thumb|floridian seagrass bed    thumb|evolution of seagrasses|alt=diagram showing seagrass evolution beginning with precambrian phytoplankton which begat silurian red, brown, and green algae,zostera  seagrasses are flowering plants (division angiospermae) belonging to four plant families (posidoniaceae, zosteraceae, hydrocharitaceae, or cymodoceaceae), all in the order alismatales (in the class of monocotyledons), which grow in marine, fully saline environments. there are 12 genera with some 58 species known. ==ecology== these unusual marine flowering plants are called seagrasses because in many species the leaves are long and narrow, grow by rhizome extension, and often grow in large "meadows", which look like grassland: in other words, many of the species of seagrasses superficially resemble terrestrial grasses of the family poaceae. like all autotrophic plants, seagrasses photosynthesize so are limited to growing in the submerged photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. most species undergo submarine pollination and complete their entire life cycle underwater. there are about sixty species worldwide. seagrasses form extensive beds or meadows, which can be either monospecific (made up of a single species) or in mixed beds where more than one species coexist. in temperate areas, usually one or a few species dominate (like the eelgrass zostera marina in the north atlantic), whereas tropical beds usually are more diverse, with up to thirteen species recorded in the philippines. seagrass beds are highly diverse and productive ecosystems, and can harbor hundreds of associated species from all phyla, for example juvenile and adult fish, epiphytic and free-living macroalgae and microalgae, mollusks, bristle worms, and nematodes. few species were originally considered to feed directly on seagrass leaves (partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass herbivory is a highly important link in the food chain, with hundreds of species feeding on seagrasses worldwide, inc
Seagrass