into a tiny flower-like organism called a polyp. In some species, this sprouts, through branching twigs, into other polyps. They filter-feed with the aid of tiny beating cilia. Eventually, the polyps bud in a different way and produce miniature medusae which detach themselves and wriggle away to take up the swimming life once more.
Portugese man o’war (Physalia physalis) split level showing float and tentacles, Indo-pacific.
This alternation of form between generations has allowed all kinds of variations within the group. The true jellyfish spend most of their time as free-floating medusae with only the minimum period fixed to the rocks. Others, like the sea anemones, do the reverse. For all their adult lives they are solitary polyps, glued to the rock, their tentacles waving in the water ready to trap prey that may touch them. Yet a third kind are colonies of polyps but ones that have, confusingly, given up their attachment to the sea bottom and sail free like medusae. The Portuguese man o’war is one of these. Chains of polyps dangle from a float filled with gas. Each chain has a specialised function. One kind produces reproductive cells; another absorbs sustenance from captured prey; another, heavily armed with particularly virulent stinging cells, trails behind the colony for up to fifty metres, paralysing any fish that blunder into it.
It seems an obvious assumption that these relatively simple organisms appeared very early in the history of animal life, but for a long time there was no proof that they actually did so. Hard evidence could only come from the rocks. Even if microorganisms can be preserved in chert, it is difficult to believe that a creature as large but as fragile and insubstantial as a jellyfish could retain its shape long enough to be fossilised. But in the 1940s some geologists noticed very odd shapes in the ancient Ediacara Sandstones of the Flinders Ranges in southern Australia. These rocks, now thought to be about 650 million years old, were believed to be completely unfossiliferous. Judging from the size of the sand grains of which they are composed and the ripple marks on the surface of their bedding planes, they had once formed a sandy beach. Very occasionally, flower-like impressions were detected on them, some the size of a buttercup, some as big as a rose. Could these be the marks left by jellyfish stranded on the beach, baked in the sun and then covered by a wash of fine sand by the next tide? Eventually enough of these shapes were collected and studied for it to be undeniable that this is just what they must be.
Since then, other assemblages of living organisms of this extreme age have been discovered in many parts of the world – the Charnwood Forest in the heart of England, Namib Desert in southwest Africa, on the flanks of the Ural Mountains and the shores of the White Sea in Russia. But the most impressive and richest of all these discoveries have been made on the Avalon peninsula in Newfoundland. There the rocks, which are around 565 million years old, are exposed in dramatic cliffs. The strata have been tilted and folded, as one might expect in deposits of such extreme age, but not so severely that they have destroyed or even seriously distorted the fossils they contain. These are so abundant that in places it is impossible to walk over the exposed surface of a layer without treading on examples that any museum in the world would regard as one of its greatest treasures. They have been preserved in extraordinary perfection, seemingly by falls of volcanic ash from nearby volcanoes which buried them almost instantaneously, so creating what have been called death masks. There is a rich variety of shapes that are still being catalogued – spindles, fronds, discs, mats, plumes and combs, by far the richest record of any of the communities that flourished in the seas of the world during this extremely ancient period. Many seem to be unrelated to anything alive today and may perhaps be regarded as evolution’s failed experiments. One or two, however, bear at least a superficial resemblance to living marine creatures called sea pens that are still common today.
The name sea pen was given them when people wrote with quills, and very apt it must have seemed, for not only are they shaped like feathers but their skeleton is flexible and horny. They grow sticking up vertically on sandy seafloors, some only a few centimetres long, some half as tall as a man. At night they are particularly spectacular for they glow with a bright purple luminescence, and if you touch them, ghostly waves of light pulsate along their slowly writhing arms.
Sea pens are also called soft corals. Stony corals, their relatives, often grow alongside them and they too are colonial creatures. Their history is not as ancient as that of the sea pens, but once they had appeared, they flourished in immense numbers. An organism that produces a skeleton of stone and lives in an environment where deposits of ooze and sand are being laid down is an ideal subject for fossilisation. Huge thicknesses of limestone in many parts of the world consist almost entirely of coral remains and they provide a detailed chronicle of the development of the group.
The coral polyps secrete their skeletons from their bases. Each is connected with its neighbours by strands that extend laterally. As the colony develops, new polyps form, often on these connecting sections, and their skeletons grow over and stifle earlier polyps. So the limestone the colony builds is riddled with tiny cells where polyps once lived. The living ones form only a thin layer on the surface. Each species of coral has its own pattern of budding and so erects its own characteristic monument.
Corals are very demanding in their environmental requirements. Water that is muddy or fresh will kill them. Most will not grow at depths beyond the reach of sunlight for they are dependent upon single-celled algae that grow within their bodies. The algae photosynthesise food for themselves and in the process absorb carbon dioxide from the water. This assists the corals in the building of their skeletons, and releases oxygen which helps the corals respire.
The first time you dive on a coral reef is an experience never to be forgotten. The sensation of moving freely in three dimensions in the clear sunlit water that corals favour is, in itself, a bewitching and other-worldly one. But there is nothing on land that can prepare you for the profusion of shapes and colours of the corals themselves. There are domes, branches and fans, antlers delicately tipped with blue, clusters of thin pipes that are blood red. Some seem flower-like, yet when you touch them they have the incongruous scratch of stone. Often different coral species grow beside one another, mingled with sea pens arching above and beds of anemones that wave long tentacles in the current. Sometimes you swim over great meadows that consist entirely of one kind of coral; sometimes in deeper water, you discover a coral tower hung with fans and sponges that extends beyond your sight into depths of darkest blue.
Purple sea pen (Virgularia gustaviana) on sandy sea bed. Rinca, Indonesia.
But if you swim only during the day, you will hardly ever see the organisms that have created this astounding scene. At night, with a torch in your hand, you will find the coral transformed. The sharp outlines of the colonies are now misted with opalescence. Millions of tiny polyps have emerged from their limestone cells to stretch out their minuscule arms and grope for food.
Coral polyps are each only a few millimetres across, but, working together in colonies, they have produced the greatest animal constructions the world had seen long before humans appeared. The Great Barrier Reef, running parallel to the eastern coast of Australia for over 1,600 kilometres can be seen from the moon. So if, some 500 million years ago, astronauts from some other planet passed near the earth, they could easily have noticed in its blue seas a few new and mysterious turquoise shapes; and from them they might have guessed that complex life on earth had really started.
Table corals (Acropora spp.) on remote reef. Komodo National Park, Indonesia.
