In the summer, my brothers, sister and I lived on the beach at the bottom of our terraced street, foraging, poking about in rock pools, making sandcastles or stone dams, levering shells off rocks to find sea creatures: creatures for dreams and for nightmares. We were always barefoot and the soles of our feet soon hardened to the sharp impress of the acorn barnacle shells that covered every surface below the tide line, millions of white cones – little volcanoes we called them – all different sizes growing over and under and on each other, competing for space, on rocks, on the wooden breakwater, on shells and driftwood. Five heads pressed together, ten feet waving in the air, our bellies on the warm sand, we’d drop a rock or a shell covered in the white cones into a bucket of sea water and wait. First, a tiny hatch opened at the top of each cone. Then a few seconds later, long, feather-like fans unfurled through the hole and began snatching at the water, rhythmically, like a pulse or a heart beat, all together. If we could have made the cone house invisible we would have seen its bizarre inhabitant, a cream-coloured shrimp-like creature, upside down, glued to the rock by its head, fishing for plankton through the hole in its cone with its feathery feet.

Barnacles take two principal shapes: the coned seashore barnacles and the stalked barnacles that cluster on driftwood. As a child I saw the stalked kind for the first time behind glass, not in an aquarium or a museum but in my grandfather’s wholesale food warehouse. His father had been a ship’s chandler on the east coast of Scotland, but the family business had moved south and was now supplying food to the restaurants and hotels of the south of England. The dimly lit, labyrinthine warehouse was another country: the smells of spices and oils, mountains of sugar and flour sacks, caves of bottled and tinned treasures, trapdoors, levers, pulleys and winches, a locked cold-room that poured out mist and in which hung the smoked carcasses of pigs.

We played hide-and-seek here. One day, seeking a new hiding place, steeling myself against imagined ghosts, I took the little back stairs that creaked up through a trapdoor into a dusty sunlit attic where the boxes of exotic foreign food were kept in jars in stacked boxes: bottled snails, frogs’ legs, okra and caviar. That day there was a case I hadn’t seen before. It was marked Perceves: twelve catering jars of pickled stalked barnacles from Portugal in a box. They looked like clawed fingers on the other side of the glass jar, one-inch prehistoric monsters with stalks the colour of glistening black elephant skins and a claw like the beak of a bird. I couldn’t imagine how anyone could want to eat such things or how they would do so. Snails were bad enough. It made my flesh creep. Why had God made such creatures; what were they for?

Thirty years later, a Portuguese waitress in a seafood restaurant overlooking a harbour wall in Viana do Castelo showed me how to deftly twist barnacle stalk from claw and pull out the slither of pink flesh hidden inside the black stalk. It tasted of the sea – mysterious, briny and a little gritty, like mussel flesh. I opened up the beak-like end on my plate, to see the little black creature inside, curled upside down, its feathered feet retracted. In the sea it would be dancing like its coned cousins, unfurled amongst plankton. Now my questions were different: how had it come to this strange shape? How had it come to be through unimaginable centuries of metamorphosis? How had it evolved?

By the time I ordered barnacles from the menu in Viana do Castelo, the bizarre creatures had become inseparable in my mind from Charles Darwin, for the great man, the author of one of the most groundbreaking books of all time, had also spent eight years collecting, dissecting, analysing and mapping barnacles. It was a passion – an obsession. It nearly killed him. But the final books, meticulously detailed, won him the Royal Society Medal in 1854 and established him as a scientist who had won his spurs. Without his barnacle spurs and barnacle contacts, On the Origin of Species by Natural Selection would have been very differently received.

The barnacle obsession which dominated all Darwin’s waking hours between 1846 and 1854 began with a discovery made on the Beagle voyage.

On a hot Chilean beach in January 1835 the twenty-four year-old Darwin picked up a conch shell covered in tiny holes, like lace, which he slipped into his pocket. Under the microscope in his Beagle cabin, with one of the holes illuminated by candlelight, he saw the squatter, the tiny cream-coloured curled creature that had dug the hole. What manner of beast was it? It looked for all the world like a barnacle, but Darwin knew that, according to the zoology text books, barnacles secrete their shelly homes; they don’t dig them. He teased out the creature from the base of the hole with a pin and then under the microscope examined its beautiful and complex anatomy. He checked the zoology books in the ship’s library just to be sure. It was indeed a barnacle anatomically, although there were unaccountable deviations from the barnacle’s archetypal norms. He didn’t know it at this point, but this barnacle, soon to be nicknamed Mr Arthrobalanus, would not be finished with him for a further twenty years. This was an encounter on a beach with a creature too small to see with the naked eye that would lead to eight years of meticulous dissection and observation of every known barnacle – fossil and living – in the world and to four published volumes with hundreds of pages of analysis.

Darwin was lucky. The barnacle he had found on the South American beach, with no cone-house or stalk, was an extremely rare form of burrowing barnacle. This squatter was highly unusual, an aberrant, because up until 1830 barnacles had been defined by the shape of their shell-houses not their soft bodies, and this one had no house of its own. For Darwin, anomalies like these raised all sorts of questions about the classification systems themselves. What makes a barnacle a barnacle? And when there is so much variation within a group like the barnacle – in terms of size, method of reproduction and life cycle – what common features hold the family together? His Chilean anomaly would help to explain barnacle evolution and adaptation.

Barnacles had colonised the shorelines, ships’ hulls and seabeds of the temperate world. They were ubiquitous. Yet they were as complex and unmapped as the Amazonian rainforest. No one had mapped the barnacle. Darwin would be the first to do so. It would be an evolutionary classification showing how, from a common ancestor, hundreds of different and spectacular barnacle adaptations had taken place over millions of years.

Darwin carried 1,529 species bottled in wine spirits back on the Beagle to London in 1836. Amongst these was a single bottle containing a dozen or so very rare minute South American barnacles, teased out from a conch shell and labelled Balanidae. He didn’t understand them yet, but he would come back to them later. The barnacle was unfinished business.

By 1844, when he was thirty-five, Darwin had formulated and sketched out his species theory in essay form – the incendiary ideas that would change human understanding of time and nature for ever. He had sealed this essay in an envelope, locked it away in a drawer in his study and put together a set of instructions to his wife on how to handle its publication in the event of his death. He now needed to flesh out the theory with evidence and careful rhetoric but, instead of doing this, he turned to the barnacle in the bottle on his study shelf – the riddle that needed to be unlocked. It would take him a month or so to solve it, he thought, and then he would complete and publish the species theory.

The baffling, ‘illformed’ creature he had found in the conch shell was the size of a pin, he remembered. It fitted barnacle body plans in some ways but was completely deviant in others. The more Darwin studied this creature, the more bizarre it appeared to be. How had it come to be this way? How did it fit into the barnacle order? Who were its near relatives?

Within days, Darwin realized that there was no way he could understand just how divergent his Chilean barnacle was until he had seen and mapped most if not all of the hundreds of varieties of barnacles clustered on rock pools, seabeds, driftwood and whale flanks around the world. He began to send for specimens, and once they began to arrive there was no going back; he was hooked. Two years later, his study piled high with barnacle specimens of all shapes and sizes labelled in pillboxes, he had committed himself to writing a definitive monograph. Slowly, he clawed his way back to the problem of the Chilean barnacle, now named Mr Arthrobalanus, via the microscopic examination of all the fossil and...