Darwin was sent specimens from across the globe. Some would, he realised, stretch the belief of his fellows and he wrote to a colleague about his discoveries that ‘You will think me a Baron Münchausen among naturalists.’ His first job was to describe what the animals looked like. As ever, he told a simple story in plain prose.
His introductory paragraph is a sober account of what most people imagine such creatures to be: ‘Almost every one who has walked over a rocky shore knows that a barnacle or acorn-shell is an irregular cone, formed generally of six compartments, with an orifice at the top, closed by a neatly-fitted, moveable lid, or operculum. Within this shell the animal’s body is lodged; and through a slit in the lid, it has the power of protruding six pairs of articulated cirri or legs, and of securing by their means any prey brought by the waters within their reach. The basis is firmly cemented to the surface of attachment.’
That statement introduced the immense variety of cirripede lives. More than twelve hundred different kinds are known and no doubt many more remain to be discovered. All live in salt water. They fall into two main groups, those with a stalk (the goose barnacles, named in homage to the ideas of Sir Robert Moray, and a delicacy in many parts of the world) and those without, many of them, like the familiar acorn barnacle, attached to rocks and other marine structures. All, or almost all, have jointed legs, often tucked away within a shell. Many use them as a net to sweep the seas, while the stalked versions depend more on the movements of the water to bring food. Unlike their relatives the crabs and lobsters, barnacles do not moult their skeletons to grow. Instead their plates increase in size as the animal gets older. Some species sit on rocks, while other kinds burrow through solid stone or into snail shells or spend most of their time afloat. Yet more are parasites of crabs, jellyfish and starfish. Some among that group are so specialised that, when adult, they look more like a fungus than an animal.
Like insects, barnacles have a head and thorax and, in a few species, what might be the remains of an abdomen. Like them, they have six pairs of jointed legs, fewer than the prawns and lobsters, who have ten. Each leg is covered with hairs and together they lash the sea. The familiar shore versions spend their lives upside down for they stand on their heads and wave their feet in the water.
Those found on rocky shores live in a fortress made of around six tough plates, based, like a snail shell, on a limestone-like substance. Different varieties have more or fewer segments of body armour and many pages of Darwin’s four books on the creatures are devoted to the minutiae of how their plates might sort out their patterns of relationship. For the common British form, an additional two plates act as a lid, which opens to let out the legs at high tide and closes to keep in water when the creatures are exposed to the air (which for some individuals means all the time except for a few days each month at spring tide). The mouth has structures that chew and grind and look a little like those of crabs and even of cockroaches. Some species excrete through their mouths as their anus has faded away. Tucked away in the dark, the adult barnacles lose their eyes. The nervous system, too, is reduced when compared with that of their free-living relatives.
Dull as a cloistered existence within a gloomy fortress might be, all barnacles have a remarkable sex life. Like all good biologists, Darwin spent a lot of time on that topic. He found a wild diversity of reproductive habit. The textbooks of his day said that all known species were hermaphrodites but many, he found, were not. Some have two sexes, some are male when young and female later, and some are true hermaphrodites - while a few among that group secrete small males around their bisexual persons in case they might be useful. Many of those with two sexes spend their adult lives fixed to a single spot. As a result every male must constantly wave his penis, erected at the cost of lots of hydraulic energy, to reach out and tap his neighbours in the hope that at least one might be a female. Those who find themselves in a sparse and scattered group must, if they are to succeed, grow a longer organ than those who live in a crowd. A female, once tapped, may copulate with half a dozen males in series and then pump out most of their seminal fluid as not up to scratch. Her fertilised eggs soon develop into the first of several larval stages.
The young biologist’s studies on cirripede sex brought forth some poetic paragraphs. The male organ of a certain species was ‘wonderfully developed . . . it must equal between eight and nine times the entire length of the animal! . . . there [it] lies coiled up, like a great worm . . . there is no mouth, no stomach, no thorax, no abdomen, and no appendages or limbs of any kind’. In another the males were reduced to parasites within the female: ‘thus fixed & half embedded in the flesh of their wives they pass their whole lives & can never move again’. The creatures hold the record for relative penis size, while a certain beetle comes next, at twice its body length. The organ comes at a price, for individuals from wave-battered shores have shorter and stouter members than do those from calmer places as the male finds it hard to control a lengthy structure in a turbulent world. So expensive are such massive genitals that many males lose them at the end of each season and grow a new set the following year.
Barnacles are remarkable for reasons that stretch beyond the penis. They stick to the rock with a sophisticated cement, a protein that repels water. The stuff is the toughest known natural glue. Like an epoxy adhesive, the material is secreted as a clear fluid with two components. When they mix, cross-links are made between the molecules and its manufacturer becomes almost impossible to dislodge. So powerful is the bond that some of the substances involved may soon be used in surgery.
The creatures cling to ships just as avidly as they do to rocks. Charles Darwin, exhausted by his years of work on them, once wrote that ‘I hate a Barnacle as no man ever did before, not even a sailor in a slow-sailing ship’, and mariners had good reason to despise the animals. The
Beagle
herself had to have her bottom cleaned several times in the cruise around South America. A ship uses 40 per cent more fuel when covered with marine organisms than when its surface is smooth - which is expensive and, in these days of the greenhouse effect, also to be deplored on ecological grounds. Poisonous paints were once used to keep the bottom clean, but as many caused sea snails to change sex most of them have been banned. The best protection is to find a finish to which the animals cannot attach. Given that they can adhere to a non-stick saucepan, the job is not simple, although paints with added carbon nanotubes offer some hope. Some corals and seaweeds manage to stay free of such creatures not with poisons but with chemicals that scare them off. Those unknown substances will make the fortune of the first scientist to extract them.
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.