The Chesapeake Bay is a shallow, brackish estuary whose marshy and sandy shorelines and mix of fresh and oceanic waters support abundant wildlife, including land and water mammals, crustaceans, mollusks, reptiles, birds, aquatic vegetation, and many other species. For the past 12,000 years, the bay has provided waterways for transportation and food for humans. Osprey migrate from South America each year to raise their young there, feeding mainly on fish, hovering in one spot, and sometimes fighting bald eagles over their catch. Blue crabs, named for the blue color on their shells, range along the east coast of the United States to Argentina, but are closely associated with the Chesapeake Bay because the estuary is important for raising young blue crabs and because their meat is especially valued in Maryland and Virginia. Three jellyfish species are commonly found in the bay—the moon jelly, the lion's mane jelly, and the sea nettle—and while they are important food sources for many animals, their stings can leave uncomfortable red marks. Horseshoe crabs, often found mating on the shore, are actually related to spiders, and their blue blood is invaluable to humans; females are large, males are smaller, and males are often seen around the female while eggs are externally fertilized.