The two different life stages of lampreys feed on very different food supplies. Ammocoetes live in tunnels in the silt and, as their teeth are not yet developed, they feed on small organic particles and algae drawn from the silt surrounding their burrow into their gut in a string of slimy mucus.
All three species spend up to five or more years growing in their burrows before transforming into adults and developing eyes and teeth. One of the differences between the species is that adult brook lampreys do not feed and as such develop very primitive, vestigial teeth. Instead of travelling to the sea they migrate straight to their breeding grounds, mate and then die. In contrast, adult river and sea lampreys swim downstream through July to September to feed in the sea with well-developed, hard, sharp teeth. The arrangement of teeth in the mouth differs between species - in sea lampreys they are arranged in concentric rows with several large teeth also on the tongue, and in river lampreys there are far fewer teeth, all found close to the mouth opening, and on the tongue.
River and sea lampreys feed on a number of fish species including herring, flounder, sprat, salmon and haddock. They attach themselves to the side of the victim using their sucker-like mouth and attack the fish by rasping away the scales and feeding on the fluids and tissues underneath. Salmon and sea trout that have returned to rivers can sometimes be found with scars from lamprey attacks. The victim may not survive such attacks if the lamprey is able to penetrate the body cavity. Yet despite this parasitic lifestyle there is no evidence that lampreys have a significant impact on any fishery in Scotland.
River lampreys feed and grow primarily in the estuary, whereas sea lampreys will venture out into the open sea where they have been recorded feeding on cetaceans and sharks. There are also historical records of sea lampreys attaching themselves to the underside of fishing boats, on which they were presumably resting, but they were assumed to have an appetite for tar with which the boats were coated. Very little is known of the behaviour of river and sea lampreys during their marine phase, but it would appear that sea lampreys can travel considerable distances from land with one caught 350km from Ireland in the North Atlantic.
Scotland provides an unusual exception to these normal feeding patterns. River and sea lampreys that breed in the Endrick Water migrate downstream into Loch Lomond and, instead of continuing downstream to the Firth of Clyde, remain in the loch to feed on the resident fish population. This is the only example of freshwater feeding in the UK and one of only three examples in Europe. In Loch Lomond the river lampreys are primarily parasites on trout and the rare powan.
When river and sea lampreys return to the rivers to breed they stop feeding and, like salmon, rely on food reserves to sustain them on their considerable upstream migration.