-living due to detachment facilitated by localized calcium dissolution, enabling asexual reproduction through budding, as observed in Fungia fungites in the Inner Gulf of Thailand.

Acidification-induced cellular changes in Symbiodinium isolated from Mussismilia braziliensis.

Coral and coralline algae become free-living to enhance resilience to environmental stressors like ocean acidification, enabling survival outside coral cells and supporting coral health in changing conditions.

Distribution, abundance and life cycle of free-living Symbiodinium

Coral and coralline algae become free-living to establish symbiosis with new coral generations, ensuring resilience and recovery after major stress events like mass bleaching.

The potential for coral reef establishment through free-living stabilization

Corals and coralline algae may become free-living to create stable habitats in poor substrate environments through 'free-living stabilization,' allowing for secondary reef colonization by other coral species.

A unique free-living geniculate coralline algal bed formation

Coral and coralline algae become free-living to create unique structural formations, providing more space, refuge, and resources for associated organisms, influencing biodiversity and ecosystem richness.