Tip:
Highlight text to annotate it
X
Most hard corals appear to be shades of yellow-brown, like this Bottlebrush acropora;
some other corals are more colourful. But in fact, the hard coral skeleton is snow white.
The colour you see comes from a tiny critter that lives in the coralʼs tissues.
This critter is part plant and part animal, or more technically a photosynthetic dinoflagellate, and its name is zooxanthellae.
This zooxanthellae uses sunlight and the coralʼs waste carbon dioxide, to create oxygen and sugars, which is coral food.
And this is the #1 mutualistic symbiotic relationship that makes coral reefs possible.
The coral gets about 80% of its food from the zooxanthellaeʼs waste,
and the zooxanthellae gets fed and gets a place to live.
Neither can survive for long without the other.
Deeper down the reef, sunlight becomes a limiting factor.
Remember sunlight is what drives the zooxanthellae food production system for the corals,
and as we go deeper thereʼs more and more water for that sunlight to have to penetrate.
The water is full of all that microscopic plankton, which en masse and over many metres, slowly starts to absorb and block the sunlight.
The depth limit for any given species of coral depends on what shapes they can grow in,
in order to catch maximum sunlight, while still being thick and strong enough to support their own weight as they grow.
In the shallow waters this Turbinaria would grow more like an upright ball of cabbage,
but here at 15m/50ft it grows more spread out and flat in order to maximize its exposure to the limited sunlight.
I filmed this shot at dusk using artificial lights.
Hereʼs a typical coral head in Indonesia—packed with colour and texture, which to me says "biodiversity".
Every square inch of this rock is encrusted with life.
The anemone in the centre looks after the anemonefish, and the anemonefish defends the anemone against certain predators like butterflyfish,
and that probably benefits other species of the patch as well.
The orange, yellow, and grey blobs are sponges. The blue bits are ascidians or sea squirts.
All around the perimeter of the rock and reaching out into the water column are the feathery arms of stinging hydroids—relatives of anemones and corals.
The sponges, the ascidians and the hydroids are all filter feeders, taking nutrients and particles out of the water.
The clearer water that they make allows more sunlight to get through,
which keeps the corals, the clams and the anemones all happy and healthy.
The clearer water also helps me to film everything, and allows us to see beyond just this rock,
out to distant rocks... rocks which each have their own collection of critters living in beneficial co-existence.
This is a what a recovering reef looks like.
All of that rubble is evidence of a disturbance.
This is Kicha Island in the Solomons, so a cyclone or a tsunami are my guesses.
But what I love about this about this scene, in fact this entire reef, is the regrowth.
This is a high energy environment where the price of good water quality is an occasional flogging from damaging exposure to the open sea.
It means the corals grow fast, but fragile branching or plating species will get smashed at fairly regular intervals—every decade or so, at least.
These small corals in the foreground are new recruits, and only a few years old, but super healthy.
The other happy news here at Kicha is the diversity of fish life.
If you scan the blue, you can see a dozen or more different species of planktivores feeding,
and that includes the Palette surgeonfish—better known as Dory to "Finding Nemo" fans.
This is a remote part of Indonesia called Alor,
where they get strong upwelling currents that bring deep clean oceanic water full of plankton up to the shallows and mid-reef.
Every space is occupied and the competition is tough between all the plankton feeders.
The fish get the first crack, then the less mobile animals.
Raised areas like this are usually claimed by crinoids which can walk up and grab on with their claw foot.
Thatʼs a black one with white frills in the middle of the shot, and there are yellow and black species above it and below as well.
Crinoids are related to starfish and sea urchins, and they catch plankton using sticky tube feet rather than stinging cells.
So they are completely harmless to humans, but very sticky and fragile so they are easily damaged.
Most crinoids are homes to minute and perfectly camouflaged commensal shrimp and squat lobsters and clingfish,
and those little guys canʼt survive anywhere else.
My guess is these commensal critters are more important than we give them credit for,
maybe helping protect their hosts from parasites or predators.
The best way to find out is through observation, and ideally through a camera lens.
I often see things in my footage that I missed with the naked eye while underwater, especially when you're shooting really tiny stuff.