In studies of habitat-forming species, those that are not spatially dominant are often considered "non-primary" habitat and may be overlooked. This is despite the fact that minority habitat formers can provide critical complexity, food, and other services that underpin ecosystem biodiversity. Octocorals and anemones are found in marine and estuarine habitats across all climate zones. Despite their potentially important ecological roles, to date there have been few studies of their specific threats and stressors or attempts at their restoration. Here we review studies of the ecology of octocorals and anemones with a focus on threats and restoration. We identify many threats including habitat damage, collection and trade, disease, predation, pollution, and the most widespread-climate change. While evidence suggests that some octocorals and anemone populations may be more resilient to disturbances than stony corals because they often recruit and grow quickly, resilience is not guaranteed. Instead, resilience or susceptibility within this large group is likely to be site and species specific. We find that the loss of octocorals and anemones has been difficult to quantify as most species have no hard structures that remain following a mortality event. Only through long-term monitoring efforts have researchers been able to document change in these populations. Due to the increasing extent and severity of human impacts in marine ecosystems, restoration of habitat forming species is becoming increasingly necessary after disturbance events. To illustrate the challenges ahead for octocoral and anemone restoration, we present two examples of ongoing restoration efforts assessed against the International Standards for the Practice of Ecological Restoration. Restoration planning and implementation progress are documented for the Mediterranean red coral Corallium rubrum and the temperate Australian cauliflower soft coral, Dendronephthya australis. This review and the detailed case studies demonstrate that while some octocorals and anemones can provide resilient habitat within reef systems, a greater research focus on their ecology, threats, and restoration potential is urgently required.

Coral reefs are amongst the most biodiverse ecosystems on earth, and while stony corals create the foundational complexity of these ecosystems, octocorals and anemones contribute significantly to their biodiversity and function. Like stony corals, many octocorals contain Symbiodiniaceae endosymbionts and can bleach when temperatures exceed the species’ upper thermal limit. Here, we report octocoral bleaching susceptibility and resistance within the subtropical Lord Howe Island coral reef ecosystem during and after marine heatwaves in 2019. Octocoral and anemone surveys were conducted at multiple reef locations within the Lord Howe Island lagoon during, immediately after, and 7 months after the heatwaves. One octocoral species, Cladiella sp. 1, experienced bleaching and mortality, with some bleached colonies detaching from the reef structure during the heatwave (presumed dead). Those that remained attached to the benthos survived the event and recovered endosymbionts within 7 months of bleaching. Cladiella sp. 1 Symbiodiniaceae density (in cells per µg protein), chlorophyll a and c2 per µg protein, and photosynthetic efficiency were significantly lower in bleached colonies compared to unbleached colonies, while chlorophyll a and c2 per symbiont were higher. Interestingly, no other symbiotic octocoral species of the Lord Howe Island lagoonal reef bleached. Unbleached Xenia cf crassa colonies had higher Symbiodiniaceae and chlorophyll densities during the marine heatwave compared to other monitoring intervals, while Cladiella sp. 2 densities did not change substantially through time. Previous work on octocoral bleaching has focused primarily on gorgonian octocorals, while this study provides insight into bleaching variability in other octocoral groups. The study also provides further evidence that octocorals may be generally more resistant to bleaching than stony corals in many, but not all, reef ecosystems. Responses to marine heating events vary and should be assessed on a species by species basis.

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