Sea Lamprey

It is a fish in the broad sense — an aquatic vertebrate with gills — but it belongs to a completely separate, far more ancient branch than the fish most people know. Sea lampreys are jawless fish (agnathans); together with the hagfishes they are the only survivors of the earliest vertebrates, which existed before jaws, bone, and paired fins evolved. So a lamprey has a skeleton of cartilage and a stiffening rod (notochord) rather than a backbone, no jaws, no scales, and no paired fins. Its eel-like shape and the old name "lamprey eel" are misleading — true eels are ordinary bony fish, and their similarity to lampreys is just the convergent result of a long, limbless, burrowing body plan.

An adult sea lamprey hunts by swimming up to a larger fish and clamping its round, funnel-shaped mouth onto the body, using the concentric rings of sharp keratin teeth to grip and the sucker to hold fast even against a struggling host. It then extends a muscular, toothed tongue like a piston and rasps back and forth to bore through the scales and skin into the flesh. Its saliva contains an anticoagulant that keeps the wound bleeding, and the lamprey feeds on the blood and liquefied tissue for hours or days before detaching. Some hosts survive with a circular scar; many die from blood loss, secondary infection, or the wound itself, especially smaller fish.

Yes, and it has been a prized food in Europe for over a thousand years. Lamprey was a delicacy of medieval royal and monastic tables — King Henry I of England is traditionally said to have died from "a surfeit of lampreys" — and it is still eaten today, especially in Portugal, Spain, France (as lamproie à la bordelaise), Finland, and the Baltic states, often stewed in its own blood or grilled. The city of Gloucester in England historically sent the monarch a lamprey pie. Because European sea lamprey populations have declined in many rivers, the fishery is now regulated and the fish can be expensive. The invasive Great Lakes lampreys are not generally eaten.

It comes down to native versus invasive. On the Atlantic coasts of Europe and eastern North America the sea lamprey is a natural part of the ecosystem, an ancient migratory fish that has coexisted with its host species for millions of years, and in parts of Europe it has declined enough from dams and pollution to be legally protected and the subject of restoration efforts. In the upper Great Lakes it is an introduced invader that arrived through shipping canals in the 1900s, encountered fish with no evolved defences, and multiplied to devastating densities, playing a major role in the collapse of the lake trout fishery. Since the 1950s a bi-national programme has suppressed it with a selective larvicide (TFM) applied to spawning streams, low barriers that block adult lampreys but pass other fish, traps, and the release of sterilised males, keeping numbers to a fraction of their peak.