Darwin's Island

by Steve Jones

  Barnacles have been passengers since long before ships sailed the oceans. Many creatures suffer their attentions. Humpback and grey whales bear large white patches of thousands, some of the individuals several centimetres across. The larvae pick up the scent of their host as they float through the sea and move towards it. Then they dig into the skin - and the huge beasts pay a price in energy as they drag their hangers-on through the sea. The whale retaliates with skin grown at a rate three hundred times that of our own in an attempt to slough off its passengers. Some marine mammals secrete enzymes that dissolve the glue and help keep their foes at bay, while grey whales come in to land to try to scrape the hitch-hikers off. Dolphins move fast and are safe from such visitors, who are washed off before they can fix on, but big sharks, who idle through the water, are also free of the pests. Shark skin is covered with tiny ridges - and a film has been developed that mimics its structure. It may find a place in the world of commerce.

  Other species of barnacle hitch lifts on the gills of fish, or live around the deep-sea vents that belch out hot rich water that nourishes a thick soup, just right for a filter-feeder. Not all cirripedes have a settled way of life, for some float blithely through the seas and never touch a solid object while yet others bore into coral reefs.

  The most aberrant kinds take up a sinister profession. From a whale’s or a matelot’s point of view, a barnacle is an irritant, but little more. For crabs faced with a marauding cirripede, the situation is far worse. A certain group live as parasites within their living bodies. Their macabre habits give an insight into the spectacular diversity of form that evolution can come up with when it generates variations upon a body plan.

  First, a female larva lands on its victim and finds a soft spot in the creature’s armour. Then she stabs it with a hollow needle and fires a few of her own cells through. She dies at once. The baleful blob finds its way to the lower part of the crab’s body and sends out fine tendrils that run through the host’s entire anatomy. They grow to make a mesh that looks more like a mould than a marine animal, and suck in food. The crab stays healthy and continues to eat as fast as it can to feed the visitor. When the time is ripe, the parasite opens up a small hole to the outside and awaits the arrival of a mate - a male larva. Should a male appear it inserts its spiny self through the hole and seals it up to prevent the entry of a rival. Now the crab is in real trouble. The male parasite fertilises his partner and she begins to pump out thousands of larvae. The victim’s whole economy is hijacked and it can no longer grow, shed its skin or even replace a damaged part. Instead, sometimes for years, it devotes its energies to its inner barnacle.

  Soon the crab, male or female, is spayed by the unwelcome visitor. A castrated male crab starts to look, and behave, just like a female. Both sexes now act as mothers - but mothers who care for another’s interests. They develop a pouch on the underside that resembles that made by a healthy female just before she releases her offspring and spreads them through the water with sweeps of her claws. The unfortunate crabs again wave a mass of newborns on their way, but now they are not their own progeny but barnacle larvae ready for the next target.

  The diversity of cirripede lives confused Victorian biologists, who could see no logic in their variety of shape and habit. Large parts of Darwin’s work turned, in the traditions of the time, on an attempt to understand how the various species are related to each other and to find where the group as a whole fits into the animal world. That pastime - taxonomy, as the science is called - was once little more than stamp-collecting, but for him it became the raw material for a deep insight into biology. His classification was based in part upon the solid plates that surround most settled barnacles and he persuaded himself that he could see a hint of order that reflected their ancient ties (even if he stayed confused by his Chilean burrower and by the parasites). The scheme has been much modified and a system based on the pattern of adult plates remains ambiguous.

  The new philately - molecular genetics - studies shared descent in DNA itself, rather than in what it makes. The confident assumption that mutations accumulate at a regular rate to give steady divergence generates a family tree of the group’s evolution. Well-dated fossils can, in principle, be used to measure how fast the changes happen, to give a molecular clock. The tree suggests that stalked forms came first and the others followed on. The double helix also hints that the protective plates emerged after a jointed-legged animal had taken the decision to settle down and wait for food to arrive rather than going out to hunt for it. Several of the classical groups, such as the naked barnacles that so confused Darwin, appear to be a mixed bunch with distinct origins, and even the rock-dwellers are an assorted lot. The deep-sea vent types and the parasites are each, in contrast, a group of true relatives. The genes also confirm his view that the cirripedes as a whole fall into the larger family of crabs and lobsters - the Crustacea - and into the wider clan of insects, spiders and other jointed-legged animals. Some claim, on the basis of their shared molecules, that insects themselves are no more than a specialised group of crustaceans that reached the land. If so, they reveal an unexpected unity between barnacles and butterflies.

  Whatever the details of their family connections, the diversity of cirripede life began long ago. Two of the great evolutionist’s books deal with their fossils. They are not, perhaps, the most riveting of his works but they make, nevertheless, a forceful case that today’s kinds descend from forms now long extinct. Darwin referred to modern times as the ‘Age of Barnacles’, and at least in terms of the number of species known he was right, for their fossils are not abundant and can be hard to identify because the plates fall apart after death. Cirripedes do not appear in the rocks in any numbers until the demise of the dinosaurs, sixty-five million years before the present. A few spots do reveal good evidence of their passage. The Red Crag deposits of East Anglia were laid down in the cool Essex seas of two million years ago. The rust-coloured rocks are still full of their protective plates, mixed in with snail shells and the teeth of the largest sharks ever to have lived. Further from home, impressive strata in the south of Spain record, in the mix of their cirripede species, the rise and fall of a vanished sea. Their petrified memorials also show that whale barnacles have been around for at least two million years, for a bed in Ecuador is filled with their remains as a hint that the whales once bred there, as they still do, just off the nation’s coast. A 164,000-year-old whale barnacle specimen from a human settlement in an African cave shows that our ancestors have long eaten those huge marine mammals. They scraped off the external parasites and may have cooked them.

  No more than a few very ancient specimens have been found. A fossil from three hundred million years ago looks rather like a modern barnacle. Another well-preserved remnant, found in Herefordshire, from a hundred million years earlier - about the time of the first land animals - resembles the larva of a modern cirripede and hints that the group was well into its stride by then. The low-growing forms found on rocks, abundant as they now are, emerged far later, perhaps no more than a hundred and forty million years before the present, when Archaeopteryx walked the Earth.

  Anatomy, genes and fossils each place the barnacles in close association with crabs and lobsters and in less intimate kinship with insects, spiders and more. That larger group makes its presence obvious early in the record, in the Cambrian, more than half a billion years before the present, the era in which life first left abundant evidence of its passing. Some of the mysterious creatures with bizarre body plans found just before that time and once claimed to represent a unique and vanished fauna may in fact have been crustaceans. A molecular clock of the whole group puts the origin of the barnacle lineage well back into the Cambrian, or perhaps earlier, even if no earlier remains have yet been found. If the clock can be trusted, the first cirripedes may have emerged as part of the vast outburst of diversity among jointed-legged animals from lobsters to insects, which began then and is still evident today.

  What sparked off the bar
nacle big bang? Why did they, like their crab and insect brethren, evolve into such diversity of form? And why did vertebrates, the group to which we and the barnacle goose belong, do the same many millions of years later? Backboned animals are less diverse in their body form than are cirripedes, but they include creatures as different as mackerel, toads, pythons and vultures. Why was their evolution, like that of barnacles, so radical while groups such as sponges or flatworms remained, in comparison, tediously conservative? The answer began to emerge from Darwin’s labours over the Down House microscope.

  Its owner was the first to identify a barnacle larva, from his strange shell-borer from Chile. As he dissected more and more species and examined their juvenile forms a great truth began to dawn: that the creatures were far more distinct from each other as adults than they were in their early stages. From Scottish rock-dweller to naked Chilean and from tasty marine snack to the sinister enemy of crabs, the juvenile forms of the various species were very similar. Even better, they looked quite like the equivalent phases in crabs and lobsters. Darwin’s excitement at this discovery is manifest: he writes of a larva ‘with six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennae’. He knew that he had hit upon a crucial piece of evidence for evolution (although his children laughed because the sentence read like a newspaper advertisement by a cirripede manufacturer).

  Most barnacles release thousands of tiny fertilised eggs into the sea. Each goes through a series of stages, in most cases as a form that floats free in the plankton. The first has jointed limbs attached to a soft and flattened body. The young animal has an eye spot, sensitive even to dim light, that allows it to choose the level at which it floats. Soon it develops jaws and antennae and starts to feed. It goes through several moults and in time becomes a strong-swimming form with a tough outer coat. Those mature larvae prefer to stay near the surface, do not eat and can be carried far from where they were born. They must find a place to settle down, or - as almost all do - they will die. Some stumble upon a rock, or a whale, or a crab, and glue themselves on with their antennae. The rock-or whale-dwelling species put out a chemical message - a protein hormone - that invites others to join the colony. For them, every visitor is welcome, for a male must land within penis-length of a female if he is to have a chance to pass on his genes and the more there are the better.

  Much as the first stages of many species might resemble each other as they float through the seas, some - like those that amused the Down House children - do have aberrant juveniles, adapted to their own special way of life. Those of the burrowers cannot swim but scuttle about on the bottom using their antennae as feet. Crab parasites have abandoned the first few stages altogether and hatch as jawed and hungry forms that search for new victims at once. Natural selection is at work on the larval stages, which have to adapt themselves to nature’s challenges just as grown-ups do. Even so, the young reveal far more about the group’s internal affinities than do the much-modified adults. They show how cirripedes and their relatives are based on a theme with variations.

  The same is true of the embryo on a wider stage. That of a barnacle goose is almost identical to the contents of a vulture egg and an embryonic human looks rather like that of a mouse or, indeed, if looked at early enough, of a goose. What emerges into the world is quite distinct from what can be seen as development begins. Now we understand why.

  Adult cirripedes apart from the crab parasites are - like lobsters and insects - arranged in obvious sections, with a head and a thorax divided into six segments, but they lack an abdomen, found in almost all their relatives. We do not often think of ourselves as segmented creatures, but the vertebrate body is, like that of a barnacle or a lobster, also based on a series of distinct units, arranged from front to back. The human head, thorax and abdomen are obvious enough but our muscles, or our brain-case, show little sign of order. A glance at the embryo, however, reveals that men and women, like their submarine relatives, are constructed from a series of modules, neatly arranged in early life but shuffled around and modified as growth proceeds.

  The remains of our watery past as primitive fish, together with the juvenile forms of our relatives among fish, snakes and birds say more. They show how the building blocks have multiplied and rearranged themselves to make the complicated creatures of today.

  Just three of the thirty or so major divisions of the animal world are organised in obvious segments; they include the worms, the jointed-legged creatures such as insects, spiders, lobsters and barnacles, and the animals with backbones. For all of them a subdivided way of life has been an evolutionary triumph.

  Segmented beings make their first appearance at - or even before - the first signs of the fossil record. They played a large part in the Cambrian explosion of diversity. Fossils from that time show how the addition of new pieces to a simple body, like beads on a string, can spark off a burst of change. Many of its strange animals were worm-like beasts, or had jointed legs and external skeletons. In time they added more and more sections. As they did, they evolved into a wild diversity of form. One ancient marine group, the trilobites (now extinct), started off with around eight segments. In time, some kinds ended up with a hundred and others with three. That process then, for some reason, reversed itself and at the peak of their success most trilobites had at most thirty-five separate elements.

  As Darwin noticed, barnacles and their relatives have been through the same process of increase, decrease and divergence. He persuaded himself that the archetypal crustacean, the ancestor of both cirripedes and lobsters, was based on twenty-one parts, divided among head, middle and abdomen. Many modern species have six elements in the head, six in the thorax (the middle part of the body) and five in the last, abdominal, section. Some have multiplied and modified particular elements while others have done the opposite. Lobsters, for example, have many more paired and jointed appendages - legs and swimmerets plus others used to mate or to help brood the young - than do crabs, while the barnacles themselves lack the whole rear segment of the body. They are the Manx cats of the crustacean world and, for that matter, are an excellent analogue of the first birds, which were dinosaurs who shook off their tails.

  Goethe - philosopher, scientist and author of Faust - had, well before the Beagle voyage, noticed hints of pattern within the bodies of fish, birds and mammals. He came up with a universal theory of anatomy, based on the notion that vertebrae - the individual sections of the backbone - were units from which many of our various parts were derived. The leaf, he imagined, had the same role in plants. Goethe saw life as emerging from a sort of biological Proteus; a simple component that could be multiplied and modified into a diversity of structures, the skull most of all. He was wrong in the details, but his idea contains an element of truth.

  Although the simplistic claim, never made by Darwin, that animals relive their ancient history as they develop from the egg is wrong, the embryo is a reminder of where we came from. The shift from fertilised egg - a formless ball of protoplasm - to man or woman looks complex but is in its basics simple. As in origami, a limited set of instructions persuades pattern to emerge from simplicity. As the embryo folds itself into being, its past unfolds before our eyes.

  Hints of order soon appear. A fertilised egg divides to form a ball of cells, which in time turns itself inside out and becomes attached to the wall of the uterus. It lengthens, and a ridge - which soon becomes a tube, the precursor of the spinal cord and brain - forms along the upper surface. The masses of tissue on either side then begin to break up into a series of evenly spaced blocks called somites. Those near the front appear first, and tissue stains show that ordered structures arranged from front to back are present long before the somites themselves become visible.

  The somites in their rows look simple, but they give rise to complex structures, some of which have no obvious hint of regularity; to vertebrae (which would have pleased Goethe), to ribs, to muscles of the back and the limbs, to skin
and tendons and even to certain blood vessels. The organised nature of vertebrae is obvious enough, but to the untutored eye the muscles of the leg or the skin on the back give no hint of segmentation. Even so, they - like many other organs - began as blocks of tissue.

  As development goes on, the front half of one somite fuses with the back of the somite ahead of it to form the precursors of vertebrae - the repeated units of the spine, the structure shared by fish, frogs, snakes, birds and humans. They surround the spinal cord with a protective and flexible sheath that solidifies as bone is formed. The process is controlled by special growth factors, which sometimes go wrong. That has an echo of Goethe, for after a failed attempt by the East Germans in the 1960s to conserve his corpse his body was stripped of flesh - and it was revealed that the great poet suffered from a debilitating fusion of several spinal bones.

  How can a uniform embryonic tissue break up into segments and then into distinct organs? In 1891, William Bateson - later the rediscoverer of Gregor Mendel’s work - came up with a ‘vibratory theory of the repetition of parts’: the notion that a flow of chemicals did the job. Just as waves on the sea create ripples on the sand, their equivalents in the body stamp order on to disorder. A century and more later, he was proved right.