Understanding the life cycle of coral is key to preserving our precious underwater worlds.
The Great Barrier Reef has been impacted by three mass bleaching events in the last five years. Foremost, it is being challenged by climate change, particularly ocean warming. There are still areas of the reef that are doing well and demonstrating quite a high level of resilience, but other areas are not. As the climate continues to warm, we predict that these bleaching events will continue to become more frequent and more severe, and this raises concerns about the reef’s long-term outlook.
Most corals already live close to what we call their upper thermal limit – the upper temperature of what they can tolerate. When seawater temperatures rise just a degree or so above that upper limit, it begins to cause stress and damage to the coral.
Corals also have symbiotic algae called zooxanthellae that live inside their tissues. When a higher water temperature is combined with a high irradiance stress on those days when it’s very calm and very sunny, those two factors together are detrimental to the algae that live in the coral tissues, and it causes a breakdown of that symbiosis. That’s what leads to coral bleaching – and if the water is hot enough, it impacts the coral animal tissue as well.
That’s the mechanism by which temperature causes damage to the corals. If that persists for long enough those corals will often die. And of course, having fewer corals left on a reef impacts the ability of that reef to recover.
Many people hear the word coral and think of colourful rocks. It’s important to understand that these are colonial animals. Hundreds or thousands of individual coral polyps make up the larger colony. Through time, as the animal grows, it lays down layers of limestone and that’s what builds the structure of the reef. The living part of the coral is actually the thin veneer of tissue covering the outside of what looks like rock.
Since childhood, I’ve always wanted to immerse myself in the underwater world. I grew up in the state of Maryland, on the east coast of the USA, and spent quite a lot of time in the summers on the Chesapeake Bay. It’s certainly not tropical but I found it fascinating and enjoyed exploring my environment. Then I did a bit of travelling to some tropical locations, and I fell in love with reefs. They are so otherworldly, with all their incredible diversity and beauty and colour. They really are quite alien. When you dive below the surface, all of the noise from the world above is quietened, and it allows you to be completely immersed in an entirely different environment that is so fascinating, with so many behaviours and life forms to observe.
I’ve been captivated by the connectedness of the reef ecosystem – how these organisms all rely on one another and work cohesively together. Of course, the flashy beautiful fish and charismatic creatures are attractive, but the corals have really enthralled me.
I realised early in my studies that corals are the foundational species of the reef ecosystem. They are the giant sequoia, if you like – the massive trees that build the “forest” of the reef. I became very interested in understanding how the system works – how it changes, recovers and maintains its communities – and also wanted to ensure that it will be maintained for future generations.
The more that I learned about these ecosystems, the more I saw the challenges that they faced. Many of the reefs in the Caribbean are degraded – I saw how they have been ravaged by diseases and bleaching in the last 15 years. Understanding the drivers of coral diseases became the focus of my dissertation work – and really prompted me to start thinking about how to apply my knowledge and skills to develop ways to help.
After my dissertation work in the Caribbean, I saw an opportunity for a postdoctoral position at the Australian Institute of Marine Science (AIMS) here in Australia. It’s been a dream to come here and study the corals of the Great Barrier Reef.
One of the key things that we’re working on is trying to overcome the bottleneck in the survival of corals – that early life history stage when the coral larvae have just settled onto the reef. It’s likely that fewer than one in a thousand survives to adulthood, maybe less. In a joint reef resilience project with BHP in Woppaburra Sea Country (the Keppel Islands) I’m working towards identifying ways to overcome that bottleneck by reducing the high mortality during a coral’s first year of life. At a broader scale, we’re really trying to nail down the know-how for generating corals reliably and consistently at scale.
Corals have a unique way of reproducing – at least from a human perspective. Obviously, they are attached to the sea floor. So, corals can’t go out and find mates. What most corals have evolved to do is synchronise the spawning of their gametes. They release their eggs and sperm into the sea in a highly synchronous event that only happens once a year for most species.
These are animals, remember, and they reproduce like animals, with eggs and sperm released into the water. When the corals on the reef are healthy and densely populated, those eggs and sperm float and then mix at the surface of the ocean – the eggs become fertilised by sperm, and those fertilised eggs develop in the sea. Over the course of a few days to a few weeks, they develop into microscopic larvae that are less than a millimetre in size. Those larvae get moved around by currents, and hopefully get taken to a reef somewhere, where they’ll settle down, attach to the sea floor, and then grow into an adult coral.
A single adult coral can release thousands to millions of eggs and sperm. They synchronise this release using a suite of cues from the environment, based on the lunar cycle, the tide and the time of sunset, down to the minute.
Once they settle onto the reef they metamorphose into a single, tiny polyp. Over time, that single polyp divides, then divides again, and grows into a larger colony. So most colonies of corals have grown from a single tiny larva that settled into one polyp that grew over many years.
At this time of year, the majority of the species of corals that live on the Great Barrier Reef will spawn. It’s interesting because they all spawn around the full moon, over several days, but they’re highly synchronous within a species. One species might spawn at 10 minutes after sunset. And then the next species will spawn 20 minutes after sunset. And then the next 30 minutes after, so that increases the likelihood that they’ll be able to fertilise eggs of the same species right around the same time.
It’s not an exact science but we’ve become pretty good at predicting those times, and we expect them to go in a certain order.
In general, the corals that we collect for the research we do are synchronised to their natural cycle, and we let them spawn naturally. But there’s a lot of interest in the research community at the moment in manipulating these spawning times. By adjusting the day length and the solar and lunar cycles, it will allow us to have a broader window of opportunity to do our work that is currently constrained to once per year.
In my research, many of the corals that are generated from spawning go back out onto the reef. We then track how those corals perform through time and across different reef sites and environments.
When a reef is supplied with trillions of larvae each year, then that reef can usually recover on its own. But problems exist when reefs are not getting adequate supplies of larvae because they don’t have adult populations producing them; the reefs may have been degraded, and there aren’t enough individuals spawning to generate the larvae required. Bleaching can also impact a coral’s ability to spawn. If a coral has severely bleached, even if it doesn’t die, it often won’t spawn, or if it does spawn, the eggs and sperm are of poor quality.
The newest research has suggested that those latent effects of bleaching can persist for several years. While it might seem that the coral has recovered, there can be lingering impacts on their ability to reproduce, which affects the recovery of the reef.
What we can do is go out and identify where the adult corals are located, put them together, spawn them, and increase the odds that each larva that’s developed in that process can then settle and survive. That overcomes the bottleneck that would be found naturally that limits the recovery potential of the reefs.
The tools that we have currently are insufficient to address the scale of coral decline globally. At AIMS, I’m involved in two big programs focused on developing new and innovative methods to improve the resilience of coral reefs affected by climate change: the Reef Restoration and Adaptation Program and the Australian Coral Reef Resilience Initiative. My hope is that we can develop a toolkit of strategies to implement on reefs that aren’t recovering naturally and that require accelerated adaptation to ocean warming, while we work to reduce greenhouse gas emissions.
Dr Carly Randall
Dr Carly Randall is a marine scientist with the Australian Institute of Marine Science specialising in coral ecology and reproductive biology. She is currently investigating drivers of post-settlement coral mortality to improve coral restoration methodologies.