THE LIFE OF CORAL / A seething soup of sex on the reef

PAUL O’DOWD

So the greatest sex show on Earth, for the smutty-minded and marine biologists at least, has come and gone. It was good to see some coral spawn, but … well…I guess I remember the good ol’ days. The spawn, unleashed, have ascended en masse to be concentrated into stinky pink slicks, so vast that they can be seen from space. Within these slicks is a seething soup of sex.

Coral planula must find naked limestone on which to settle and there are not many spots on a reef which are not already covered in life.
Parrotfish clean the rocks and provide habitat for baby coral to establish themselves after spawning.
Healthy reefs are noisy places and this noise is critical in attracting baby coral to suitable sites.

The fertilised eggs divide and divide again, and over the coming few days they form into barely visible specks of minute coral larvae, called planulae. A coral planula looks like minuscule potato covered with tiny little beating hairs called cilia. At this stage, they are simply a ball of cells with very little other than the automatic beating of their cilia to distinguish them from sea snot. They have no eyes, nor do they have ears. They don’t even have a nervous system. All they possess by way of sensors, is a chemical detector on their surface which probably tells their genes to activate ‘stage two’ in their life cycle, where they attach and form a polyp when they come into contact with clean limestone rock.

Image Credit / Coral Brunner / Stony coral (Acropora millepora) releasing egg-sperm bundles during the annual coral spawn on the Great Barrier Reef .

Coral begin their lives in the currents that wash over and between the reefs. The time between fertilisation and settlement on an appropriate substrate can be considerable, potentially many days. By the time they are ready for stage two, they may be kilometres away from appropriate habitat. During this time, they must somehow navigate out of open water and towards a reef, in time to find some good real estate and stick to it.

Critically, the only substrate the coral larvae can chemically adhere to, and thus grow upon, is clean limestone.

You may have noticed, there is not a lot of that kind of real estate available on a busy reef. Pretty much every square centimetre of a living reef is already occupied. There are many resources, beyond suitable real estate, that the growing coral will also require, so there is no point ending up on a dead, slime covered reef…

Baby coral are very hungry. They need clean, well lit water with just the right amount of suspended protein and nutrients in it. Plus, if the reef is not alive, something must have killed it. What if it’s still uninhabitable or hostile? A living reef is one in which the ecosystem still functions, which means it should provide all the things the young coral needs.

A living reef is what the baby coral needs to find.

The question is; how do you detect and navigate toward a healthy reef when you are literally an insentient blob of cells tumbling randomly in the water under the impotent influence of a coat of flapping cilia?

Image Credit / Coral Brunner / Acropora coral spawning

It turns out that a healthy living reef is a very noisy place. There are the communicative clicks, grunts and pops emitted by many reef creatures, and the cringe inducing crunches that reveal sudden acts of violent carnivory… But by far the loudest and most constant noise on the reef, is that of the unremitting grazing of parrotfish.

Image Credit / Richard Whitcombe / Parrotfish feeding on coral in the Great Barrier Reef

These colourful and charismatic critters eat enormous amounts of algae. They do this by scratching it off the furry rocks and chomping into the living corals, which of course, are loaded with symbiotic algae.

With every powerful bite of their birdlike beak, they create a loud cracking sound, leaving a livid bite mark of clean limestone surrounded by the incidental chaos of ripped algae, sponge and coral tissues.

This damage itself feeds a multitude of opportunists who profit wildly by following the grazing parrotfish herds, cleaning up the mess left in their wake.

This activity serves to prevent the reef from becoming overwhelmed by a viscous slick of slime and asphyxiating under a carpet of algal goo.

So a noisy reef, is a reef under the management of parrot fish, and therefore a reef with an abundance of the first critical requirement in the life of a baby coral; clean limestone, which, if they do not find and stick to, they will be lost.

It now appears that it is to this noise that the young coral are attracted.

We can demonstrate this in the lab by playing various sounds through open ended collection tubes in tanks full of planulae. The tubes that play healthy reef recordings are the tubes which the coral larvae move to, and become collected in.

At this point, some readers might remember my earlier description of baby coral, specifically the bit about them having “no ears”…

So, if there are no ears and no nervous system, how can they be attracted by a sound?

This is where it gets magic.

It just so happens that the tiny beating hairs on the surface of our coral planula are exactly matched in their physical dimensions to the wavelengths and frequencies of the dominant sounds being generated by the living reef.

As this strongly parrot fish-powered cacophony washes over the coral larvae, the sound waves interfere with the beating of the cilia in such a way as to cause the tumbling ball of cells to become oriented to the signal. Once locked into this sonic influence, they drive themselves down the acoustic gradient, and into the safety and opportunity of the vibrant ecosystem that is its source.

There is no detection and no decision. They do not excitedly declare “hey fellas, it’s this way. Let’s go!” The process is completely mechanical.

And we’ve only just figured this out.

So the next time you are out on the reef, don’t ignore the parrot fish. Do yourself a favour and listen to what is arguably the most important sound on Earth.

This constant crackling is the sound that enables the tiniest of creatures to produce and sustain a living sea-wall, thousands of kilometres long, which protects our coast against the violence of the open Pacific. And this is to say nothing of the incalculable number of other processes and services delivered to the world by its coral reef systems.

Without this sound, the coast you live on would be unrecognisable, and Port Douglas as we know it, would simply not exist.