Lampreys are a very primitive form of fish. They are superficially eel-like, but can be readily distinguished by having a row of 7 gill openings on each side of their head, and by the absence of paired fins. They have a cartilaginous skeleton, no scales and no jaws. The mouth of adults takes the form of a sucking disc lined with horny teeth. They feed by attaching themselves to other fish and rasp away their flesh.

Adults spend most of their life at sea and migrate into rivers to spawn. Migration occurs over an extended period but the peak is from September to November. Lampreys are only active when it is dark, and burrow into the stream bed at other times. Because of this behaviour, and the fact that they do not feed on their upstream migration, they are seldom seen and little is known of their spawning activities.

It is thought that spawning takes place in shallow fast water on sand and gravel substrates. A spawning "nest" of pebbles may be constructed in the stream bed, and females produce up to 13,400 eggs. Adults probably die after spawning. The larvae are called ammocoetes. They are worm-like and blind and live in the fine sediments in the substrate of slow flowing areas of the stream. They filter feed on small organisms and over the next 3-5 years grow to about 150mm in length. They then metamorphose into miniature adult lampreys complete with eyes and rasping mouths and migrate to sea in the late spring.