Most people, in fact, most biologists, are unaware that there are two submarine, totally submerged habitats for flowering plants, i.e., hidden beneath the waves, along tropical and temperate coastlines, where one expects to find only marine algae. The habitats are called marine meadow and surfweed. The surfweed community is more commonly known as kelp forest. Both habitats and three species of special seed-bearing halophytes occur submerged below tide along the California coastline.
In all, 58 species of monocotyledons, collectively termed seagrasses or marine phanerogams, live in such habitats, especially in marine meadows, where they are the primary producers of, probably, the most productive community on Earth. Marine meadows (another example) are especially famous as the feeding grounds of grazing, herbivorous dugongs and sea turtles, which is why certain species are called turtlegrass.
Seagrasses thrive in shallow sedimentary environments of mainly warm tropical waters, especially in flats of mud or sand ("meadows") with depths of less than ten meters. These habitats must be sheltered from intense surf and oceanic swell, and often the best habitats occur behind offshore coral reefs and atolls, or in lagoons with small tidal effects and very clear water. In places where rock outcrops occur, seagrasses typically are replaced by algal macrophytes (seaweeds), although a few seagrass species occur in the surfweed community, attached to rocks below the level of lowest tide. Eelgrasses, Zostera marina and Z. pacifica, are species that occur in intertidal estuarine mud flats, but have also been located at a depth of 30 meters off the California coast; Zostera occurs as well in New England and northern Europe bordering cordgrass saltwater marsh (Spartina). Around Caribbean islands, turtlegrass (Thalassia testudinum) commonly occurs on the outer edge of Rhizophora, red mangrove. The remarkable record for depth is held by Halophila, found once at 90 meters in exceedingly clear water.
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