Grass in the sea?
Ever heard of grasses growing in the sea-bed? But, believe it or not,
there are plenty. Letís find out.
They are called sea-grasses because they resemble grasses growing on
land. Like terrestrial plants, sea-grasses have leaves, roots,
conducting tissues, flowers and seeds, and produce some of their food
via photo-synthesis. Unlike terrestrial plants, however, sea-grasses do
not possess the strong, supportive stems. Sea-grass blades are supported
by the natural buoyancy of water, remaining flexible when exposed to
waves and currents.
Sea-grasses form extensive beds or meadows under water. They can be
either mono-specific (made up of one species) or multi-specific (where
more than one species co-exist). In temperate seas, usually one or a few
species dominate the environment, whereas tropical beds usually are more
diverse, with up to 13 species recorded in the Philippines.
Like all other green plants, sea grasses also photo-synthesize. They
need sunlight to grow. Therefore, they are limited to growing submerged
in the upper marine zone and most occur in shallow and sheltered coastal
waters, anchored in sand or mud. They undergo pollination while
submerged and complete their entire life cycle under-water. There are
about 60 species worldwide.
Sea-grass beds are highly diverse and productive ecosystems, and can
harbour hundreds of associated species from marine genera. These include
juvenile and adult fish, epiphytic and free-living macro- and
micro-algae, shell-fish, bristle, cylindrical worms. Juvenile stages of
many fish species spend their early days in the relative safety and
protection of sea-grasses.
The complexity of sea-grass habitats is increases when several species
of sea-grasses grow together. Many hundreds of species feed on
sea-grasses ocean-wide, including dugongs, manatees, fish, geese, swans,
sea-urchins and crabs.
Sea-grasses are sometimes labelled eco-system engineers, because they
partly create their own habitat. The leaves slow down water-currents,
increasing sedimentation, and the sea-grass roots and shoots stabilize
the seabed. Their importance for associated species is the shelter they
provide and for their extraordinary high rate of primary production.
Sea-grasses are subject to a number of stresses such as storms,
excessive grazing by sea creatures, disease, and human threats due to
pollution, decreasing water clarity, excessive nutrients in run-offs, by
propellers and scarring.