What is Sea grass?
[[File:Floridian seagrass bed.jpg|thumb|250px|alt=|right|A sea grass bed in Florida Bay]] [[File:FangoCarb1.jpg|thumb|250px|alt=|Young mangroves and carbonate mud in the internal part of the lagoon {{ndash}} Florida Bay]] [[File:Evolution of seagrasses Pengo.svg|thumb|250px|alt=|Diagram of plant evolution from the [[Precambrian]] to the sea grasses.]] [[File:Zostera marina - National Museum of Nature and Science, Tokyo - DSC07663.JPG|thumb|250px|''Zostera marina'' – the most common seagrass species in the Northern Hemisphere]] '''Sea grasses''' (or ''seagrass'') are [[flowering plant]]s which live in the sea. They come from four plant families in the order [[Alismatales]]. They are [[monocotyledon]]s which grow in [[Marine biology|marine]], fully [[Salinity|saline]] [[environment]]s. Sea grasses evolved from plants which went back to the ocean 70 to 100 million years ago, in the long, warm, [[Cretaceous period]]. Sea grass is a key part of [[continental shelf]] [[ecosystem]]s where [[phytoplankton]] produce [[carbonate]] [[sediment]]. This kind of ecosystem occurs in [[subtropical]] places like [[Florida Bay]], and around [[Bermuda]]. Sea grass beds often contain many [[species]] from various [[phyla]]. Apparently, seagrass [[Herbivore|herbivory]] is a highly important link in the [[food chain]]. Many species feed on sea grasses, such as [[green turtle]]s, [[dugong]]s, [[manatee]]s, fish, geese, swans, [[sea urchin]]s and [[crab]]s. The surface of the seagrass is the place where ''[[Melobesia]]'', a small [[alga]], makes chalk as a by-product of its metabolism. That is not the only chalk-producing organism in the seagrass beds.<ref>Westbroek, Peter 1991. ''Life as a geological force'', p158{{ndash}}165. W.W. Norton, New York. ISBN 0-393-30817-0</ref> == Largest plant == A huge bed of seagrass has been found off the coast of [[Western Australia]]. Researchers collected shoots from the bay and looked at 18,000 genetic markers. It is ''all from the same plant'', and covers about 77 square miles (200 square k