Sea lampreys are parasites native to the northern and western Atlantic Ocean that suck blood and other vital fluids from their fellow fish. They have the distinction of possibly being the first destructive invasive species in North America; they entered the Great Lakes in the 1830s through the Welland Canal and have been killing trout there ever since.
They also have the distinction of having split off from the rest of the vertebrate lineage very early on, about 550 million years ago, before the evolution of jaws. This makes lampreys useful as a model organism for shedding light on the evolution of various vertebrate traits. But our studies have also revealed some strange features of the lamprey, including the fact that they get rid of hundreds of genes early in development.
Sea lampreys have several traits that other (jawed) vertebrates do not, suggesting that these traits either (a) were present in our last shared ancestor and lost by us or (b) arose since lampreys diverged from the jawed vertebrate lineage. One of these traits is a real oddity: programmed genome rearrangement. During this process, sea lampreys jettison about 20 percent of their genome during embryonic development. A few cells don't undergo this process, and these go on to pass on the otherwise missing DNA to another generation.