Freshwater accounts for only three percent of the world's water - the rest is saltwater. But despite that tiny amount, freshwater habitats are homes for more than 100,000 species of plants and animals. Fish living in freshwater habitats have plenty of company. Snails, worms, turtles, frogs, marsh birds, mollusks, alligators, beavers, otters, snakes, and many types of insects live there too. Some unusual animals, like the river dolphin and the diving bell spider, are freshwater creatures. Plants such as algae, cattails, water lilies, and aspen and willow trees help keep the water clean by using their root systems to filter pollution and excess nutrients from the water. Rivers, creeks, lakes, ponds, and streams are all freshwater habitats. So are wetlands like swamps, which have woody plants and trees; and marshes, which have no trees but lots of grasses and reeds. Wetlands connect land and water, serving as natural filters, reducing pollution, controlling floods, and acting as nurseries for many aquatic species. Rivers, lakes, and estuaries serve as important transportation, recreation, and wildlife hubs. A recent study by an international research team used satellite photos and computerized mapping technologies to inventory all of the lakes on Earth. The team found about 117 million lakes, covering almost four percent of the world's land surface, not counting the glaciers on Greenland and Antarctica.