Coral reefs hold 25% of all marine biodiversity. They’re a wonderland of ecological hotspots filled with a multitude of weird and wonderful ocean creatures. Each resident provides an important function and service for maintaining the health of our coral reefs and, in turn, our oceans. And, the coral reef structure itself acts as a natural wave breaker, preventing large waves crashing into delicate coastal ecosystems. Algae, aquatic plants and seagrasses on and around coral reefs act as the rainforests of the sea. They absorb large amounts of carbon dioxide and help regulate our global climate. And what’s more, with all of this genetic diversity, there could be a compound or two of medicinal significance to treat many currently incurable diseases.
However, their fragility means years’ worth of growth can easily be broken in a very short period of time. Their sensitivity means mortality is high when water conditions change. And their resistance and resilience are low in combination with a myriad of global threats. For instance, when higher than average water temperatures persist for sustained periods of time, corals expel their photosynthetic symbionts and go into a low-energy state, with a looming fate. And with excess nutrients, algae can grow quickly, smothering corals from sunlight.
It’s time to pay recognition to the saviours of our precious ocean hotspots. The unlikely underdogs doing their bit to keep coral reefs healthy and resilient. With reef ecosystems suffering global decline, maybe it’s time for us to shift our focus of conservation onto some of these unsung heroes.
1) Seabirds – Well, seabird poop!
Recent research is suggesting that seabird poop may be stimulating the growth of healthy algae for coral reefs. The poop, which is incredibly rich in nutrients, provides the perfect ratio of nitrogen to phosphorus for crustose coralline algae to thrive. These crunchy seaweeds are an indicator of a healthy reef. They help to strengthen the reef-building hard corals skeleton – acting almost as a cement for coral bricks! Coral reefs adjacent to bird colonies have more lively fish populations than reefs lacking the nutrient-saturated bird droppings. And fish are also more likely to bounce back from stressors, such as coral bleaching, than those living on coral reefs lacking this key ‘support’ from bird poop.
2) The ‘Sponge Loop’
Coral reefs hold 25% of all marine biodiversity and are among Earth’s most dynamic environments, yet they thrive in low-nutrient waters. It’s a phenomenon known as Darwin’s Paradox. One of the most basic life forms, an invertebrate with just two cell layers, the sponge or porifera has undergone millennia of evolution to produce individuals with thousands of spherical chambers within. Each of these chambers acts as a filter feeding machine, catching small food particles in the water column. So to put a perspective on how powerful these filter feeders are, research discovered that one 10cm tall (1cm diameter) sponge contained 2.2 million chambers and pumped 22.5 litres of seawater a day!
The commonly-held belief was that bacteria were the main eco-warriors recycling nutrients on coral reefs. But this ‘microbial loop’ can’t be responsible for handling the high levels of carbon, nitrogen and phosphorous produced by corals. It’s the unsung superhero sponges who recycle 10 times more organic matter and plankton from their reef neighbours than bacteria do. They keep waters clean and clear for corals to thrive.
To uncover these heroes, scientists used labelled sugars to trace their journey through the reef food web. The sponges filtered the sugars then quickly shed their dead filter cells, producing detritus which sunk to the sea floor. Within just two days, these tracers were found in snails and other bottom-dwelling creatures feeding on the sponge waste. Bigger animals will in turn eat the bottom-feeders and the cycle continues!
Check out a sponge at work.
3) Snail saviours
Invasion of the Crown of Thorns (COT) starfish on coral reefs is becoming an increasingly prevalent event on many reefs already under threat from global stressors. These spiky sea creatures prey on vulnerable corals. They can eat up to ten square metres of living coral tissue per year. And they’re super fertile – one female can produce up to one million eggs each year! With reported outbreaks of thousands to millions arising on the Great Barrier Reef, this starfish can be an immense risk for the survival and resistance of even the largest of coral reefs.
There is, however, hope to this thorn-filled despair in the shape of a snail! Growing up to half a metre long, the giant triton (Charonia tritonis) are one of few natural predators of the venomous COT. The Australian Institute of Marine Science (AIMS) is investigating a potential method of counterattack on COT infestations. This massive snail species is rare. Because of their beautiful shell design and slow-moving pace they’re an easy catch for the decorative shell trade (more on why we shouldn’t take shells from the sea next time!).
AIMS is working on a breeding program with the aim of deploying the giant tritons as a crisis taskforce to mitigate COT populations. In 2017, their 8 adults bred and released 100,000 swimming larvae! Their egg-laying adults were so starved afterwards they were eating 1-2 COT a day! What’s more, these carnivorous snails are such relentless predators, the very smell of tritrons close by causes the COT to flee in panic! Watch their reaction here.
4) Happy clams
Have you ever wondered how giant clams (Tridacna gigas) can grow so large? They’re the biggest living members of the bivalve class, with the clam span of the largest known specimen reaching 1.37m (4ft 6in)! The weight of the shell, discovered in the early 19th century off the coast of Sumatra, Indonesia, was 230kg (510 lb)!
A possible explanation for this vast growth is their feeding potential. Juvenile and adult giant clams can feed in 4 different ways, so they never go hungry:
- Feeding via photosynthetic symbiotic algae, Zooxanthellae, in the creature’s colourful mantle, the same algae that acts as a powerhouse for corals.
- Digestion of Zooxanthellae.
- Absorbing particulate organic and inorganic matter in the water.
The last three feeding behaviours are related to water pumping and waste removal, which assists in keeping coral reef ecosystems pristine. Clumps of clams will increase water flow rate and hydrodynamics around a coral reef. They pump, transport and filter huge amounts of phytoplankton from the water. Even after death, their giant valves remain and keep affecting the sea water’s flow!
What’s more, these shells aren’t just for decoration. Giant clams can shelter smaller fishes, helping species such as anemonefish evade predation in the absence of anemones. And the shell ridges too can provide a private, safe spot for egg layers. Or even a home for a multitude of encrusting reef inhabitants and small mobile creatures. Their presence also supplies construction materials for reef-builders. With lots of clams in the coral reef city, together they can produce up to 80 tonnes of calcium carbonate shell material per hectare annually. This helps to keep coral reefs supplied with the chemical building blocks they need to get bigger and stronger!
6) Gardeners of the coral reefs
Pop quiz! What’s the most colourful and arguably one of the most important fish families on coral reefs? Hint! The scales of these fish have the same vibrant colours as one species of spectacular, mimicking tropical bird. Yes, it’s the parrotfish!
We’re all certain a parrot is a pirate’s best friend, but did you know the parrot fish are a coral reef’s best friend too? Without parrotfish and other herbivorous fish munching on surface algae, the coral reef can become an algal-dominated system. This can result in a lack of free space for coral larvae to attach. As a result corals become smothered and starved of oxygen, and ultimately there can be too much algae for our fishy friends to consume. What’s then left behind is an altered ecosystem that’s lacking all the habitats and niches for hosting the wide range of biodiversity we expect from our colourful coral reefs.
During grazing, a certain amount of coral is corroded by the parrotfish’s beak-like teeth. This contributes to an essential process called bioerosion, the breaking down of hard ocean substrates, producing and distributing sands. Their coral poop is what creates our picturesque, holiday-destination white sandy beaches! Yep, you read that correctly. Our dream beaches are parrotfish poop!
Did you know this is precisely why we shouldn’t feed fish? Imagine being offered a tasty, exotic snack instead of a healthy, natural diet – you’d swarm to trial the new cuisine too! Fish-feeding creates a downside for the coral reefs with corals and bare space remaining uncleaned of ever-creeping algal growth. Parrotfish in particular spend up to 90% pecking and consuming algae, so the corals depend on these natural lawn mowers to keep them clean.
And the rest…
Of course, there are more herbivorous fish. Surgeonfish and damselfish for example are two small and beautiful reef fish responsible for coral gardening. However, their numbers are in decline because of their attractiveness and our desire to home them in aquariums. You’re probably most familiar with the regal blue tang surgeonfish, Paracanthurus hepatus, A.K.A Dory! The Finding Nemo sidekick’s popularity had a massive and detrimental impact on wild fish stocks when the demand sky-rocketed for this charismatic, forgetful fish in our homes. What you might not know from the movie is that this species is incredibly helpful at keeping our coral reefs clean as their small and sharp teeth are perfectly adapted for chewing algae.
Spread the word!
So next time you are watching a nature documentary, snorkelling around a shallow reef or diving deep into coral crevices, take a note of these dynamic and essential organisms. It might be easy to overlook the vital role they play in sustaining healthy coral reef ecosystems from their appearances. Sing their praises, appreciate their understated beauty and tell your fellow Netflix/snorkeller/diver buddies about these spectacular species. The more we know, the better we can conserve our delicate and fragile coral reefs!
- Benkwitt, CE, Wilson, SK, Graham, NAJ. Seabird nutrient subsidies alter patterns of algal abundance and fish biomass on coral reefs following a bleaching event. Glob Change Biol. 2019; 25: 2619– 2632.
- Jasper M. De Goeij, Dick Van Oevelen, Mark J. A. Vermeij, Ronald Osinga, Jack J. Middelburg, Anton F. P. M. De Goeij, Wim Admiraal. Surviving in a Marine Desert: The Sponge Loop Retains Resources Within Coral Reefs. Science. 2013; 6154: 108-110.