Scallop

• 1. Introduction

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  A scallop that senses a predator nearby does something few shellfish can do. It leaves. Scallops belong to the family Pectinidae, a group of marine bivalves found in every ocean, though never in fresh water. Most rest on sandy ground, and when a starfish approaches, they clap their shells together and jet away in erratic bursts.

  The animal doing this has no brain. Like every bivalve, it relies on clusters of nerve tissue scattered through its body. So how does a creature without a brain spot danger, steer, and survive? How did a family that appeared hundreds of millions of years ago spread so widely? And how did one humble shell end up on the coats of arms of popes and princes, on the road to Compostela, and in the name of a famous company?

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• 2. Mirror Eyes

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  Up to 200 small eyes, each about 1 mm across, ring the edge of a scallop's mantle. Few animals see the way they do. Instead of a lens, each eye uses a concave, parabolic mirror built from guanine crystals. The mirror focuses light and reflects it back across the retina.

  The retina itself has two layers. The outer layer responds most strongly to light, while the inner layer responds to sudden darkness. Together they cannot resolve sharp shapes, but they give the scallop exceptional contrast and a sense of shifting light and motion. The animal uses them mainly as an early-warning system, scanning for moving shadows. Some scallops even change how they swim or feed depending on how cloudy the water is, judging it from the drift of particles.

  Three pairs of yellowish ganglia take the place of a brain. The visceral pair is the largest, sitting as a nearly fused mass near the center of the body. Proportionally, it is the largest and most intricate set found in any modern bivalve. From it, nerves run out to a ring that loops around the mantle and connects every tentacle and eye. In some species that ring is so well developed that it could count as another ganglion.

  The cerebral ganglia lie a good distance above, linked to the visceral mass by long connectives. They run the mouth through the palp nerves and connect to statocysts, which help the animal sense its position. The pedal ganglia sit close together near the midline and serve the small muscular foot. Whatever these eyes report, the answer comes from a pair of muscles packed inside the shell.

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• 3. Clap and Glide

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  Two different muscles sit side by side inside a scallop, divided by a sheet of connective tissue. The striated one contracts very quickly and powers swimming. The smooth one has no striations and can stay contracted for long periods, holding the shell shut while spending very little energy. Like true oysters, scallops have a single central adductor, which leaves a round scar inside the shell. A scallop's version is larger and more developed than an oyster's, because scallops are active swimmers.

  Swimming and jumping send the animal in opposite directions. To swim forward, a scallop draws water in through the gape between its valves and pushes it out through small holes near the hinge. To jump backward, it expels the water the way it came in. A curtain-like fold of the mantle called the velum directs the flow and can abruptly change the course. A jumping scallop usually lands between contractions. A swimming one stays up for most of them, though seldom higher than 1 m above the seabed or farther than 5 m.

  The effort is enormous. Most scallops manage only four or five movements in a row before they are completely exhausted and need several hours of rest. One that lands on its left side can flip itself back onto its right using the righting reflex, another clapping motion. So-called singing scallops are rumored to make a soft popping sound underwater, although whether that happens is debated.

  Not every scallop swims. Chlamys distorta and Hinnites multirigosus cement themselves to hard ground, and the family sorts into six life habits. Pedum nestles, Crassadoma cements, Chlamys attaches by byssal threads, Pecten digs a recess, Argopecten lives free, and Amusium glides. Chlamys hastata often wears sponges on its shell. The sponges interfere with the grip of sea-star tube feet and keep barnacle larvae from settling. Barnacles are costly, since a scallop carrying them needs more energy to swim and goes shorter distances.

  Age also slows the escape. In older scallops the density of mitochondria in muscle falls, which lowers stamina and makes capture more likely.

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• 4. Spat and Spawn

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  Sex in this family is unusually flexible. Some scallops are separate males and females, some are hermaphrodites with both sexes in one body, and a few begin life as males and later become female. In species with separate sexes, the female's roe is red and the male's is white. Both are released freely into the water during mating season.

  Spawning usually happens in late summer and early autumn, with spring spawning possible in the Mid-Atlantic Bight. A female can produce hundreds of millions of eggs in a year. Fertilized eggs sink, and after several weeks they hatch into transparent miniature versions of the adult, called spat. The larvae drift among plankton for four to seven weeks. When they finally settle on the bottom, an event called spatfall, they usually grip the ground with byssal threads.

  Scallops reach sexual maturity at around the age of two but contribute little to egg production until four. Growth is fastest in the first few years. Shell height rises by 50-80 percent, while the weight of the meat quadruples. A scallop reaches commercial size at about four or five years old. The Atlantic bay scallop, Argopecten irradians, is short-lived, but others live 20 years or more, and the rings on the shell can be counted to estimate age.

  The seasons leave a mark on the muscle too. In Euvola ziczac, the adductor of summer scallops in May shows lower oxidative capacity than at any other time of year. Throughout all these stages the scallop feeds on plankton. It has no siphons, traps particles in mucus on a filtering structure, and sweeps them to its mouth with cilia.

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• 5. Deep Family Tree

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  About 240 million years ago, in the Middle Triassic, the family Pectinidae began. The oldest true scallops, those with a ctenolium, appear in Triassic rocks more than 200 million years old. The ctenolium is a comb-like structure beside the byssal notch of the right valve, and the Latin word pecten, meaning comb, gave the family its name. Many scallops lose it as they become free-swimming adults, but every one has it at some point. No other bivalve has anything like it.

  The direct ancestors were the Entoliidae, scallop-like bivalves with ears and a byssal notch only when young. They had no ctenolium, and neither did the Aviculopectinidae, another putative ancestor. The earliest true scallops split into two groups. Pleuronectis von Schlotheim, 1820, had a nearly smooth exterior, while Praechlamys Allasinaz, 1972, had radial ribs and ears.

  The fossil record shows dramatic swings. In the Mesozoic era, Pectinidae was the most diverse bivalve family, yet it nearly vanished by the end of the Cretaceous. The survivors then branched into many new species during the Tertiary. Nearly 7,000 species and subspecies names have been proposed for fossil and living scallops. Today the family holds over 300 living species in 60 genera. Raines and Poppe listed nearly 900 names, but most are questionable or invalid.

  Sorting this family has been hard. Waller's studies in 1986-1991 and 1993 brought some order by naming three subfamilies, Camptonectinidae, Chlamydinae and Pectininae. A 2012 cladogram by Yaron Malkowsky and Annette Klussmann-Kolb used mitochondrial and nuclear gene markers. A 2008 paper by Puslednik and colleagues found that similar shell shapes among gliding scallops evolved repeatedly, not once. The Spondylidae and the glass scallops of the Propeamussiidae both sit beside Pectinidae on the tree. Yet nearly all earlier molecular work leaned on mitochondrial data alone, which can give a misleading picture of the true species tree.

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• 6. Boom and Bust

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  The largest wild scallop fishery targets the Atlantic sea scallop, Placopecten magellanicus, off the northeastern United States and eastern Canada. Boats harvest them with scallop dredges or bottom trawls. Most of the world's remaining production comes from Japan, through wild, enhanced and farmed stocks, and from China, mostly cultured Atlantic bay scallops. In the D'Entrecasteaux Channel in southern Tasmania, dredging was banned in 1969, and divers have collected them there since. A trial with lighted pots meant for lobster and crab showed that the lights also drew in scallops.

  New Zealand shows how fast a fishery can fall. The catch dropped from 1246 tonnes in 1975 to just 41 tonnes in 1980, and the government ordered the fishery closed. Seeding the seabed with spat in the 1980s helped, and catches in the 1990s reached 684 tonnes. Tasman Bay was shut to commercial harvest from 2009 to 2011 after numbers fell again. By 2015 the commercial catch was only 22 tonnes, and the fishery closed once more. The suspected causes include fishing, climate effects, disease, pollutants, and sediment washed off farms and forests. The conservation group Forest and Bird lists scallops as the Worst Choice in its Best Fish Guide.

  On the east coast of the United States, bay scallops have shrunk in number over the last 100 years. The likely main cause is the loss of seagrass, where their spat attach, due to coastal development and nutrient runoff. Overfishing of sharks may also play a part. Sharks once fed on rays, and rays are the main predators of bay scallops, so fewer sharks means more rays eating scallops. The Atlantic sea scallop tells a different story. After recovering from overfishing, it now stands at historically high levels of abundance.

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• 7. On the Plate

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  Every scallop shell holds two textures. The meat, called the scallop, is firm and white. The roe, called the coral, is soft and often reddish-orange. Outside the United States, scallops are often sold whole, and in the UK and Australia they come with or without the coral. Sellers also separate scallops by treatment. Dry-packed ones have no additives. Wet-packed ones are treated with sodium tripolyphosphate, which makes them absorb moisture before freezing and so adds to their weight. The freezing itself takes about two days.

  In France, cooks often sear them in a hot buttered pan, sometimes with calvados, and serve them with creamed leeks or in a white wine sauce. Galician cooks bake them with breadcrumbs, ham and onions. In Scotland, king scallops are a delicacy, usually pan-fried and served with black pudding or bacon. In the northeastern United States they are breaded and deep-fried with coleslaw and french fries, sometimes on a fisherman's platter. New England restaurants also offer scallop rolls, with breaded scallops on a grilled, split-top hot dog bun.

  Japanese cooks serve them in soup, as sashimi, or on rice as sushi, where hotategai is the traditional scallop. Cantonese cuisine knows the dried form as conpoy. Smoked scallops show up as appetizers and as an ingredient in other dishes.

  The word scalloped began here. It first described seafood creamed and served hot in the shell. Today it names a creamed casserole such as scalloped potatoes, which contains no seafood. The wavy edge of the shell also gave its name to a decorative pattern on furniture and fabric.

  Scallops do sometimes make pearls, though they lack the translucent nacre layers that make feather oyster pearls valuable. They tend to be small, dull and uneven in color, but a few are prized for their looks.

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• 8. The Pilgrim's Shell

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  At Compostela, medieval Christians collected a scallop shell as proof they had made the journey. The shell is the traditional emblem of St James the Great and of the Way of St James, the Camino de Santiago. The link most likely traces to a legend in which the apostle rescued a knight covered in scallops. A second version says that as the ship carrying his remains from Jerusalem to Galicia neared land, a wedding was under way on shore. The groom's horse spooked at the ship and plunged into the sea. By miraculous intervention, horse and rider rose from the water alive, covered in seashells.

  Language preserves the connection. In French the mollusc and its cream-sauce preparation are both called coquille St. Jacques. In German they are Jakobsmuscheln, literally James's shellfish. The Mediterranean scallop is named Pecten jacobeus, while the larger scallop native to Galicia is Pecten maximus. At St James' Church in Sydney, shells appear in several places, including the mosaics on the chancel floor. A shell for James is shown with its convex outside up. A shell for the goddess Venus shows the concave inside.

  That goddess appears with a scallop in Botticelli's 15th century painting The Birth of Venus. Since antiquity, hinged shells have stood for the feminine principle. One legend even says the Way of St James was a fertility pilgrimage for couples wanting children, and that pagans carried the shell first. Another reading compares the shell to the setting sun in pre-Christian Celtic ritual. In that account the route led west to Finisterra, the End of the World, on the Coast of Death, beside the Sea of Darkness.

  Heraldry carried the shell onward. It began as a badge of those who had gone to Compostela and later stood for pilgrimage in general. Scallops appear in the arms of the Spencer family, of Prince William and Prince Harry, of Pope Benedict XVI, and of John Wesley, which is why the shell became an emblem of Methodism. Sometimes it is only a pun on a name, as with Jacques Coeur. The Shell company is represented by a scallop, and in 1988 the State of New York chose the bay scallop as its state shell.

  On the beach at Aldeburgh in Suffolk, a steel sculpture by Maggi Hambling called The Scallop was erected in 2003. It memorializes the composer Benjamin Britten, who had a long association with the town.

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