These freaks of nature that are neither fish, worm nor eel have survived for 360 million years, have long snake-like bodies and a sucker mouth adorned with sharp teeth worthy of any Hollywood sci-fi movie.
Many of the parasites feed by sucking the blood of fish, attaching to their prey with a suction disk and teeth. The Portuguese prefer to eat them cooked in their own blood.
At this time of year, for some communities on Portugal's northern rivers, lampreys are big business. They also make their way onto menus at top-notch Lisbon restaurants.
Between January and April, when sea lampreys move up river from the sea to spawn, fishermen head to the northern rivers while cities stage festivals dedicated solely to the creature.
In the small village of Montemor-o-Velho, with a population of about 3 000, the annual lamprey festival draws 30 000 visitors over two weekends. Around 2 500 lampreys are cooked and served.
"We have a big area of influence, we get visitors from all over the place. Yesterday I met people who travelled more than 400 km from the south of the country," said Luis Leal, Mayor of Montemor-o-Velho.
But the gastronomic feasts may be tempered this year by the falling lamprey population, which has been affected by the building of dams and pollution. The National Institute of Conservation classifies the species as "vulnerable", estimating there were only 100 000 lampreys in Portuguese rivers in 2006.
The shortages have left many Portuguese restaurants with no choice but to import lampreys from France.
In the United States and Canada, dwindling lamprey populations would probably be welcome, where they are considered a pest which threatens the trout population of the Great Lakes.
But in Portugal they are cooked in at least 10 different ways. The most common is the lamprey stew, made with lumps of lamprey boiled in its own blood and mixed with rice.
"They look rather different when they sit on a plate with rice," said Martinho, grinning as a lamprey twirled in the bottom of his boat with a hissing sound.