The free-loading sea lamprey

The free-loading sea lamprey

As an unsuspecting prey fish (coalfish, herring, shark, salmon and so on) swims past, the lamprey uses its suckerlike mouth to latch on to its side.

The sharp tongue and mouth (filled with rows of long, hook-like teeth) work together to rasp a hole in the prey’s flank or belly. The lamprey then secretes a substance that dissolves the body tissues and prevents the blood from clotting. It feeds on the prey’s blood and tissue, often killing the host after a short time. Even if the host isn’t actually killed by the lamprey, the damage caused lays the victim open to fatal infections.

Drag-free body shape

The largest of the lampreys, the sea lamprey can grow up to 90cm (3ft) long. The long, slender body is well suited to its lifestyle — its slim outline offers minimal resistance to a weakened host fish as it swims through the water.

The lamprey’s colour varies from olive to yellow-brown with a mottled black or dark brown back. Some lampreys are much darker in overall colouring. The belly is light grey or white. The lamprey also has two separate dorsal fins and a single paddle-shaped tail. It doesn’t have a swimblad-der, so it isn’t affected by changes in depth. There are seven window-like gill openings stretching down the fish’s sides behind the small eyes. Each opening contains a separate gill pouch. The lamprey can take in water through the mouth and expel it through the gills like other fish, but if the mouth is attached to a host, the openings can function independently, taking in and expelling water at will.

Freshwater beginnings

A sea lamprey spends only one to two years at sea. When sexually mature, it migrates into rivers to spawn. In some fast-flowing rivers, the lamprey uses its sucker-like mouth to attach itself to rocks to avoid being swept away downstream.

The male lamprey arrives first, from February to June, and prepares a nest along a gravelly river bottom. The female soon joins him and then lays about 200,000 eggs which the male quickly fertilizes. Both parents partially cover the eggs with sand and gravel. The smaller particles stick to the eggs and add weight. By this time the mature lampreys are physically exhausted, and most die, although a few may make it back to the sea to recover.

After hatching one to two weeks later, the larvae, blind and toothless, live like worms along the bottom. They move downsteam and burrow into silt. Here they stay for a few years, feeding on microscopic organisms and organic particles by filtering them through an efficient apparatus located in their gullet.

When the young are 15-20cm (6-8in) long they begin to take on the shape and features ofmature lampreys. After this they migrate to the sea and begin their hitch-hiking, free-loading life-style.