Lamprey

• 1. Introduction

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  The lamprey is a jawless fish, and its adult mouth is a funnel of teeth that works as a sucker. The name probably comes from the Latin lampetra, which may mean stone licker. Scholars admit that the etymology is uncertain. People sometimes call these animals lamprey eels, which is inaccurate.

  About 38 species live today, and roughly seven more are known only from fossils. Together they raise a handful of puzzles. How did a fish with no jaws survive for so long? What happens in the mud and gravel where it hides? Why do cooks, brain scientists and fisheries agencies all keep a close eye on the same animal?

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• 2. Ancient Lineage

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  Priscomyzon, the oldest known lamprey, comes from South Africa and dates to the very end of the Devonian, around 360 million years ago. Its relatives Pipiscius, Mayomyzon and Hardistiella left fossils in the Carboniferous rocks of North America. These early stem-lampreys were small next to living species. Their oral discs held a few teeth arranged in rays, but not the heavy toothed discs with plate-like laminae that later lampreys carried. They may have scraped algae off animals rather than preying on them. Their young also looked like miniature adults, with no distinct larval stage.

  In the Carboniferous of France, the fossil Myxineidus is usually called a hagfish. Some studies, however, classify it as a lamprey.

  About 163 million years ago, in the Middle Jurassic of China, Yanliaomyzon appeared with the specialised toothed disc that modern lampreys use. Researchers think it was a predator and probably passed through a larval stage like its descendants. Mesomyzon, from the Early Cretaceous of China, clearly shows that larval stage. One study placed it closer to the large northern family than to other living lampreys. Other studies put it outside the group that contains every living lamprey.

  Genetic evidence points to hagfish as the closest living relatives of lampreys. These are the only other jawless fish alive today, and the pair form the superclass Cyclostomi. That reverses an older arrangement, which treated hagfish as the sister of true vertebrates. Lampreys have a skeleton of cartilage. In place of true vertebrae they carry arcualia, small cartilaginous structures arranged above the notochord.

  Three families hold all living species. Geotriidae and Mordaciidae are small and southern, while Petromyzontidae is large and northern. Studies disagree about when the last common ancestor of living lampreys existed. One points to about 175 million years ago, in the Middle Jurassic, and ties the northern and southern split to the breakup of Pangea. Another favours the Late Cretaceous and suggests that modern lampreys emerged in the Southern Hemisphere. Most of today's diversity appeared in the Cenozoic, especially within the last 10-20 million years.

  For most of the 20th century, two spellings of the order's name circulated, one with a t and one without. Even the same author sometimes used both in later publications. In the mid-1970s the ICZN was asked to settle the matter, and after much debate it resolved the question by voting. The spelling with a t won in 1980. From 1981, every higher-level group built on the genus Petromyzon has had to begin with Petromyzont.

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• 3. Mud, Sea and Spawn

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  Up to eight years can pass while a lamprey lives as an ammocoete, the larva that hatches from eggs laid in a nest of sand, gravel and pebbles in a clear stream. The young drift downstream with the current until they find soft, fine sediment in a silt bed. There they burrow in and live as filter feeders, collecting detritus, algae and tiny organisms. Ammocoetes grow from about three to four inches to around eight inches. The Arctic lamprey may spend only one to two years in this stage.

  Their eyes are underdeveloped, yet they can still tell light from dark. Many species change color through the day, turning dark in daylight and pale at night. Light-sensitive cells in the skin, mostly in the tail, help the larva stay buried.

  The water that crosses an ammocoete's feeding apparatus moves at the lowest rate recorded in any suspension-feeding animal. That slow flow demands rich water. Most invertebrate suspension feeders do well below 1 mg of suspended organic solids per litre. Ammocoetes need at least 4 mg per litre, and their habitats have been measured at up to 40 mg per litre. A muscular flap near the jaws, called the velum, drives the current toward the mouth. It serves both feeding and breathing.

  Metamorphosis usually lasts three to four months, and the animal eats nothing during it. The gallbladder and biliary tract disappear, and the endostyle turns into a thyroid gland. Some species never leave fresh water. They spawn and die soon after they transform. Many others are anadromous. These begin preying on animals while still swimming downstream, as soon as metamorphosis has given them eyes, teeth and a sucking mouth. They may spend up to four years at sea.

  Back in fresh water, the adults build nests called redds by moving rocks. Sexually mature males carry a ridge of fat cells near the front dorsal fin that produces heat. After a male draws in a female with pheromones, she feels that warmth through body contact, and it encourages her to spawn. The male winds around her and fertilizes the eggs as they are released. A female may release thousands of eggs, sometimes up to 100,000. Lampreys breed only once, and both parents die after fertilization.

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• 4. Built to Latch

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  Only 18 species of lamprey live as predators. They bore into the flesh of other fish, or in rare cases marine mammals, to eat flesh or drink blood. One of the 18, Caspiomyzon, is suggested to feed on carrion instead of live prey. Nine of them migrate from salt water to fresh water to breed, and some of these also have freshwater populations. The other nine live only in fresh water. Every species that does not hunt lives in fresh water too, and the adults of those species never eat. They live on reserves stored while they were ammocoetes. Attacks on people have been documented, but a lamprey generally goes after humans only when starved.

  The mouth does two jobs. Suction holds the animal to a stone or to its prey. The tongue then rasps algae off the stone or tears at flesh to draw blood. At the tip of that piston-like tongue sit three horny plates, one placed across and two placed lengthwise. They scrape through surface tissue until they reach body fluids. The teeth on the oral disc mostly serve to hold on. They are made of keratin and other proteins, and each has a hollow core that leaves room for a replacement tooth growing beneath.

  The last common ancestor of lampreys seems to have specialised in blood and body fluids. Some descendants now take both blood and flesh. Others eat flesh alone and may even invade the host's internal organs. Flesh-eaters have smaller buccal glands, because they do not need a constant supply of anticoagulant. They also have ways of keeping solid material out of the branchial pouches, where it could clog the gills. Stomach contents from some lampreys have included remains of intestines, fins and vertebrae.

  Lampreys are sometimes said to be the only living backboned animals with four eyes. Two are ordinary eyes, and two are the parietal eyes, the pineal and the parapineal. Members of the genus Mordacia are the exception. In juveniles the eyes are eyespot-like structures under translucent skin, while adults have well-developed ones. To focus, the eye flattens its cornea, which pushes the lens toward the retina. The family Mordaciidae has one kind of photoreceptor, Petromyzontidae has two, and Geotriidae has five.

  The sea lamprey, the largest species, reaches a body length of about 1.2 m. Adults are scaleless and eel-like, with no paired fins, a single nostril on top of the head and seven gill pores on each side. They rank among the most energy-efficient swimmers. Their movements create low-pressure zones along the body that pull it through the water. Suction also lets a lamprey climb ramps and rocks on its way upstream, even though it grips at only one point. Scientists have studied how that works against a current.

  Countershading gives most lampreys a dark back and a pale belly. Markings can reveal where an individual stands in its life. The adult pouched lamprey, Geotria australis, has two bluish stripes along its body, and it loses them as it nears the breeding phase and heads upstream. The sea lamprey, Petromyzon marinus, shifts toward orange when it reaches that same stage.

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• 5. A Window on Early Vertebrates

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  Carl Rovainen began extensive work on lampreys in the 1960s, using them as a model system to uncover the basic principles of vertebrate motor control. He started in the spinal cord and worked toward the brain. In the 1970s, Sten Grillner and his colleagues at the Karolinska Institute in Stockholm carried the work on. The lamprey suits the job because its relatively simple brain is thought to resemble that of early vertebrate ancestors.

  Rovainen and his student James Buchanan examined the spinal cord cells that form circuits able to generate the rhythmic patterns behind swimming. Grillner has claimed the network is fully characterised, but details are still missing. Those spinal circuits answer to locomotor areas in the brainstem and midbrain. Higher structures, including the basal ganglia and the tectum, control those areas in turn. A 2007 study stimulated the tectum electrically and got eye movements, lateral bending or swimming, depending on the spot. The researchers read this as support for the idea that the tectum generates goal-directed movement.

  The lamprey also serves in biomedical research because its reticulospinal axons are unusually large. That size allows scientists to inject substances into them and study synaptic transmission. Lampreys can recover full function after their spinal cord is cut completely through.

  Anatomists place lampreys and hagfish in the subphylum Vertebrata of the phylum Chordata. The same phylum holds sea-squirts and the fish-like lancelets. Lampreys show every defining chordate trait at some stage. These are a notochord, a hollow nerve cord along the back, pharyngeal slits, an endostyle and a muscular tail behind the anus. Anatomists often compare lamprey larvae with adult amphioxus. Both have a pharynx with slits, a notochord, a dorsal hollow nerve cord and a row of somites reaching forward of the otic vesicle.

  The genome holds odd surprises. Northern lampreys have 164-174 chromosomes, the highest count among vertebrates. Their immune system works differently from ours. Leukocytes build surface receptors by recombining leucine-rich repeat gene segments. Lymphocytes carrying them act as the T cells and B cells of higher vertebrates, a trait that evolved independently. Pouched lamprey larvae tolerate large amounts of free iron and have biochemical systems to detoxify it. Lampreys also delete about 20% of their DNA from their somatic cell lines. Those genes matter in the embryo, but in humans they can cause cancer later in life. How the cell picks the genes to delete is not yet known.

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• 6. At the Table

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  The ancient Romans prized lampreys, and in the Middle Ages the upper classes across Europe ate them widely. They were especially popular during Lent, when meat was forbidden, because their taste and texture were so meaty. King Henry I of England is claimed to have loved them so much that he kept eating them late in life and in poor health, against his physician's advice. He is said to have died from "a surfeit of lampreys." Whether the dish really killed him is unclear, but the phrase survives in British culture.

  For the coronation of Elizabeth II in 1953, a lamprey pie was baked. Sixty years later, for her Diamond Jubilee, the city of Gloucester had to import fish from North America. Too few lampreys could be found in the River Severn.

  In Portugal, Spain and France, in Finland and in Latvia, lampreys are a highly prized delicacy. Latvians sell them routinely in supermarkets. The river lamprey is the local symbol on the coats of arms of Nakkila in Finland and Carnikava Municipality in Latvia. In 2015 the European Commission added the Carnikava lamprey to its Protected designation of origin list. In Portugal the sea lamprey is the most sought-after species. Government regulation no. 587/2006, from the 22nd of June, allows only two species to carry the commercial name lampreia. The other is Lampetra fluviatilis, the European river lamprey. Arroz de lampreia, meaning lamprey rice, and Lampreia à Bordalesa, in the Bordeaux style, rank among the most important dishes in Portuguese cuisine.

  Sweden, Russia, Lithuania, Estonia, Japan and South Korea also eat lampreys. Finns cook them grilled or smoked, and also pickled or in vinegar. Caution is needed, because the mucus and serum of several species are toxic. The Caspian lamprey, the river lampreys and the sea lamprey all need thorough cleaning before cooking. In Britain, dead lampreys are commonly used as bait. Northern pike, perch and chub can all be caught on them, and tackle shops sell them frozen.

  Indigenous American tribes of the Pacific Northwest have traditionally fished the Pacific lamprey, Entosphenus tridentatus. Because the fish is valued as food there and must travel upstream to reproduce, some scientists hope to design ramps that make its climb easier.

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• 7. Rivers and Invaders

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  Three species, Geotria australis, Petromyzon marinus and Entosphenus tridentatus, travel long distances in the open ocean. The evidence is a lack of reproductive isolation between their populations. Lampreys live mostly in coastal and fresh waters across most temperate regions, and other species stay in land-locked lakes. Their larvae tolerate high water temperatures poorly, which may explain why lampreys are absent from the tropics.

  At the time of the Norman Conquest in the 11th century, lampreys swam up the River Thames as far as Petersham. Pollution, overfishing and loss of river habitat have all harmed them since. Cleaner water in the Thames and the River Wear has brought recent sightings in London and in Chester-le-Street. Dams and other construction can block migration routes and cut off spawning grounds.

  Artificial channels, by contrast, have opened new habitats. In North America the sea lamprey has become a significant introduced pest in the Great Lakes. It is generally believed to have entered through canals in the early 20th century, but that theory is controversial. The sea lamprey has no natural predators in the lakes and preys on commercially valuable species such as lake trout.

  Today the lampreys are found mostly in the streams that feed the lakes. Managers block adults from moving upstream with special barriers, and they apply toxicants called lampricides, which are harmless to most other aquatic species. The programs are complicated and expensive. They keep lampreys in check but do not remove them from the lakes. Concerns about drinking-water quality in some areas are pushing changes to these programs.

  New approaches are in development. One uses chemically sterilized male lampreys, in a method similar to the sterile insect technique. Another draws on pheromones that are critical to migration. Their chemical structures have been determined and their effects studied in the laboratory and in the wild. Work continues on sourcing them chemically and on the regulatory issues that could allow their use.

  The U.S. Fish and Wildlife Service and the Canadian Department of Fisheries and Oceans carry out control in the Great Lakes, and the Great Lakes Fishery Commission coordinates it. Lake Champlain, which borders New York, Vermont and Quebec, also has high populations that warrant control. Its program is run by the New York State Department of Environmental Conservation, the Vermont Department of Fish and Wildlife and the U.S. Fish and Wildlife Service. In New York's Finger Lakes, the state's Department of Environmental Conservation runs the program alone.

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• 8. Folklore and Fiction

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  The name "nine-eyed eels" comes from a miscount. Seven gill pores sit behind each eye, and folklore treated them as extra eyes. It did the same with the nostril on top of the head, even though there is only one. The German word for lamprey, Neunauge, means nine-eye. In British folklore, the Lambton Worm may have been modeled on a lamprey, since it is described as an eel-like creature with nine eyes. In Japanese, the lamprey is yatsume-unagi, the eight-eyed eel, because the count leaves out the nostril.

  Vedius Pollio kept a pool of lampreys and threw into it slaves who displeased him. Once, in front of Augustus, one of his slaves broke a crystal cup. Vedius ordered the man seized and fed to the huge lampreys in his fish pond. The boy slipped free and ran to Augustus, begging only for a different way to die, because he did not want to be eaten. Augustus was moved by the strangeness of the cruelty. He freed the slave, ordered all the crystal cups broken before Vedius's eyes and had the pond filled in. Robert Harris adapted the incident in his 2003 novel Pompeii, where Ampliatus feeds a slave to his lampreys.

  Lucius Licinius Crassus loved a pet lamprey in his fish pond. Gnaeus Domitius Ahenobarbus, consul in 54 BC, mocked him for weeping when it died. Plutarch records Crassus replying by asking whether Domitius had buried three wives without shedding a tear. Aelian and Macrobius tell the story too. Hugo von Hofmannsthal used it in the Chandos Letter, where Crassus's answer mentions a first and a second wife. Hofmannsthal's narrator calls the lamprey "a dumb, apathetic, red-eyed fish." He sees Crassus as a mirrored image of himself, "reflected across the abyss of centuries."

  In George R. R. Martin's series A Song of Ice and Fire, enemies mockingly call Lord Wyman Manderly "Lord Lamprey." The nickname plays on his rumored fondness for lamprey pie and his striking obesity. Kurt Vonnegut's late short story "The Big Space Fuck" imagines a future America so polluted that the Great Lakes teem with massive, man-eating lampreys that walk on land.

  In season 3, episode 5 of The Borgias, Cesare Borgia's mercenary Micheletto goes on a hunting trip. He kills the King of Naples by pushing him into a pool full of lampreys, a pool that King Ferrante had built during his reign.

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