that the lamprey has unusual guanine-cytosine content and amino acid usage patterns compared to other vertebrates. The full sequence and annotation of the lamprey genome is available on the Ensembl genome browser. The lamprey genome may serve as a model for developmental biology and evolution studies involving transposition of repetitive sequences. The lamprey genome undergoes drastic rearrangements during early embryogenesis in which about 20% of the germline DNA from somatic tissues is shed. The genome is highly repetitive. About 35% of the current genome assembly is composed of repetitive elements with high sequence identity. Northern lampreys have the highest number of chromosomes (164–174) among vertebrates. Two genes important to immune function - CDA1 and CDA2 - were first discovered in P. marinus and then found to be conserved across lampreys. See §Immunology above. Invasive species Sea lampreys are considered a pest in the Great Lakes region. The species is native to the inland Finger Lakes and Lake Champlain in New York and Vermont. Whether it is native to Lake Ontario, where it was first noticed in the 1830s, or whether it was introduced through the Erie Canal which opened in 1825 is not clear. Improvements to the Welland Canal in 1919 are thought to have allowed its spread from Lake Ontario to Lake Erie, and while it was never abundant in either lake, it soon spread to Lake Michigan, Lake Huron, and Lake Superior, where it decimated indigenous fish populations in the 1930s and 1940s. In its original habitats, the sea lamprey coevolved with its hosts, and those hosts evolved a measure of resistance to the sea lampreys. However, in the Great Lakes, the sea lamprey attacks native fish such as lake trout, lake whitefish, chub, and lake herring, which historically did not face sea lampreys. Elimination of these predators allowed the alewife, another invasive species, to explode in population, with adverse effects on many native fish species. The lake trout plays a vital role in the Lake Superior ecosystem. The lake trout has traditionally been considered an apex predator, which means that it has no predators. The sea lamprey is an aggressive predator by nature, which gives it a competitive advantage in a lake system where it has no predators and its prey lacks defenses against it. The sea lamprey played a large role in the destruction of the Lake Superior trout population. Lamprey introduction along with poor, unsustainable