There are many interesting anatomical features of sea lampreys. They are eel like in shape, with 7 gill openings and an inconspicuous median fin. This median fin becomes quite pronounced in the males as they ascend the river in breeding condition. Though the lamprey's eyes are small they are certainly functional. There is a third, pineal, eye located on top of the head which functions to regulate circadian rhythms. The entire skeleton is made of cartilage. The spinal cord is supported by a notochord, rather than surrounded by a bony vertebral column. Sexually mature adults average 2 to 2.5 feet in length..
The anadromous life history strategy involves breeding in fresh waters and migrating to the sea. At some point the seagoing adults return to the natal streams to breed. Lamprey, like many anadromous fishes, are selmelparous, they come upstream once to spawn then they die. The adults start moving up the river in late March and early April.