Our daughter Eviie was born in December, not long after our return. Following a brief period of anxiety on my part that it could all happen again, we had a time of celebration the likes of which I hadn’t felt in many a year.
My sister Jan lived to see Eviie, but only for a couple of months. Fourteen years had passed since Noni’s death, yet Jan was still dealing with her private sorrows by drowning them in alcohol. I went to see her in Sydney soon after getting back from our trip and it was obvious to me, if not to her, that she was in a bad way. I persuaded her to sell her house and move to Rivendell where we could look after her. She became terminally ill while with us and died after two weeks in hospital. She was fifty-seven. What a waste of a life. It was only in the last few years, after she’d moved out of the family home, that she had a chance to do anything she herself wanted to do.
Time and Place
Invading deep time
I first visited the Caribbean in 1992 with Mary, who had worked there several times before and had a good knowledge of its corals. On arriving in Belize, we went on a tour of Mayan ruins, then had a stunning journey along freshwater rivers that wound their way into the jungle. The trees were dripping with epiphytes I knew existed but had never seen. Then, joy of joys, we found juveniles of the cichlid fish that lived in our biggest aquarium at Rivendell. I could have spent months in that place, but, duty calling, we headed for Glover’s Reef. There, Mary no doubt enjoyed giving the expert his lessons, but at the risk of sounding tedious there wasn’t much to learn and I knew the species anyway. A patch of central Indo-Pacific reef the area of our kitchen is likely to have more species than the whole of the Caribbean combined. Thus it was not so much the coral species that interested me, but why they were so distinctive, and the story of how they arrived.22
The birthplace of modern corals doesn’t exist today, for it was the Tethys Sea, a place with origins going so far back in geological antiquity – over 200 million years – that the face of the earth was then unrecognisable. Around 100 million years ago the continents of the Southern Hemisphere had started to relinquish their hold on Gondwana (of which Antarctica is the last remnant), so there were several east–west seaways circling the tropical world. The most important of all these as far as marine life is concerned was the enormous gap between North Africa and the place we now call Europe. At the end of the Cretaceous, around 70 million years ago, with dinosaurs contemplating their demise, the Mediterranean was connected with the Indian Ocean to the east and with the Atlantic Ocean to the west. Further north, with the northern movement of Africa unstoppable, the western Eurasian plate had buckled and sunk so that eastern Europe and the Middle East were also joined to the founding Mediterranean. This was the Tethys Sea, with ever-changing shorelines that usually extended from Africa, at that time still well south of its present position, all the way to Scandinavia.
With the southern continents colliding with those of the north, the earth continually shuddered. That meant strings of volcanoes erupted, which in turn meant carbon dioxide belched forth. The earth, by today’s standards, was hot. With the southern Tethys, now called the Mediterranean, getting deeper and the northern Tethys, now Europe, getting shallower, a large expanse of high-latitude warm sea developed. This ‘Super-Tethys’ was reef-building country at its best, and the corals that built them abounded in both number and diversity. Like reef corals today these were powered by sunlight-capturing zooxanthellae, but these zooxanthellae, unlike those of today, were heat-resistant, having had millions of years of selection to become so.
Of course the Tethys, the birthplace of so much marine life, was doomed. By the Middle Miocene, around 15 million years ago, Europe was undergoing one of the greatest land reclamation projects in the Earth’s history. As the map on page 214 shows, this reduced the Tethys to a seaway north of Saudi Arabia, but that too was being choked off as the Red Sea deepened to the south. When that process was complete, all that was left of the Tethys was the Mediterranean, where there are no reefs and almost no zooxanthellate corals.
Continental positions during the Middle Miocene, about 15 million years ago. Surviving reef limestone is shown in dark grey.
Just how corals moved out of the Tethys attracted some bizarre theories before biogeographers realised that their larvae could make long-distance journeys. Some corals travelled east to discover east Africa and northern India, before the latter slammed into Asia. Some travelled west to reach the Atlantic, and some travelled nowhere and went extinct. This is a drastic simplification of 100 million years of the Earth’s history, for that history was anything but simple. No doubt there were many migrations and many reversals back to the Tethys, but the main events relevant to this story were the extinctions, for carbon dioxide repeatedly acidified the oceans, and each time it did corals were decimated; some survived and some didn’t.
Most of the corals that reached the Atlantic crossed it and so came to the Caribbean. That crossing was not as far as it is today, but more importantly it was warmer; the Atlantic wasn’t the barrier it would be now be. Those corals that made the crossing found almost another Tethyan Garden of Eden, for the Caribbean was not isolated. There was no Isthmus of Panama, so the Caribbean was a passage that linked the Atlantic with the Indo-Pacific. And so the journey of the Tethyan corals continued, up and down the Pacific coast of Mesoamerica.
When I went for my first dive in Belize I couldn’t help imagining I was diving in the Tethys (ignoring passing Plesiosaurs). All the species and about half the genera were new to me in their living state, and many were strikingly different from any I had seen before. This, I imagined, was what the cradle of modern corals might have looked like. To some extent this was true: the average geological age of Caribbean coral genera is twice that of the Indo-Pacific, but in the distant past the diversity of the former was far greater than it now is.
Several million years ago the gradual formation of the Isthmus of Panama started separating the tropical Pacific from the Atlantic, eventually leaving the corals on the Pacific side completely cut off from their Caribbean cousins. That would have left time enough for new species to evolve on the Pacific side, and indeed they probably did until disaster, in the form of the ice ages, intervened. The cold Humboldt Current, running north along the west coast of South America, wiped out most if not all corals of the entire eastern Pacific. On the Caribbean side conditions were generally better, for the corals were kept warm by the westward flow of water from the tropical Atlantic, just as they are today.
The history of all coral reefs is one of disasters and the Caribbean is no exception. Their greatest disaster was yet to come, caused by the ice ages, but not by temperature. To see this we must jump forward to about 25 000 years ago, when a massive ice sheet built up over much of North America. A large portion of the sheet (or shelf, as it is called) was over 2 kilometres thick. At that time several Milankovitch cycles kicked in and the sheet melted, and did so quickly. A glance at a map shows what this meant for the Caribbean, for the Mississippi River drains most of North America and even some of southern Canada. A volume of fresh water beyond imagination poured into the Gulf of Mexico and thence the Caribbean. The fate of the corals there is not known, but the event would have been catastrophic, perhaps wiping out most of them. The most likely explanation for their survival and subsequent return is that the corals hung on in the south-east, protected from freshwater by the incursion of tropical water from the Atlantic.
The corals of Belize that I saw would have had to struggle with the sea level changes, as all corals did, but I can’t imagine they survived all that fresh water, which is lethal to corals. The corals there today must be immigrants, and geologically recent ones at that.
After leaving Belize we headed for Discovery Bay Marine Lab on the northern coast of Jamaica. Marine biologist Tom Goreau (1924–1970) had made the station famous, partly because of his historic studies of reef ecology (John Wells, who was anything but an ecologist, had a photo of him on his living-room mantelpiece), but
also because he was the first person to use scuba (in the mid-1950s) for coral research. That’s a long time before I did.
When we saw it, Discovery Bay had almost no coral; we dived mostly on bare rock. Mary had worked there before and was dismayed by the change. The destruction was well known and much studied, but the real shock came when we went through the black and white photos Tom had taken during his time there. The corals were once lush and abundant, as good as any we ever saw anywhere in the Caribbean. It was a stark reminder that such disasters can and do happen within a single human lifetime.
I have worked in many other parts of the Caribbean at one time or another. Some students claim they can recognise geographic variations in some species there. If so, good for them. I never could; at least, not remotely on the scale we see in the Indo-Pacific.
The fossil record of corals is by far the most telling of all animal groups because corals build reefs and the fossils these reefs contain give us windows back to the very beginnings of animal life. This makes reefs Nature’s historians, places that can be dated and which keep track of changes in all the organisms they preserve, something they have been doing for hundreds of millions of years.
The history of all reefs goes in boom-and-bust cycles, but the end of the Palaeozoic Era, 250 million years ago, was a bust like no other. Every coral on our planet, along with so much else, went extinct because our oceans became lethally acidified by a massive spike in atmospheric carbon dioxide and as a result were almost devoid of anything that had a carbonate skeleton. These conditions took millions of years to subside, and it was many more millions of years before new groups of animals evolved to replace those that had gone extinct. This evolution included modern corals – the Scleractinia – which came from what were probably soft-bodied, anemone-like ancestors only very distantly related to the corals that built the reefs of the Palaeozoic.
From the mid-1980s I took a good deal of interest in what palaeontologists had to say about the fossil record of scleractinian corals, a time that spans the Mesozoic (the era of the dinosaurs) and the Cenozoic (our own era). With the help of my computer, such as it was then, I built a detailed compilation of that record, and this became the start of a very big problem, because a high proportion of Cenozoic families and some genera are alive today and these had to be amalgamated with the older, extinct taxa from the palaeontological literature. The result was a family tree, which I first published in Corals in Space and Time (1995), that looked nothing like that of its widely accepted predecessor produced by John Wells thirty years earlier. His had one trunk and main branches, whereas mine was coppiced, a tree of many trunks all growing from the bottom. Some of my branches were cut off, representing families that went extinct, while others reached the top of the drawing, representing families still alive. My tree made some European palaeontologists angry, this time because I was invading their territory. What right did I have to do that?
In 2007 I was delighted to accept an invitation to an international symposium in Austria on fossil corals. When I was asked to be the opening speaker I knew the organisers wanted to kick off on a controversial note, so I talked about my concept of reticulate evolution and then about my family tree of corals. This soon had some of the audience in the front rows shaking their weary grey heads and scowling at me: the organisers got their money’s worth. However, most delegates seemed happy with what I had to say and several specialists later went over some of the finer points of the fossil record with me. The symposium was great fun and I appreciated talking with palaeontologists, whom I seldom got a chance to meet. I especially enjoyed the field trip, to a place full of Mesozoic fossils on the slopes of a massive reef they’d once built, the calcareous Austrian Alps. I doubt that the thousands of people who ski down these slopes every year realise they’re skiing down the face of a coral reef.
Late Triassic reefs of the Austrian Alps.
One day, I hope someone will revise the old coral palaeontology catalogue published by John Wells in 1956, a daunting task that has been talked about for decades.23 If they do, they’ll hopefully heed what is being done with modern corals and use decision-making technology that can separate fact from opinion or, dare I say it, fiction. A program I have been using for the past fifteen years does just that. Every newly added taxon needs justification in the form of characters that separate it from every other taxon. This was the notion that most outraged the old palaeontologists: how dare I say that a computer was smarter than they?
I wrote a paper for them, laying out the facts of the matter. It was rejected, of course.
I give up. These old guys should too.
While I was attending a seminar at James Cook University in 1987, Russell Kelley, then a coral palaeontology student, passed me a coral – a small Porites – and asked me what it was. I knew that he knew what it was, so what was he on about?
He whispered that it was about 2 million years old, but it looked as if it had been collected yesterday. It was from the coast west of Port Moresby, Papua New Guinea. Enough said; we had to go there.
Today, Port Moresby is among the most dangerous cities in the world, and although back then it wasn’t so bad, some of the surrounding districts were, and unfortunately they included the piece of coast we needed to get to. Nevertheless, Russell persuaded the University of Papua New Guinea to lend us a Land Rover and off we went, north-west from Port Moresby along a rough track through the forest. A couple of times some ‘rascals’, as they’re called, tried to stop us, so we were relieved when we arrived at a small Roman Catholic mission, where one of the two inhabitants offered to take us to meet the local villagers. All was well after that, especially when a mob of children joined us, collecting coconuts for us to drink and helping to carry our fossils back to their village.
The fossils were, as expected, exceptionally well preserved and the place turned out to be the richest coral fossil deposit in the world. We had no means of determining the age of the fossils from radioisotopes at that time, but as the whole region had been stratigraphically mapped in detail we were reasonably confident that they were 2–3 million years old. That’s not enormously old by most geological standards, but it does come into the category of time out of mind for most people. The fossils were of particular interest for me because by then I’d worked on the living corals of the region and so had a good basis for seeing what changes had taken place over that amount of time. With rare exceptions, fossil corals cannot be reliably identified to species, but this collection was exceptional because the corals had been encased in mud, which turned to stone, shielding them from deterioration. Extraordinarily, sixty-five species of fossil corals seemed identical to their modern counterparts, or nearly so; nine species were clearly different, and a further nine had almost certainly gone extinct.
When all this was tallied up in detail we concluded that the average geological age of the coral species we studied was about 20 million years.24 That’s a huge span of time compared with most animal life. Our study showed that scleractinian corals are a slowly evolving group, although nowhere near as slowly evolving as some ‘living fossils’, of which blue coral (that I’d seen in Shiraho Lagoon) is among the oldest, dating back more than 70 million years.
This study seemed satisfying at the time, and from a palaeontological perspective could hardly be bettered, but I had doubts that lingered. Our work was based on the morphology of a few time-eroded specimens of each species. I now believe that studies using DNA, which can give a significantly improved insight into the longevity of species, would indicate a much shorter time span.
Even though this trip resulted in my getting another dose of malaria, I would love to go back again and explore further. I have tried a couple of times, but it remains too dangerous.
The first scleractinian corals (Middle Triassic). These have structures like corals of today, but did not all occur together at the same time and place.
Common growth forms of modern scleractinian corals.
Hell’s atoll
In 1994 I had a chance to visit Clipperton Atoll, the only atoll of the entire eastern Pacific. By this time the notion that corals could disperse over very long distances was firmly established, something that had become obvious when I first plotted species distribution maps.
When writing Corals in Space and Time I was a little irritated to read a condemnation of what was then a novel proposal, by T.F. Dana, that the corals of the far eastern Pacific (the west coast of Mesoamerica) were recent immigrants from the central Pacific. I took sides, as much on behalf of Dana as the corals.25 A few species, which appear to have gone extinct elsewhere, might have survived the ice ages in the east, but most are indeed immigrants that crossed the vast empty space of the eastern Pacific from islands in the west. That’s saying something about the endurance of the larvae of these species, for these journeys must have taken months, the larvae floating on the surface of the east-flowing North Equatorial Counter Current. However, they took their zooxanthellae with them; they didn’t go hungry, and several species probably cheated by growing on floating objects like pumice, spawning asexually, or releasing egg and sperm bundles during monthly lunar cycles rather than annual mass spawnings. The chances of colonising locations as remote as the Mesoamerican coast, let alone Clipperton Atoll, would have been extremely remote. Nevertheless, this is what happened.
A Life Underwater Page 22