As stony corals decline from heat and pollution, soft-bodied octocorals are taking over and fundamentally changing these vital habitats. Powered by unique algae partnerships, their resilience is helping scientists understand which species might endure warming seas — and what that means for the future of reefs.
Coral reefs, while only a small portion of the world’s oceans, offer enormous benefits to people and the planet. These ecosystems contain vast biodiversity, fix carbon on scales comparable to rainforests, provide coastal protection from storms, and supply massive economic benefits in the form of ecotourism, seafood production, and pharmaceutical discovery. However, as oceans warm due to climate change, many coral reefs are dying.
What happens during coral bleaching
Since the early 1980s, heat waves have become more frequent and have made ocean water warmer. This affects coral because it disrupts a crucial and mutually beneficial, or symbiotic, relationship between coral and microscopic single-celled algae known as zooxanthellae that occur at millions of cells per square inch of the coral’s tissue.
As a part of their relationship, the zooxanthellae use sunlight to produce nutrients, which they share with the coral. Heat stress disrupts this relationship causing corals to lose their symbionts (the zooxanthellae partner) and much of their brown pigmentation (the color of the zooxanthellae).
When this process happens, corals appear bleached, revealing their white skeleton as their tissues become more transparent. The loss of these symbionts often initiates mass mortality events through starvation and the spread of opportunistic diseases.
The rise of soft corals (octocorals)
Yet while many stony coral populations are on the decline, a distantly related coral is on the rise. The soft-bodied octocorals are proliferating in places vacated by reef building corals. These soft corals are a diverse and ancient group of tentacle-bearing animals that divide as they grow to produce large, interconnected colonies similar to their stony reef coral relatives.
Octocorals are defined by their signature eight-fold radial symmetry and include leather corals, sea fans, sea whips, sea pens, blue corals, and organ pipe corals.
How soft corals withstand stress
Unlike stony corals, most octocorals lack dense calcium carbonate skeletons and therefore do not build reefs. However, like stony corals, they also depend on zooxanthellae for survival and growth. While also experiencing stress during marine heat waves, octocorals seem better adapted to tolerate and recover from episodes of warming and even appear to tolerate turbid (murky) and nutrient-enriched waters from human pollution better than stony corals.
Tolerance to heat and high nutrients may explain why octocorals are thriving where stony corals are declining. For example, in the Caribbean Sea, many reefs are dominated by sea-fans and sea-whips. Not only this, but several octocorals from the Indo-Pacific and Red Sea have been unintentionally introduced to the western Atlantic and are rapidly proliferating, including coastal sites in Brazil, Venezuela, and Puerto Rico, where their unchecked growth is smothering stony corals and other endemic reef organisms. These shifts in community dominance toward octocorals appear to also be occurring on Indo-Pacific reef systems, but much less is known about these animals and the identity of their essential symbionts.
Investigating octocoral symbiosis
With this large knowledge gap in mind, and our laboratory’s expertise on the ecology and evolution of animal-zooxanthellae mutualisms, we endeavored to characterize the diversity, biogeography, and host specificity of octocoral zooxanthellae by examining samples collected across the Indo-Pacific (e.g. Great Barrier Reef, Thailand, Taiwan, Zanzibar, Palau) with a focus on octocorals in the family Sarcophytidae (the “toadstool” or “devil’s hand” leather corals), family Xeniidae (the pulsing and pom-pom corals), and the genus Rhytisma (the yellow-tinted “sulphur leather” corals).
By sequencing the DNA of the zooxanthellae inside these samples, we discovered that octocorals host symbiont species different from those found in stony corals. Furthermore, symbionts specific to the leather corals (Sarcophytidae) were different from those common to the pulsing pom-pom corals (xenids) and the sulphur leather corals (Rhytisma), indicating high ecological specialization inherent in these partnerships.
The efforts of this research also led to the formal description of five new symbiont species, some of which occur in octocorals across the Indo-Pacific; and importantly, these findings on ecology and diversity should facilitate future research and consistent scientific reporting with the use of formal species names.
What this means for the future of reefs
In stony corals, knowing the range of symbiont compatibility that a coral possesses helps predict how well colonies of a given species in a certain location can withstand heat stress. Since these soft corals harbor novel kinds of symbiont not found in reef-building corals, this difference may partially explain why certain symbiotic reef animals better resist or recover from bleaching and why they grow in abundance on reefs subjected to greater turbidity and nutrient pollution.
Yet, this research is only a first step. Hypotheses regarding differential tolerances to heat stress or pollution require verification with experimental testing. Ultimately, having a better understanding of the symbiont’s identity and host specificity facilitates future research on octocoral physiology and ecology.
Todd LaJeunesse is an IEE faculty member who focuses on the evolutionary ecology of mutualistic symbioses, mainly coral-dinoflagellate associations. He examines ecological, biogeographic, and phylogenetic patterns in order to deduce fundamental ecological and evolutionary processes involving microbial eukaryotes. Part of this research seeks to understand how coral communities globally are responding to climate warming.
Caleb Butler is a marine ecologist from Dallas, Texas, specializing in coral-symbiont associations. Caleb completed their bachelor's degree at the University of Texas at Arlington and recently defended their Ph.D. in the Huck Institute's Inter-College Ecology Program.
