Life on Earth
Other molluscs, such as mussels and clams, have shells divided into two valves lke those of a brachiopod and thus are known as bivalves. These creatures are much less mobile. The foot is reduced to a protrusion that they use to pull themselves down into the sand. For the most part, they are filter feeders, lying with valves agape, sucking water in through one end of the mantle cavity and squirting it out through a tubular siphon at the other. Since they do not need to move, great size is no disadvantage. Giant clams on the reef may grow to be a metre long. They lie embedded in the coral, their mantles fully exposed, a zigzag of brilliant green flesh spotted with black, which pulsates gently as water is pumped through it. They can certainly be quite big enough for a diver to put his foot into, but he would have to be very incautious indeed to get trapped. Powerful though the clam’s muscles are, it cannot slam its valves shut. It can only heave them slowly together, and that gives plenty of notice of its intentions. What is more, even when the valves of a really large specimen are fully closed, they only meet at the spikes on the edge. The gaps between them are so big that if you plunge your arm through into the mantle, the clam is quite unable to grip it – though the experiment is a little less unnerving if it is tried first with a thick stick.
Some filter feeders like the scallops do manage to travel – by convulsively clapping their valves together and so making curving leaps through the water. By and large, however, adult bivalves live rather static lives and the spreading of the species into distant parts of the seabed is carried out by the young. The molluscan egg develops into a larva, a minuscule animated globule striped with a band of cilia, which is swept far and wide by ocean currents. Then, after several weeks, it changes its shape, grows a shell and settles down. The drifting phase of its life puts it at the mercy of all kinds of hungry animals, from other stationary filter feeders to fish, so in order that its species can survive, a mollusc must produce great numbers of eggs. And indeed it does. One individual may discharge as many as 400 million.
One branch of the molluscs, very early in the group’s history, found a way of becoming highly mobile and yet retaining the protection of a large and heavy shell – they developed gas-filled flotation tanks. The first such creature appeared about 500 million years ago. Its flat-coiled shell was not completely filled with flesh as is that of a snail, but had its hind end walled off to form a gas chamber. As the animal grew, new chambers were added to provide sufficient buoyancy for the increasing weight. This creature was the nautilus, and we can get an accurate idea of how it and its family lived because a few nautilus species, just like Lingulella and Neopilina, have survived to the present day.
A nudibranch (Eubranchus tricolor) on the seabed of a Scottish loch.
One of these species, the pearly nautilus, grows to about 20 centimetres across. A tube runs from the back of the body chamber into the flotation tanks at the rear so that the animal can flood them and adjust its buoyancy to float at whatever level it wishes. The nautilus feeds not only on carrion but on living creatures such as crabs. It moves by jet propulsion, squirting water through a siphon in a variation of the current-creating technique developed by its filter-feeding relatives. It searches for its prey with the help of small stalked eyes and tentacles that are sensitive to taste. Its molluscan foot has become divided into some ninety long grasping tentacles which it uses to grapple with its prey. In the centre of them it has a horny beak, shaped like that of a parrot and capable of delivering a lethal, shell-cracking bite.
About 400 million years ago, after some 100 million years of evolution, the nautiluses gave rise to a variant group with many more flotation chambers to each shell, the ammonites. These became much more successful than their nautilus relatives, and today their fossilised shells can be found lying so thickly that they form solid bands in the rocks. Those of some species grew as big as lorry wheels. When you find one of these giants embedded in the honey-coloured limestones of central England or the hard blue rocks of Dorset, you might think that such immense creatures could do little but lumber massively across the seabed. But where erosion has removed the outer shell, the elegant curving walls of the flotation chambers that are revealed remind you that these creatures may well have been virtually weightless in water and able, like the nautilus, to jet-propel themselves at some speed through the water.
About 100 million years ago, the ammonite dynasty began to dwindle. Perhaps there were ecological changes that affected their egg-laying habits. Maybe new predators had appeared. At any rate, many species died out. Other lines gave rise to forms in which the shells were loosely coiled or almost straight. One group took the same path as the sea slugs did in more recent times and lost their shells altogether. Eventually all the shelled forms except the pearly nautilus disappeared. But some shell-less ones survived and became the most sophisticated and intelligent of all the molluscs, the squids and cuttlefish and the octopus. These are the cephalopods.
The relics of the cuttlefish’s ancestral shell can be found deep within it. This is the flat leaf of powdery chalk, the cuttlebone, that is often washed up on the seashore. The octopus has no trace of a shell even within the flesh of its body, but one species, the argonaut, secretes from one of its arms a marvellous paper-thin version shaped very like a nautilus shell but without chambers, which it uses not as a home for itself but as a delicate floating chalice in which to lay its eggs.
The squid and cuttlefish have many fewer tentacles than the nautilus – only ten – and the octopus, as its name makes obvious, has only eight. Of the three creatures, the squids are much the more mobile and have lateral fins running along their flanks which undulate and so propel the animal through the water. All cephalopods can, like the nautilus, use jet propulsion on occasion.
Several nautilus (Nautilus pompilius) on a coral reef at night, Pacific.
Cephalopod eyes are very elaborate. In some ways they are even better than our own, for a squid can distinguish polarised light, which we cannot do, and their retinas have a finer structure, which means, almost certainly, that they can distinguish finer detail than we can. To deal with the signals produced by these sense organs they have considerable brains and very quick reactions.
Some squids grow to an immense size. The aptly named colossal squid lives in the seas around Antarctica. It can reach nearly 100 kilos in weight and measure six metres from the end of its body to the tip of its outstretched tentacles. Its rival for the claim to be the largest species of all is the giant squid. The biggest so far discovered have in fact been slightly smaller and substantially lighter. Although there are records of even larger specimens of this species, it seems that these were not accurate. Nevertheless, we are unlikely to have discovered the biggest individuals of either species, so the record may yet be broken. The eyes of these huge cephalopods are even larger than might be expected. The biggest recorded were 27 centimetres across and are the largest known eyes of any kind of animal, five times bigger, for example, than those of the blue whale. Why the squid should have such gigantic eyes is a mystery.
It could be, however, that they need extremely sensitive eyes to detect the presence of their great enemy – the sperm whale. Squid beaks are often found in the stomachs of sperm whales, and their heads often carry circular scars with diameters that match a squid’s suckers. So there seems little doubt that squids and whales regularly fight in the dark depths of the ocean. Maybe the squids’ huge eyes help them to detect the presence of the only animal big enough to hunt them.
The intelligence of all the cephalopods – octopus, squid and cuttlefish – is well known. Octopus have been observed disguising themselves from an approaching enemy by covering themselves with shells or picking up two halves of a coconut and hiding within. Many species in all three groups have an extraordinary ability to change their colour and shape. They can camouflage themselves by matching almost any environment and also signal to one another with patterns and shapes that sweep across their bodies. A female squid has even been filmed signalling to a male lying alongside her that she is not ready to mate, while at the same time displaying a pattern on the other side of her body to summon another male. Octopus and squid, two of the most advanced animals in the ocean which least resemble human beings, are among the few, it seems, that can rival mammals in their intellectual abilities.
But what of the second great category of animals without backbones, the one represented in ancient rocks by the flower-like crinoids? As these are traced upwards through the rocks, they become more elaborate and their fundamental structure becomes clearer. Each has a central body, the calyx, rising from a stem like the seedhead of a poppy. From this sprout five arms which, in some species, branch repeatedly. The surface of the calyx is made up of closely fitting plates of calcium carbonate, as are the stems and branches. Lying in the rocks, the stems look like broken necklaces, their individual beads sometimes scattered, sometimes still in loose snaking columns, as though their thread had only just snapped. Occasionally gigantic specimens are found with stems 20 metres long. These creatures, like the ammonites, have had their day, but a few species, sometimes called sea lilies, still survive in the ocean depths.
Bigfin squid (Sepioteuthis lessoniana) hovering in open water above a coral reef at night. Dampier Strait, Raja Ampat, West Papua, Indonesia. Tropical West Pacific Ocean.
Crinoid (feather star, centre) on a gorgonian (sea fan, red) with a Dendronephthya soft coral in the background, Andaman Sea, Thailand.
Sea lilies show that the calcium carbonate plates, in life, are embedded just under the skin. This gives their surface a curious prickly feel. In other families, related to the crinoids, the skin has spines and needles attached to it so the creatures are known as echinoderms, ‘spiny-skins’. The basic module on which the echinoderm body is built has a fivefold symmetry. The plates on the calyx are pentagons. Five arms extend from it, and all the internal organs occur in groups of five. Their bodies work by a unique exploitation of hydrostatic principles. Tube feet, each a thin tube ending in a sucker and kept firm by the pressure of water within, wave and curl in rows along the arms. The water for this system circulates quite separately from that in the body cavity. It is drawn through a pore into a channel surrounding the mouth and circulated throughout the body and into the myriads of tube feet. When a drifting particle of food touches an arm, tube feet fasten on to it and pass it on from one to another until it reaches the gutter that runs down the upper surface of the arm to the mouth at the centre.
Tube feet of a red cushion sea star (Oreaster reticulatus), Singer Island, Florida.
Though stalked sea lilies were the most abundant crinoid in fossil times, the commonest forms today are the feather stars. Instead of stalks, they have a cluster of curling roots with which they attach themselves to coral or rocks. In places on the Great Barrier Reef, they swarm in huge numbers, covering the floor of the tidal pools with a tufted coarse carpet of brown. When disturbed, however, they can suddenly swim away, writhing their five limbs like Catherine wheels.
The fivefold symmetry and the hydrostatically operated tube feet are such distinctive characteristics that they make other echinoderms very easy to recognise. The starfish and their more sprightly cousins, the brittle stars, both possess them. These creatures appear to be crinoids that have neither stalk nor rootlets and are lying in an inverted position with their mouths on the ground and their five arms outstretched. Sea urchins too are obviously related. They seem to have curled their arms up from the mouth as five ribs and then connected them by more plates to form a sphere.
The sausage-like sea cucumbers that sprawl on sandy patches in the reef are also echinoderms, although in most species their shelly internal skeletons are reduced to tiny structures beneath the skin. Most lie neither face up nor face down, but on their sides. At one end there is an opening called the anus, though the term is not completely appropriate for the animal uses it not only for excretion but also for breathing, sucking water gently in and out over tubules just inside the body. The mouth, placed at the other end, is surrounded by tube feet that have become enlarged into short tentacles. These fumble about in the sand or mud. Particles adhere to them and the sea cucumber slowly curls them back into its mouth and sucks them clean with its fleshy lips.
One highly specialised deep-sea sea cucumber, called a sea-pig, lives in the mud of the deep seabed at depths of up to 5,000 metres. They are rotund little creatures about 15 centimetres long and have large tube-like structures on their underside with which they rootle about in the mud. They have been filmed in the deep sea, assembled in herds, perhaps for reproduction or attracted by the smell of a new source of food drifting down from the surface.
If you pick up a sea cucumber, do so with care, for they have an extravagant way of defending themselves. They simply extrude their internal organs. A slow but unstoppable flood of sticky tubules pours out of the anus, fastening your fingers together in an adhesive tangle of threads. When an inquisitive fish or crab provokes them to such action, it finds itself struggling in a mesh of filaments while the sea cucumber slowly inches itself away on the tube feet that protrude from its underside. Over the next few weeks it will slowly grow itself a new set of entrails.
The echinoderms may seem, from a human point of view, to be a blind alley of no particular importance. Were we to imagine that life was purposive, that everything was part of a planned progression due to culminate in the appearance of the human species or some other creature that might rival us in dominating the world, then the echinoderms could be dismissed as of no consequence. But such trends are clearer in the minds of people than they are in the rocks. The echinoderms appeared early in the history of life. Their hydrostatic mechanisms proved a serviceable and effective basis for building a variety of bodies, but were not susceptible in the end to spectacular development. In the areas that suit them, they are still highly successful. A starfish on the reef can crawl across a clam, fasten its tube feet on either side of its gape and slowly wrench the valves apart to feed on the flesh within. The crown-of-thorns starfish occasionally proliferates to plague proportions and devastates great areas of coral. Crinoids are brought up in trawls from the deep sea several thousand at a time. If it is improbable that any further major developments will come from this stock, it is also unlikely, on the evidence of the last 600 million years, that the group will disappear as long as life remains possible at all in the seas of the world.
Panamic cushion sea stars (Pentaceraster cumingi) group on seafloor, Galapagos Islands.
The third category of creatures on the reef contains those with segmented bodies. In this instance, we do have fossil evidence of even earlier forms than the trilobites found in the Moroccan hills. The Ediacaran deposits in Australia which contain the remains of jellyfish and sea pens also preserve impressions of segmented worms. One species, a 5-centimentre-long animal named Spriggina after Reg Spriggs who first discovered the Ediacara fossils, has a crescent-shaped head and up to forty segments, fringed on either side by leg-like projections. What exactly it was, nobody can agree. No legs have been identified, but this may be a limitation in the process of fossilisation. Some scientists think it may represent a completely extinct group. One widely accepted possibility is that it was a kind of annelid worm related to the bristle worms that are so common on a reef and the earthworms that you can find in your garden.
Annelids have grooves encircling their body that correspond to the internal walls that divide its interior into separate compartments. Each of these is equipped with its own set of organs. On the exterior and on either side, there are leg-like projections sometimes equipped with bristles, and another pair of feathery appendages through which oxygen is absorbed. Within its body, each segment has a pair of tubes opening to the exterior from which waste is secreted. A gut, a large blood vessel and a nerve cord run from front to end through all the segments, linking and coordinating them.
Fossils can only tell us so much. Even the exceptionally well-preserved remains of Ediacara offer no clue about the connection between the segmented worms and the other early groups. However, there is one further category of evidence to be looked at – the larvae. The segmented worms have spherical larvae with a belt of cilia round their middles and a long tuft on top. These are almost identical to the larva of some molluscs, a strong indication that back in time the two groups sprang from common stock. The echinoderms, on the other hand, have a larva that is quite different, with a twist to its structure and winding bands of cilia around it. This group must have separated from the ancestral flatworms at a very early stage indeed, long before the split between the molluscs and the segmented worms. Geneticists, analysing the DNA of each of these groups, now confirm these deductions and reveal that there are two major groupings of bilaterally symmetrical animals. Octopus, crabs and flatworms form one group, while echinoderms, tunicates and all the backboned animals make up the other.
Segmentation may have developed as a way of enabling worms to increase their efficiency as burrowers in mud. A line of separate limbs down each side is clearly a very effective structure for this purpose and it could have been acquired by repeating the simple body unit to form a chain. The change must have taken place long before Ediacaran times, for when those rocks were deposited the fundamental invertebrate divisions were already established The Ediacaran fossils, in Australia where they were first discovered and in Britain, Newfoundland, Namibia and Siberia, now confirm these deductions. Thereafter their history remains virtually invisible for a 100 million years. Only after this vast span do we reach the period, 600 million years ago, represented by the Moroccan deposits and others throughout the world. By that time many organisms had, as we have seen, developed shells from which we can deduce their existence and shape, but not much more.
However, there is one exceptional fossil site dating from only a little later than those of Ediacara that provides far more detailed information about the bodies of animals than can come from mere shells. In the Rocky Mountains of British Columbia, the Burgess Pass crosses a ridge between two high snowy peaks. Close to its crest lies an outcrop of particularly fine-grained shales, and in these have been discovered some of the most perfectly preserved fossils in the world. The shales were laid down about 530 million years ago, close to the beginning of the Cambrian period in a basin of the seafloor at a depth of about 150 metres. It must have been sheltered by a submarine ridge, for there were no currents to disturb the fine sediments on the floor or to bring down oxygenated water from nearer the surface. Few animals lived in those dark stagnant waters. There are no signs of tracks or burrows. Once in a while, however, mud from the ridge above slipped down in a turbid cloud, carrying with it all kinds of small creatures, and dumped them there. Since there was neither oxygen to fuel the processes of decay nor any scavenging animals to feed on the bodies and destroy them, many of the tiny carcasses remained complete as the settling mud particles slowly entombed them, preserving even their softest body parts. Eventually the entire deposit became consolidated into shale. Earth movements elevated and folded great areas of these marine deposits during the building of the Rocky Mountains. Many parts of them were distorted and crushed until most traces of life in them were obliterated. But miraculously, this one small patch survived virtually undamaged.
Velvet worm (Peripatus novaezealandiae). Velvet worms are known as ‘living fossils’, having remained the same for approximately 570 million years.
The range of creatures it contains is far wider than that found in rocks of a similar age at any other site so far discovered. There are the jellyfish that Ediacara would lead us to expect. There are echinoderms, brachiopods, primitive molluscs and half a dozen species of segmented worms – further representatives of the lineage that stretches from the beaches of Ediacara to the Barrier Reef of today.
There are also several creatures which were rather more mysterious. Among the most abundant of these was a strange segmented creature with what seemed to be a line of legs on its underside. It looked rather like a shrimp, though mysteriously none of the species had a head. It was given the name Anomalorcaris: strange shrimp. There were also small disc-shaped fossils marked with lines radiating from its centre that looked somewhat like a tiny slice of pineapple, which was initially thought to be some kind of jellyfish. Perhaps strangest of all, there was an elongated segmented animal that appeared to have seven pairs of spiny stilt-like legs, and seven flexible tentacles along its back, each ending in a tiny mouth. It seemed so strange as to be almost nightmarish, and the researcher who studied it accordingly called it Hallucigenia.
Subsequent work, however, showed that these oddities were not the founder members of some wholly unsuspected animal groups. A very exceptional specimen of Anomalocaris showed that the ‘strange shrimps’ were not complete animals but just the forelimbs belonging to a much bigger creature that used them to grab its prey. And the pineapple slice was eventually shown to have in its centre minute teeth. It was a mouth that belonged to the same animal as the tentacles. Both these pieces of Anomalorcaris’ body apparently had a more heavily strengthened exoskeleton and so regularly became separated from the animal’s more easily decayed body. As for Hallucigenia, further research on other specimens showed that it had been reconstructed in an upside-down position. The spindly legs were in fact protective dorsal spines, and what had been considered tentacles were in reality its legs. It is now thought that it may be the first known member of a strange group called the lobopods which today includes odd little creatures called velvet worms.
The great variety of creatures in the Burgess Shales is a reminder of how incomplete our knowledge of all fossil faunas actually is. The ancient seas contained many more kinds of animals than we can ever know. In this one site, conditions allowed a uniquely large proportion to be preserved, but even this is only a hint of what must have once existed.