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Seagrasses evolved from terrestrial plants which recolonised the ocean 70 to 100 million years ago. The name seagrass stems from the many species with long and narrow leaves, which grow by rhizome extension and often spread across large "meadows" resembling grassland; many species superficially resemble terrestrial grasses of the family Poaceae.
When submerged, new leaf growth has been found to have thinner leaves and thinner cell walls than the leaves on the plant that grew while above water, along with oxygen levels being higher in the portion of the plant grown underwater versus the sections that grew in their terrestrial environment. [32]
Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant. Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.
This was an important use in the Aveiro Lagoon, Portugal, where the plants collected were known as moliço. In the early 20th century, in France and, to a lesser extent, the Channel Islands , dried seagrasses were used as a mattress ( paillasse ) filling – such mattresses were in high demand by French forces during World War I .
Marine plants can be found in intertidal zones and shallow waters, such as seagrasses like eelgrass and turtle grass, Thalassia. These plants have adapted to the high salinity of the ocean environment. Light is only able to penetrate the top 200 metres (660 ft) so this is the only part of the sea where plants can grow. [77]
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, including green turtles ...
Plants often have to produce aerenchyma, while the shape and size of leaves may also be altered. [26] Conversely, oxygen is fatal to many kinds of anaerobic bacteria. [22] Nutrient levels are important in controlling the abundance of many species of algae. [27]
"Seaweed" lacks a formal definition, but seaweed generally lives in the ocean and is visible to the naked eye. The term refers to both flowering plants submerged in the ocean, like eelgrass, as well as larger marine algae. Generally, it is one of several groups of multicellular algae; red, green and brown. [7]