learn only 3 things about them ...
Seaweeds are different from the plants we are more familiar
Many animals eat seaweeds, while few eat seagrasses.
have different textures, colours and shapes. Feel them!
are seaweeds? Seaweeds
are algae that live in the sea or in brackish water. They are also
called macroalgae (which means 'big algae') to differentiate them
from the microscopic algae are common in the sea.
Algae are everywhere! Algae grow
everywhere in the sea, in freshwater and even on land. They even grow
in your bathroom, if you don't clean it often! And in your home aquarium.
In the sea, large seaweeds grow attached to the ground or hard surfaces.
Smaller algae grow on seagrasses or even bigger seaweeds. They also
coat rocks, snail shells and other hard surfaces. Microscopic algae,
also called phytoplankton, play a vital role in the sea, but are often
ignored because they usually can't be seen with the naked eye. Microscopic
algae is also found in soil. Macroalgae are also found in freshwater;
these are usually called water weeds.
How are seaweeds different from plants?
Algae are very different from ‘normal’ land plants. Although both
have chlorophyll, algae don't have true roots, stems and leaves. Algae
don't have a system of channels to move water around their body (called
a vascular system) that 'normal' plants do. Seaweeds can take all
kinds of shapes from filaments to sheets, encrusting layers to branching
Seaweed parts: Although some seaweeds
may look like some land plants or seagrasses, the body parts of these
seaweeds work differently. So seaweed body parts are named differently.
Some seaweeeds have a leaf-like portion. This is called the blade.
Sometimes, reproductive structures are also found on the blade. The
blade does not contain veins that transport water and nutrients, so
it is not a true leaf. The blade can take on a wide variety of shapes;
from flat large surfaces that resemble pieces of plastic bags; to
narrow strips, hairy filaments or thick juicy tubes.
The stem-like portion that holds up the blade is called the stipe.
The part that anchors the seaweed is called the holdfast and not the
root. The holdfast simply grips the surface and does not absorb nutrients
or grow extensively into the ground like true roots do.
The whole seaweed is called the thallus. Unlike land plants, photosynthesis
takes place in all parts of the seaweed and not just the 'leafy' parts.
Nutrients are also absorbed with all parts of the seaweed.
Sometimes confused with seagrasses.
Green seaweeds are easily confused for seagrasses. Here's more on
how to apart seagrasses
and green seaweeds.
Why are seaweeds slimy? A slimy
coating reduces water loss when exposed at low tide. Being leathery
also helps. Some seaweeds are hard or crunchy because calcium carbonate
is incorporated into the thallus. Grazers may also be deterred by
distasteful and even toxic chemicals.
Blooming seaweed: Some seaweeds
can grow very fast. Sometimes, one or a few species of seaweeds can
suddenly be seen in vast quantities, carpeting the shores or getting
washed up in huge piles. Amazingly, after a few weeks, these seaweeds
can just as suddenly disappear. They are then not seen in huge quantities
again for some months. Various species seem to 'take turns' blooming
and dominating the shore at different times of the year. These 'blooms'
may be due to changes in nutrients or other conditions on the shores.
may also bloom, colouring the seawater. Red
tide is a bloom of harmful algae.
Seaweed reproduction: Algae do
not have flowers, fruits or seeds. Instead, they have a complex method
of sexual reproduction involving the formation of some kind of spore.
It is usually impossible to tell if a seaweed is reproductively mature
without examining it under a microscope. Seaweed can also grow from
fragments of the parent plant.
Role in the habitat: Unlike seagrasses,
seaweeds are eaten by a wide range of creatures from snails to fish,
crabs to sea
urchins and special animals such as sea
turtles. Small algae provide food for tiny grazers such as snails.
Larger herbivores such as sea
hares munch on bigger seaweeds. During a seaweed 'bloom' there
can be a corresponding explosion in the number and variety of animals
that eat that particular seaweed. As well as the predators that eat
Seaweeds also provide shelter for small animals so that they can hide
from their predators. Some animals like squids, cuttlefishes and octopus
may also lay their egg capsules on seaweeds.
Seaweeds can also affect the growth of other animals. Studies have
shown that some
seaweeds release chemicals that prevent coral larvae from settling
down. Thus animals that eat seaweeds play a role in the balance
of life on a reef.
Intimate friends of algae: Many
animals harbour microscopic, single-celled algae (called zooxanthallae)
within their bodies. The algae undergo photosynthesis to produce food
from sunlight. The food produced is shared with the host, which in
return provides the algae with shelter and minerals.
This symbiotic relationship between hard
corals and zooxanthallae provides the corals with extra nutrition
needed to build their skeletons more quickly in the clear, nutrient-poor
waters of the tropics. Thus algae are important in reef building!
Other animals that harbour zooxanthallae include carpet
anemones and giant
Algae may also have a symbiotic relationship with fungus, to form
Human uses: Many seaweeds are
edible to humans too. You have probably eaten some seaweed recently!
Extracts of seaweeds are widely used to colour, stabilise and thicken
packaged food. Fresh seaweed is part of the traditional diets of many
people. Seaweeds are a good source of iodine, and many species are
used in traditional medications. Unfortunately, they are low in protein
and other nutrients so fresh seaweed is often used merely as a garnish
or side dish.
Other uses of seaweeds including as fertiliser to grow food crops,
and as part of livestock feed. In the past, Sea
lettuce (Ulva sp.) were collected in boatloads in the Straits
of Johor, washed in freshwater then cooked and fed to pigs.
Some seaweeds are cultivated for commercial uses. It is estimated
that around two million tonnes of marine plants (fresh weight) are
consumed worldwide, valued at more than US$3billion (as at 2000).
The single most valuable seaweed is probably Porphyra sp.,
which is used in Japanese sushi. Seaweed cultivation can be an important
source of income for poor fishermen in more remote areas.
More recently, there has been increasing interest and research focus
on developing seaweeds as a source of biofuel.
Colours of the Weed: Seaweed species
are generally classified by their colours! All seaweeds have green
chlorophyll, but some also have other pigments.
Green seaweeds (Division Chlorophyta)
are usually found closer to shore because they generally can tolerate
more sun and drying out.
Red seaweeds (Division Rhodophyta)
can grow in deeper waters where green seaweeds may not survive. This
is because their red colour allows them to photosynthesise in lower
light levels. The ‘Nori’ used in Japanese sushi is a red seaweed (Porphyra
spp.). Red seaweeds are a source of agar extracts which are used to
make delicious jellies (agar-agar); and carrageenans, natural gums
used to gel and stabilise food such as chocolate milk and yoghurt.
Brown seaweeds (Division Phaeophyta)
are more common in colder waters. They are a source of alginates used
to make water-based products thicker, creamier and more stable. For
example, alginates make smoother ice cream.
Blue-green algae (Division Cyanophyta)
form fine hair-like filaments or mats.
Seaweeds species are difficult to positively identify without close
examination under a microsope, especially of their reproductive parts.
On this website, they are grouped by external features for convenience
Status and threats: Like other
plants of the intertidal zone, seaweeds are affected by reclamation
and human activities that result in pollution or increased sedimentation.
Some human activities may actually promote seaweed growth. For example,
nutrient rich water from sewage and other outflows might trigger an
explosive growth of seaweeds. This, however, can result in an imbalance
in the ecosystem.
Red, brown and green seaweeds
St. John's Island, Jan 06
Various brown seaweeds
Sentosa, Apr 04
Some green seaweeds are
easily mistaken for seagrasses.
Pulau Sekudu, Apr 06
Red encrusting coralline algae
plays an important role in 'cementing' the reefs.
Sentosa, Jun 05
Large piles of green seaweed sometimes
form on the shores during a seasonal 'bloom'.
Chek Jawa, Feb 02
Seaweeds provide shelter for small animals
such as this tiny octopus
Sisters Island, Jul 04
Egg capsules of a cephalopod laid
on brown sargassum seaweed
Pulau Semakau, Nov 05
This slug looks exactly like the
green seaweed that it probably feeds on
Sentosa, Nov 03
Giant clams contain zooxanthallae,
microsopic algae that share products
of photosynthesis with the host.
Pulau Hantu, Feb 06
Hair-like cyanobacteria growing on rocks.
St. John's Island, May 05
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