Sea lamprey

Petromyzon marinus is a large, anadromous jawless fish (agnathan) native to the Atlantic coasts of North America and Europe. Adults can reach 90–120 cm in length and are characterized by a cylindrical, eel-like body, a cartilaginous skeleton, a single nostril on top of the head, seven pairs of gill openings, and a distinctive oral disc lined with concentric rows of sharp, keratinous teeth used to attach to host fish and rasp through their skin to feed on blood and body fluids. They lack paired fins and jaws. Sea lampreys are parasitic as adults, targeting large fish such as salmon, trout, and marine species. They undergo a complex life cycle: eggs hatch into blind, filter-feeding larvae called ammocoetes that burrow in river sediments for 3–7 years before metamorphosing into the parasitic adult form. Adults migrate to the sea (or large lakes), feed for 1–2 years, then return to freshwater rivers to spawn and die. P. marinus became notoriously invasive in the North American Great Lakes after gaining access via canals in the early 20th century, devastating native fish populations. Its genome has been sequenced and is a key reference for understanding vertebrate genome evolution, adaptive immunity origins, and neural crest development.