Seagrass is an unusual example of an entirely aquatic marine flowering plant. It grows in large meadows in areas of shallow water, providing a habitat to numerous species from all phyla, including free-living macroalgae and microalgae, bristle worms, mollusks, nematodes, and many fish, especially white-spotted puffers. Its green meadows are either monospecific (containing one species) or multispecific (many). Temperate meadows are more likely to be dominated by one or a few species, while tropical meadows, like those in the Philippines, may have as many as 13 or more. It is estimated that there are about 60 species of seagrass worldwide.
Like other grasses, to which they are not closely related, seagrasses are monocots, the smaller and simpler group of flowering plants. Seagrass is a fairly recent evolutionary innovation, having evolved from saltwater-tolerant land plants, especially mangroves, which are believed to be its immediate ancestor. Seagrass didn't likely exist prior to about 60 million years ago, shortly after the extinction of the dinosaurs, but since then, it has spread to most of the temperate and tropical coasts of the world. Oddly, seagrass is considered to be part of the "land plants," because it evolved from them and has much more in common with land plants than algae, from which land plants evolved about 450 million years ago.
Like the true grasses, seagrass has relatively little nutritional content, meaning that only specially adapted animals can eat it, but recent findings have determined that seagrass is more commonly consumed than once thought. In fact, it likely forms a foundation of the food chain for hundreds of species. Dugongs, manatees, swans, fish, geese, sea urchins and crabs have all been observed eating seagrass. Even if a species of animal can't consume seagrass directly, it may be able to consume other animals that feed upon the seagrass.
Seagrass is an example of a group called "ecosystem engineers," like coral, because they significantly modify their surrounding environment, attract certain species, and discourage others. The seagrass slows down water currents, encouraging the buildup of sedimentation, which can serve as a hiding spot for small bottom-dwelling creatures. Seagrass has an extraordinary rate of primary production, meaning that it converts sunlight and nutrients into biomass more effectively than most other marine plants.