At the time, I found this hard to believe because most corals are exceptionally good at catching their own food. During the day, polyps usually hide inside their limestone skeletons, but at night they show themselves, body and tentacles, looking, as I have said, just like little anemones. They don’t do this for fun, they do it to capture food, and that’s something easily seen. Corals in fact have voracious appetites, ensnaring tiny zooplankton in their tentacles, which are often armed with batteries of stinging cells. They stuff plankton into their mouths one after another, just like the anemones at Long Reef did with the worms I fed them.
But Muscatine was right – a habit of his. Experiments have shown that most nutrients corals use do come from their algae. It’s an intimate symbiosis. At least, this is the case with zooxanthellate corals, but there’s another group of hard corals, the azooxanthellate species, which have no algae, and these are mostly found in caves, where they only need reefs to supply zooplankton to eat. Azooxanthellate corals also occur in very deep, near-freezing water where there is no light at all. These groups are usually easy to tell apart: most zooxanthellate corals form colonies whereas most azooxanthellate corals live as tube-shaped solitary individuals. The symbiosis between coral and their algae is of enormous importance: it is the key to their capacity to build reefs, and as we will see, when that symbiosis breaks down it can cause mass death.
Reef corals, like forest trees, live in highly competitive ecosystems where corals vie for sunlight, often by forming sunlight-capturing plates, architectural masterpieces that make the most of the building material available and permit rapid growth. Branching staghorn corals do likewise, sometimes growing as much as 30 centimetres a year. These corals can be readily broken by storm waves or eaten by predators, so other species have a different strategy, forming solid structures that waves seldom break and predators seldom eat. While they’re small, these ‘massive’ corals, as they’re called, might be outgrown by the plates and staghorns, but they bide their time, waiting until storms and predators clear away the competition. Eventually they grow large enough to avoid being overgrown and then they get a continuous share of sunlight.
All this can readily be observed by divers, but what happens if a zooxanthellate coral is deprived of light? One of the first observations I made when I first dived on reefs was that the poorer the light (whether from turbidity, shading or depth), the slower the corals grow, until they don’t grow at all. Consequently the number of species dropped off the deeper I dived. Green plants do much the same thing where sunlight becomes scarce, but most green plants can’t capture live food whereas corals can. So what’s going on? Why are corals so totally addicted to their algae? I still ponder that question. The bottom line is that if any zooxanthellate coral is deprived of light they eventually die, no matter how much food they have. Corals are not on their own here; giant clams also capture food by filtering seawater, and like corals can grow on nothing other than seawater and sunlight because they also have photosynthetic symbiotic algae, albeit of a very different sort.
Light is not the only thing corals compete for; they vigorously compete with each other for space. At first glance they appear to grow harmoniously together, but on closer inspection, especially at night, they can be seen attacking each other, using their tentacles or, more commonly, bundles of long filaments extruded through their mouths. As soon as a filament touches another coral the battle is on, for each tries to sting or digest the tissues of the other. Peaceful-looking coral gardens are anything but peaceful; there are war zones everywhere.
But corals aren’t always competing. When it comes to reproduction they go to extreme lengths to co-operate. Across the entire Great Barrier Reef, one or two nights each year are like no other, for then mass spawning takes place. Like most plants and animals, corals need to be able to reproduce sexually, but as they can’t mate like mobile animals, they release eggs and sperm, usually in bundles, which float to the surface where cross-fertilisation can take place. This must be synchronised to within just a couple of hours, something governed by the time of sunset, the phase of the moon, and changes in water temperature. When every polyp of every colony of a whole coral garden spawns in synchrony it’s show time, for egg and sperm bundles are released in such quantity that the water column looks like a pink snowstorm, except that everything goes up, not down. Fish in frenzies eat their fill but still the show goes on, and not just with corals; other organisms, picking up on chemical cues, cash in on the timing of the coral spawning when fish are full of food and release their gametes also. The ocean surface becomes covered in slicks of pink spawn, all to be swept away by surface currents to another reef, or to oblivion. Mass spawning is a sight never to be forgotten, at least not for a year, when the whole show is played out once again.
Azooxanthellate coral. The bright colour comes from the coral itself, as these corals have no zooxanthellae.
Batteries of stinging cells on the tentacles of a coral.
A coral releasing egg and sperm bundles at night during a mass spawning event.
Expedition north
At the time of the Stoddart expedition (1973), it’s fair to say, nobody had advanced any clear idea about why the Great Barrier Reef is where it is or how long it has been there. But by the mid-1980s, about a hundred articles on its origin had been published, the consensus being that it was of recent age, mostly post-Pleistocene, meaning post the last ice age, as any pre-existing reef would have been removed by erosion. This idea followed work done in the Caribbean by the American geomorphologist Ed Purdy, who in 1974 hypothesised that reefs are composed of successive layers of limestone, each layer growing in response to a sea level rise.7 Not only did this create the internal morphology of reefs, as seen in seismic profiles, but it also controlled their modern shape and position.
As soon as Ken Back gave us the go-ahead for the expedition, a small group of us immersed ourselves in planning. We needed to go in December, when the monsoons came and the seas were usually calm, despite it being cyclone season. Calm seas or not, our plans were a bold move, for the charts of the time were frighteningly archaic: from Capt’n Matthew Flinders as amended (1802).
The places most of us wanted to visit were the ribbon reefs, the outermost reefs of the northern Great Barrier Reef, which form a spectacular chain starting east of the Torres Strait and running south almost to Cairns, a distance of over 700 kilometres. Tijou Reef looked especially interesting because, judging from depth soundings of the time, the abyssal depths of the Queensland Trough come closest to The Reef at that place. Just how far from the ribbon reefs was the Trough? This question was very Darwinian in scope and thought, for it begged a connection between the origin of the Great Barrier Reef and Darwin’s theory of the origin of atolls.
Ribbon reefs. The outer face (right) plunges into the Queensland Trough. The reef front is wave-hammered for most of the year.
Tijou Reef. This long ribbon reef, part of the outer Barrier, is about 1000 metres wide. The white line to the right shows the 1000-metre depth contour; the numbers to the left show depths in metres.
On the first leg of our expedition, we stopped at several islands where divers could work on corals and others could sample beach rock, reef flat corals and clams suitable for isotope ageing. We then headed up to Lizard Island, where the building of the now famous research station had just commenced, then on into almost unknown territory. After anchoring the Kirby on the western side of Tijou Reef, we crossed to the outer face in a tinny for a dive, and so became the first people, as far as I know, to dive anywhere on the outer face of the entire northern Great Barrier Reef. With the water crystal-clear we could see the reef face, spectacular in its grandeur, plunging steeply down and down. It was like diving down a mighty dam wall and it soon became obvious that this outer face of The Reef was the western edge of the Queensland Trough: the two were one. This did not sit well with any of the hypotheses later put forward about the age or origin of The Reef. To have maintained themselves
in such a position, the ribbon reefs had to be old, very old, even as old as the Trough itself. It was during that first dive that I knew the northern Great Barrier Reef had a history nobody had even vaguely contemplated.8
That outer face is a deathtrap for divers, at least it was until dive computers, which sound warnings, came into use. On that first dive, I was so absorbed in the splendour of the reef face that by the time I checked my depth gauge I discovered I’d reached 50 metres, nearing the maximum safe depth for scuba, and I was going down rapidly. This caught me unawares because even at that depth the coral communities looked like those found on wave-hammered upper reef slopes. I’ve never been on the outer face of a ribbon reef when the sea is very rough, but judging from the corals the turbulence must be extreme. It’s only during the monsoon season, at the height of summer, that the sea goes calm, and then it may go so calm that it’s completely flat, without even a ripple. At that point the horizon disappears and the water surface reflects the sky, hiding everything beneath. If this happens at high tide the reefs are at their most dangerous, for boats get glassed in; it’s as if they’re floating on a gigantic mirror. Many times we drove a zodiac in front of the Kirby to warn Davie of a reef ahead.
At low tide, when the outer reef flat is high and dry, the ribbon reefs look very unlike most other reefs, for they’re so wave-pounded that there are no corals, just hard, consolidated, flat limestone pavement. So hard and flat that I imagined it might be possible to land a jet on it with impunity.
Diving on the outer face of Tijou Reef was memorable for another, absolutely extraordinary reason. A couple of us were down about 20 metres or so when we were hit – there’s no other word for it – by a blast of the deepest and most intense sound imaginable, as if we were in front of the biggest pipe of a giant organ. The same thing happened again the following year. I had no idea what it could possibly be until, many years later, I talked to a researcher who worked on whale sounds. He said it would have been a whale, probably a sperm whale, checking us out, doubtless because they hadn’t come across divers before. I’d never heard of this happening to anyone else, and nor had he. Maybe most whales today know about scuba divers and ignore them? The answer still eludes me.
We left Tijou Reef in mid-December and headed north to the remotest island of the Great Barrier Reef. Perhaps some historians would have known about Raine Island, but all we could find out about it was that it had a stone beacon built by convicts in 1844 to warn sailing ships away.
When we arrived we felt we’d rediscovered the island: 32 hectares of low sandy cay with the most incredible birdlife any of us had ever seen, as well as the biggest green turtle rookery on Earth, as far as we knew, with at least 280 turtles heaving their massive bodies up the beach at dusk. (Our discovery was dwarfed the following year at the same location when we counted nearly twelve thousand turtles in a single night, a world record for green turtles.) We watched these beautiful animals all night, for there is something compellingly emotional about a turtle nesting. Everything these animals do requires enormous effort, and they constantly seem exhausted as they search for a good nesting spot before they start digging, then laying over a hundred eggs in a pocket scooped out by their hind flippers. Finally, these nests must be covered up before the mother slowly makes her way back to the ocean and swims away.
Underwater, we saw turtles stacked three-deep on reef ledges, waiting for the night to come, and from the Kirby we saw several thrown high out of the water as tiger sharks ripped their fins off, the exhausted females being easy prey.
Our accounts of this place on our return attracted a lot of attention and led to the start of the Raine Island Corporation, resulting in the island being given the strictest conservation protection in all Australia. I returned to Raine Island several times, having been given special permission to go whenever I chose, a privilege that ended when the work of the corporation was turned over to the Queensland government in 2005. No grumbles there, I’m just happy to see the place protected, since anybody who goes ashore sends up clouds of birds and disturbs nesting everywhere.
Sadly, I believe this spectacular place and the life it supports is doomed because it has little chance of surviving the sea level rise now under way. This view is unpopular with most of the scientists and managers engaged in a battle to save the turtle rookery by building up the cay with sand from the mainland. The island is surrounded by deep water; will the sand survive a cyclone? I think not, especially at higher sea levels, but I suppose anything that might buy precious time deserves a try.
The beacon on Raine Island built by convicts in 1844.
Turtles starting their journey up the beach at Raine Island.
Further north we discovered the most formidable and spectacular reefs I have ever seen. These are the deltaic reefs that form the northern limit of the ribbon reef chain, and which in aerial photos look like a string of wide river estuaries, opening east into the depths of the Coral Sea. To the west there are vast areas of deltas, complete with progressively smaller tributaries.9 We had a set of World War II photos, the sum of all knowledge of them at that time. The mudflats forming these deltas, as we initially supposed them to be, turned out to be solid limestone with the tributaries, in the form of deltaic patterns, carved into them. The channels were all U-shaped in cross-section, with vertical sides and flat, rubble-covered floors.
What was most spectacular of all about these reefs were the currents on the ebb tide, for they were ferocious, the current in the channel we were in almost bringing the Kirby to a standstill, engine roaring at full throttle. Davie Duncan was a great skipper; we found an anchorage.
The deltaic reefs north of the ribbon reefs. The white parts look like mudflats but are solid limestone; the dark ‘rivers’ are channels cut by tidal currents.
‘Bloody hell, Charlie, coming here was dumb,’ he said as he wiped sweat from his brow.
Davie’s got no sense of history. Nobody’s ever been here before. Or if they have, they never lived to tell the tale.
Our first job was to confirm that these reefs were indeed solid rock and not mud; the second was to check out the channels. There was nothing for it but to start a scuba dive from one of the small back tributaries and let the current take us – three of us, with little marker buoys attached and followed by a zodiac – down the channels as they became bigger. We soon found ourselves going faster and faster with no chance of stopping. Although wildly turbulent, the water was very clear, giving no sense of movement, but in it were unseen forces of compelling strength, pulling us up then down, this way then that, sometimes spinning us full circle. The channel edges, almost devoid of coral, whizzed by and even the bottom, which we could glimpse deep below, appeared to be on the move. By the time we reached the main channel things were getting a little scary, but the sound of the zodiac’s outboard above us was reassuring. I was thinking it was time to call it quits and surface when a huge ridge of reef loomed ahead, forming a sort of sill, which we later discovered was the outer lip of the channel, about half a kilometre wide. Over the sill we went, at terrific speed, and then down down down, losing sight of each other as we went. Even the bubbles from my regulator went straight down. I inflated my vest at about 30 metres and, relieved that this arrested my descent, swam for the surface, searching for my buddies as I went. It was a daunting place, especially as there were hundreds of sharks all moving very fast, as if it were a feeding ground.
The surface, when I reached it, was a lonely place. My buddies were nowhere to be seen: all I could see were big, foam-topped standing waves, holding their position against the current, which was pushing me, and presumably my buddies, relentlessly away from Australia. There was nothing for it but to wait, and hope the others had reached the surface. Fortunately, our boatman had his wits about him. Our marker buoys had disappeared when we were pulled down, so he zigzagged back and forth until he found each of us. I have seldom been so pleased to see a boat, with the others safely in it, in my life.
Ev
en for a zodiac with a powerful outboard, the return trip over the waves and against the current was slow, and once back in the channel we had traversed it was fascinating to see whirlpools and smooth mounds of upwelling water forming here and there and then disappearing as we sped along.
Diving when you have control of what you’re doing is one thing, but when you have none it is quite another. I know of nobody else who has dived in these channels. No surprises there, but at least we gained an impression of what they looked like. Later this gave me some ideas about how they had formed: from tidal currents, erosion, sea level changes and a subsiding continental shelf.10
Mer Island, known as Murray Island when we were there, lies to the north of the deltaic reefs. Legend had it that it was a dangerous place to visit as its occupants were in the habit of inviting unexpected arrivals for dinner. I had little doubt that they once did, especially after their chief, a massive man the colour of obsidian, bellowed with laughter as he told us how his father delighted in cooking and eating the tongues of shipwrecked sailors, or nearby Darnley Islanders when no sailors were around, while their owners were made to look on. Still chuckling, he invited us ashore. It was Christmas Eve and the island’s children were going to stage a concert.
We made our way to the village hall, keeping in mind the chief’s parting hope that we wouldn’t be turned into a traditional feast. The hall was a large corrugated-iron shed and there we waited, and waited. We were about to give up when the distant sound of children singing came wafting through the coconut palms. I bolted for our zodiac, not for safety, but to get my tape recorder. I decided to sacrifice Beethoven in favour of the children’s songs, but at first I had no idea how good that concert was going to be. The children filed in, dressed from head to foot in their best traditional Papua New Guinea-like regalia, and for hours – almost until dawn – they sang and danced, and I recorded the whole thing. I have never forgiven myself for throwing out, twenty years later, what I thought were a lot of old Beethoven tapes. I had in fact thrown out hours of children singing in Meriam Mir, a language that at one stage looked like it was on a path to extinction.
A Life Underwater Page 10