This piece is written in collaboration with Reef Life Restoration and Foundation, an organization using breakthrough nanotechnology to restore damaged reefs and regenerate coral colonies, creating homes for the ocean’s homeless.
Coral reef ecosystems are like underwater cities—bustling hubs of activity where thousands of marine creatures find shelter, food and protection. Colourful coral structures, living animals themselves, form the intricate buildings of these fantastical metropolises and, much like in our human cities, critters of all shapes and sizes roam the deep blue streets, interacting with one another, carrying out their business, commuting to and fro and building a home for themselves.
Despite only covering a tiny fraction of the ocean, coral reefs support almost a quarter of all marine life. Everything from sea turtles and sponges to clownfish and crustaceans rely on the reef and its resources and, in turn, each play an important role in creating a healthy and productive ecosystem. By providing a source of food and safety for such a diverse range of life forms, coral reefs are able to maintain carefully balanced relationships between all marine inhabitants and are irreplaceable in conserving the diversity and abundance of our oceans. These colourful ecosystems are also incredibly valuable to us humans (we are often just not very good at showing our gratitude). They provide nutrition and resources for local communities and revenue for small-scale fisheries and commercial fleets, act as a natural barrier that protects shorelines from storms and surges, and are a tourism hotspot, often serving as the backbone of their local economies.
However, current environmental conservation efforts are biased towards creatures that are most relatable to us humans, namely the mammals. Polar bears, orangutans, and elephants are all in need of our protection but the alien world beneath the waves deserves our love too. Coral reefs are home to a myriad of different plants and animals, and it is time we shine the spotlight on some of these lesser-known organisms, to understand the important role these underdogs play in the health of the ocean and the planet. Let’s dive in.
Sponges: the planet's most primitive animals keep coral reefs alive
Photo by Ellen Cuylaerts
Ever since Darwin’s early observations of coral reefs, scientists have been perplexed by the mere existence of these vibrant underwater ecosystems. How can one of the world’s most productive and diverse communities prosper in such a nutrient-barren landscape? Sponges, it turns out, are the answer.
The unsung heroes of the reef, marine sponges are among the oldest known multicellular organisms on earth, with fossil records dating back to 580 million years ago. These fascinating animals come in all colours, shapes and sizes, with thousands of different species working in symbiosis with the coral to facilitate a healthy reef. Sponges are filter feeders, pumping water through their bodies to feast on the dissolved organic matter (DOM) released by corals and algae. DOM is the largest source of energy produced on the reef but, in this dissolved state, it is impossible for most reef fauna to ingest. This is where sponges come in. After consuming the DOM, the energy and nutrients the sponge receives are quickly recycled as it sheds its old filter cells. This detritus is then gobbled up by particle-feeding critters such as snails, hermit crabs and marine worms who are, in turn, eaten by the reef’s larger inhabitants, creating a sponge loop that nourishes the entire ecosystem.
The rate at which sponges are able to recycle this DOM is no small feat. It has long been known that bacteria play a similar role, with the microbial loop providing sustenance to higher trophic levels throughout the ocean. But the same amount of water that bacteria filters in 30 days, sponges are able to pump through in an outstanding 30 minutes. The humble sponge also has a rapid cell replacement cycle, with one particular species, the Halisarca caerulea, able to produce new filter cells every five to six hours—the fastest cell turnover rate seen in any multicellular organism. This highly efficient sponge loop stops DOM from being lost to the waters of the open ocean, keeping these resources cycling within the reef ecosystem to produce a thriving community.
Algae: the organisms said to absorb more carbon dioxide than trees
When we hear the name ‘algae’, thoughts of slimy green goop probably flood our brain. This usually results in an involuntary nose wrinkle, instead of inspiring a compassionate movement for their protection. But algae are an incredibly diverse and beautiful group of organisms, and in ocean ecology, they have a very important role to play.
Algae can be broadly categorised into three groups: turf algae, crustose coralline algae and macroalgae. In a healthy ecosystem, turf algae serve as a decadent meal for reef grazers, quickly converting the sun’s energy into a rich source of nutrients. The crustose coralline variety also serves a unique and critical function on the reef; but before we explore their lives we must take a quick detour, and venture beyond the coral confines of the underwater metropolis, to understand the important role that algae play in the health of the entire planet.
Forests of kelp bloom in cold, nutrient-dense water, anchored to the rocky seafloor as their arms stretch up to the nourishing sun. While coral reefs and seaweed forests are physically separated, the landscape ecology of the ocean is heavily interlinked, and they depend upon each other in numerous ways. Both provide a rich habitat to marine creatures, who often travel between the two ecosystems throughout their lives, engaging them in an intricate web of nutrient sharing. These towering canopies of kelp also absorb huge amounts of carbon dioxide as they grow—and they grow fast, increasing in size by 30-60 times the rate of terrestrial plants. As bits of kelp break off and drift out to sea they carry the stored carbon with them, eventually sinking down, deep down, to the ocean floor. Here the carbon is removed from the atmosphere completely, sequestered for thousands of years. Coastal ecosystems, such as kelp forests, mangroves and seagrass beds, are highly efficient carbon sinks, sequestering up to 20 times more carbon per acre than their land-based equivalent. Seaweed aquaculture is particularly special, due to the way that it carries carbon all the way down to the ocean floor. In terrestrial forests, the carbon stored in tree biomass is vulnerable to re-release through deforestation, and the carbon captured by mangroves and seagrass, which is buried in their rich soil, is at risk of being disturbed by human activity, runoff and storms.
The rapid growth rate of seaweed aquaculture, combined with the permanence of their carbon sequestration, makes these underwater forests an ideal investment. Seaweed farms are already proving to be a successful industry, with the harvested crop offering even more opportunities for climate change mitigation as it can be used for biofuel production and as a replacement for synthetic fertilizer. In order to combat climate change, we must both lower the amount of carbon we are pumping into the atmosphere and remove the excess of what is already there. By supporting or restoring ecosystems that naturally sequester carbon, we can work with nature to sustainably amplify their effect.
Crustose coralline algae: the foundation that facilitates growth and the armour that protects
Photo by Anderson B. Mayfield
Back on the reef, there lives another very different type of algae; unlike the plant-like structures of turf algae and seaweed, crustose coralline algae (CCA) resemble a pink rock. But don’t be fooled by their unassuming appearance—CCA species are essential players in the formation and maintenance of reef ecosystems. If coral structures are the buildings of these underwater cities then CCA are the base infrastructure; it's the mortar that binds the bricks together, the strong foundation which facilitates the growth of a healthy community. These inconspicuous algae grow in the form of encrusting veneers, cementing together coral substrate to create new sites for colonisation. CCA even chemically prepare these sites for settlement, making their calcified encrustation the most attractive real estate on the reef. The rock-hard cover of these algae also serves to protect the reef from breakdown, reinforcing coral skeletons and forming a shield on wave-exposed crests, which reduces the rapid erosion of underlying substrate.
Marine worms: the guardian angels healing coral tissue
Photo by Jayne Jenkins
There are hundreds of different marine worm species and these critters often serve as a popular food source for larger organisms, helping healthy reefs blossom. The dazzling Christmas tree worm, pictured above, makes a permanent home for itself on the reef by creating a tube of calcium carbonate in the body of live coral, where it can reside for up to 40 years. The beautiful festive display you see is just the crown of this fascinating creature, a spiral of appendages used for feeding and respiration, as the rest of it’s segmented body remains burrowed inside it’s tube. But don’t be fooled by this worm’s good looks—this mesmerizing creature is first and foremost a protector. Christmas tree worms guard their coral against the predatory Crown of Thorns Starfish, and researchers have observed the burrows they inhabit facilitating the fast recovery of coral tissue following coral bleaching events or the overgrowth of turf algae.
Photo by Francesco Ungaro
These are a few standout members of an incredibly valuable community. Coral reefs are a kaleidoscope of unsung heroes: creatures of all different colours, shapes and sizes fitting together in complex ways to create something spectacular. Unfortunately, human activity and anthropogenic climate change are having a serious impact on these marine ecosystems, destabilising these carefully balanced relationships and leaving many of the ocean's inhabitants without food and shelter. In order to protect coral reefs, we must give these underdogs the attention they deserve and mobilise conservation efforts that safeguard their future. Even our individual actions can have a serious impact—we need to all be conscious consumers, reducing the amount of carbon we emit and choosing sustainably sourced products, treating the reef with reverence and respect when diving or fishing in these environments, participating in beach clean-ups or advocating for reef conservation, and supporting initiatives like Reef Life Restoration, who are providing homes for the ocean's homeless. There is a wonderful world of life beneath the waves, and these strange and colourful creatures need our love, just as much we need them.