Largest lamprey species

The world's largest species of lamprey (aka vampire fish) is the sea lamprey (Petromyzon marinus), which can attain a maximum body length of 120 cm (3 ft 11 in), and a maximum published weight of 2.3 kg (5 lb). Native to the north and western Atlantic Ocean around the coasts of Europe and North America, it is also an invasive species in North America's Great Lakes. The primitive fish get their vampiric reputation from their habit of sucking blood from largest marine creatures (including fish and cetaceans) using circular mouths filled with several rows of sharp teeth.

The sea lamprey's larvae, known as ammocoetes, live in freshwater, migrating to the sea down rivers, and are much smaller, also lacking functional eyes and the familiar jawless, multi-toothed, blood-sucking mouth of parasitic lampreys until they metamorphose into the adult form.

Lampreys belong to the taxonomic group Agnatha (usually categorized as a class, but sometimes as an infra-phylum), within the subphylum Vertebrata of the phylum Chordata. Agnathans are known colloquially as jawless fishes (Agnatha translates as "without jaw"), because they lack the true jaws of the bony fishes, cartilaginous fishes, placoderms and acanthodians.

Today, the only living agnathans are the lampreys and the hagfishes, but in distant prehistoric times there was a much greater diversity of species, including spectacular armoured forms during the Palaeozoic era, known as ostracoderms.