Ask most people about lampreys and the chances are you will be greeted with a blank stare. And even those that do know something about these strange fish are unlikely to know more than that they are supposed to lie behind one of Britain’s more bizarre royal fatalities.
Let’s start with the facts. Lampreys are one of the oldest vertebrate species. They first evolved around 450 million years ago and have barely changed since. They are also one of that relatively small selection of fish which can metabolise enough to move from freshwater to saline conditions – and back again. At first glance this doesn’t seem such a huge move (we, after all, are quite happy to swim in river, sea or swimming pool and temperature appears the only significant difference). The shift between freshwater to saline and back again puts huge stresses on a fish, particularly at the cellular level. So while quite a few species can manage in the brackish water or estuaries, only a relatively small number can shift from the fast-flowing cold relatively sterile waters of mountain streams to the nutrient-rich, salty, constant balmy conditions beneath the waves.
Salmon and eels are obvious examples, but two of our three lamprey species also make the transition. All begin their lives as eggs buried in the silt of a river bed and spend their first few years using their primitive mouths, which are armed with rasping tongues rather than teeth, to feed on organic matter and bacteria.
But things change when they reach maturity. The 20cm brook lamprey responds by fasting and from now on its mouth is used only to hang onto rocks in the strong currents, particularly while mating. Their bigger relatives, the river and sea lampreys, respond differently. When about seven or eight years old their bodies metamorphose to cope with the huge physical changes of a move from fresh- to salt water and they move downstream.
Beneath the waves both prey on larger bony fish, latching onto their sides and rasping their way into their victim’s flesh with their sandpaper tongues to feed on the bloody pulp. Their weakened victims usually die from blood loss or infections, so small wonder they are hated by fishermen. Indeed, sea lampreys are a major pest in the Great Lakes where numbers have exploded thanks to the manmade canals which now link these huge inland waters. The lake trout, an important apex predator and valuable commercial fish, has been particularly badly hit.
After months fattening on cod, haddock and hake (about a year in the case of the ‘river’, up to three for the ‘sea’ relative), the lampreys return to spawn in fresh water before dying. It is at this point that they can become commercially valuable in their own right. Today most people find their rather pulpy soft flesh unpalatable, but in the past they were considered a delicacy. In York archaeologists have found millions of fossilised microscopic scales from lamprey tongues in Viking latrines and according to the Mediaeval chronicler Henry of Huntingdon (c. 1085 – c. 1154), Henry I of England died after eating a ‘surfeit of lampreys’ in 1135.
