As coral reefs face threats, UB scientists study the future of soft corals

Lasker: When I started studying reefs in the 1970s, we were all focused on complex and fascinating questions about how reefs work. The role of corals, fishes, hurricanes, sea urchins and other organisms were all being studied in systems that seemed to have been around ‘forever’ and which we expected would continue ‘forever.’

While some researchers were already raising the alarm about the effects humans were having, many, including me, thought of those as concerns for specific places with especially large human populations or especially uncaring approaches to using reefs. 

We have steadily seen the effects of humans spread through all of the world’s oceans, and the effects of ocean warming pays no attention to local policies. Now it is the rare scientist who does not have to include our altered environments in their research.

Lasker: When people hear the word coral, they usually think of stony corals. Those are corals that produce hard skeletons. Stony corals — called scleractinian corals in the vernacular of researchers — create the framework of the reef.

Soft corals, also known as octocorals, are the sea fans and sea plumes one sees waving to and fro in videos of reefs. Their name, octocorals, comes from each polyp having eight tentacles. Like their scleractinian cousins, they create three-dimensional structure on the reef, which is used by fishes and other small organisms. Unlike their scleractinian cousins, they do not have a solid skeleton, and when they die they break down into sand.

Octocorals have always been present on reefs, but as hard corals have steadily declined in abundance, the octocorals have increasing importance on reefs. And in some places, octocorals, unlike the hard corals, have actually increased in abundance.

Stony corals, also called hard corals, have been in decline for at least the last 50 years, and sadly, many reefs are only a pale shadow of the reefs of 50 years ago.

Octocorals have been more resilient to stresses that have killed stony corals, and some reefs have transitioned from a mix of hard corals and octocorals to predominantly octocorals. The soft corals’ upright, tree-like structure creates a “forest” that provides many, but not all, of the ecosystem services that hard corals provide.

We have been studying this transition with the goal of understanding why octocorals have been resilient and the important question of whether we can expect that to continue.

The first thing to understand is that coral reefs have always been affected by hurricanes, just as fire has been an important component of the dynamics of forests. Historically, hurricanes have caused damage, which over the course of years and decades reefs recover from. The difference now is that stony corals have been reduced to such low numbers that they do not recover.

What we discovered in the Virgin Islands is that while octocorals were adversely affected at our study sites, the damage was not as great as we feared and, more importantly, the following year we saw the development of new colonies which with time should lead to the recovery of the octocorals.

Reef corals and octocorals form a symbiosis with single-celled algae that live in the coral tissue. These algal symbionts, in the family Symbiodiniaceae, use energy from the sun to produce nutrients that are passed to the coral, and the coral in return provides the algal symbionts with nitrogen, CO2 and a safe place to live. This symbiosis is a true mutualism where both partners benefit.

Much of the normal coloration of corals and octocorals is due to the brownish algal symbionts that they harbor. Under periods of stress, such as elevated temperatures, the stony corals and octocorals may lose the algal symbionts on which they depend. Then the coral appears white, and this is called coral bleaching.

We have found that, in many cases, octocorals do not bleach as readily as stony corals, and if they do bleach, they generally recover. Given that there are many species of symbiodinian algal symbionts which have different physiologies, we sought to determine if the symbionts harbored by Caribbean octocorals were more thermotolerant. 

Our laboratory studies demonstrated that the symbiont types that are found in Caribbean octocorals can grow at temperatures where many stony corals exhibit bleaching. This suggests that at least some of the resilience seen in octocorals may be due to this symbiosis.

Lasker: This is the big, and unknown, question. If conditions continue as they are, octocoral forests may persist. They will not build the reef the way stony corals have, and in the long run that will lead to changes on the reef. 

Reef scientists refer to ‘flattening of the reef,’ which occurs as the dead skeletons of stony corals erode. However, in the short term, octocoral forests will provide habitat for fishes and other organisms, and if conditions improve, their effects might even facilitate recovery of stony corals. 

However, that requires a big improvement in environmental conditions. If environmental conditions continue to deteriorate due to warming sea temperatures, overfishing, onshore land use policies and other anthropogenic effects, then octocorals too will suffer.

Lasker: If humans do not reverse CO2 emissions and eliminate other stressors to reefs, then the fate of reefs is rather bleak. Some researchers are working on finding and propagating more resistant corals, but that, too, requires us to stop the decline in environmental conditions.

We cannot simply turn back the clock and recreate the reefs of 50 years ago, but we may be able to set the stage for recovery if we can reverse CO2 emissions, eliminate overfishing and adopt land use policies that will not further degrade reefs.