Report on

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Report on
Biology
Field Study Trip 2003
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Study Site Kei Ling Ha Lo Wai
Date 30th June 2003
Time 300pm (Low Tide)
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Physical Environment
of Mangrove
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Hong Kong Mangroves
Mangrove is unique inter-tidal wetland
ecosystem found in sheltered tropical and
subtropical shores, which receive inputs from
regular tidal flushing and from freshwater
streams and rivers.
It is characterized by high temperature,
fluctuating salinity, alternating aerobic and
anaerobic conditions, periodically wet and dry,
unstable and shifting substratum.
Mangrove communities are made up of
taxonomically diverse groups of plants and
animals, and each community has its own
distinctive flora and fauna species. 
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Mangrove Trees
Common mangrove tree species in Hong
Konginclude Kandelia candel, Avicennia marina,
Acanthus ilicifolius, Bruguiera gymnorrhiza and
Aegiceras corniculatum, which are located near
the seaward areas of the mangrove, and Lumnitzera
racemosa and Excoecaria agallocha, which are
commonly found near the landward side where there
is more freshwater input. In the landward areas
there are also a variety of shrubs and trees
which are never immersed by the high tides which
can be described as mangrove associates. Common
mangrove associates in Hong Kong include
Hisbiscus tiliaceus, Clerodendron inerme and
Pandanus sp.
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Animals found in Mangrove
Mangroves provide a complex, 3-dimensional
habitat for organisms to live in. Organisms can
inhabit the mangrove tree trunk, leaves, root
surface, the mud surface and also burrow into the
mud. The mud snail Terebralia sulcata, for
example, inhabits both the mud surface and the
mangrove tree trunk. Sessile organisms, including
the oyster Saccostrea cucullata and barnacles
Euraphia withersi, attach onto the mangrove tree
trunks and root surfaces. Bivalves such as
Anomalocardia squamosa and Gafrarium pectinatum
burrow inside the mud. Crabs such as Scylla spp.
and Perisesarma bidens burrow inside the mud but
to feed they emerge from their burrows, and
Perisesarma actually climb into the trees!
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Graph showing zonation patterns
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Adaptations of
Mangrove Trees
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Introduction
Mangroves are a challenging environment
for plants to live in. Different species have
unique ways of adapting to such a difficult
growing environment. In this session, we are
going to study their ways of adaptation in
detail.
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(i) Mode of attachment
a)Buttress roots
The roots of the mangroves expand horizontally as
buttress roots to enable them to have firm
anchorage on the ground.
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Buttress roots of Kandelia candel
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b) Cable roots and Prop roots
Prop roots that extend the plant base
over a greater area and cable root that runs
horizontally a long distance below the mud
facilitate mangrove plants to stand upright in
the soft mud and survive against the tidal flow.
Avicennia marina has under ground roots called
'cable roots' which expand in a horizontal
direction to gain anchorage.
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Prop roots of Kandelia candel
Prop roots provide additional stability to the
trees in the soft unstable substratum by
broadening the tree base
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(ii) The struggle for oxygen
Pneumatophores and knee joints
To facilitate roots to breathe in the
waterlogged environs with low oxygen content
,certain mangrove plants produce breathing
roots(pneumatophores)which protrude in the air to
enhance gaseous exchange.
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As the mud of mangroves has a very low oxygen
content, Avicennia marina has roots that grow
vertically from the ground called Pneumatophores
to obtain oxygen. Pneumatophores have loosely
packed tissues to allow effective diffusion of
gases into the pith.
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The roots of some mangrove species (e.g.
Bruguiera gymnorrhiza, Kandelia candel)
periodically form into "knees" that project above
the mud surface to facilitate gaseous exchange.
"Knee" of roots of Bruguiera gymnorrhiza
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(iii) Salinity stress
Salt glands on leaf-excretion of salt
Like all other living plants, mangroves need to
obtain fresh water for growth and other
activities. All mangrove plants develop a very
high internal osmotic pressure, which enable them
to absorb and transport water from the salty
environment. As the water available to them are
staline in nature, excess salt may be excreted
through glands on the leaves and discarded
through the shedding of leaves.
To get rid of the salt content inside the plant,
Aegiceras corniculatum has salt glands on its
leaves to secrete salt out of the body.
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Concentrate and actively secrete the absorbed
salts so as to regulate the salt concentration
inside the mangroves.
Salt glands of Acanthus ilicifolius
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(iv) Desiccation stress
To reduce water loss from evaporation, leaves of
Lumnitzera racemosa have a thick cuticle. The
leaves also have loose packed cells to store
water vapour inside the leaves.
Leaves of Lumnitzera racemosa
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(x) Reproduction
Germinated seeds that attach to the parent
plant of mangroves (e.g. Bruguiera gymnorrhiza)
after germination.
This kind of viviparous reproduction allows rapid
growth and establishment of mangrove seedlings
once they come into contact with the substratum.
It increases the survival rate of the mangroves
in the unstable substratum and the intertidal
environment.
Dropper of bruguiera gymnorrhiza
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Adaptations of animals
living in Mangrove
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(i)Periophthalmus cantonensis
Mangroves are semi-submerged in
seawater at high tide and completely expose on
their muddy platform in low tide. Mudskipper is
an amphibious fish which has developed means for
short period survival out of the sea. Mudskippers
have a dark grey-brown or green coloring, with
several darker dots or oblique stripe across
their bodies. They have large bulbous eyes
protruding from the top of their heads and two
dorsal fins which can be raised or lowered in a
sail-like fashion.
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Mudskippers (another name for
Periophthalmus cantonensis) have strong pectoral
fins, which are jointed halfway down and
well-muscled, to enable them to hop along the mud
surface. The mudskippers feed on either insects
or mud (both algae and detritus). They are, in
turns, prey of numerous wading birds. More
importantly, mudskippers are prized as a delicacy
and are caught and consumed locally in a
considerable quantities.
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(ii)Uca
Fiddler crabs(another name for Uca)
are the most distinctive species living in the
mud banks of sluice channels. They are featured
with prominent different sized chelae one is for
attack and defence while the other is for
feeding. The channels provide good shelter,
protection and constant nutrient supply to the
crabs that are abundant in the marshes.
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Conservation of
Mangrove in Hong Kong
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Mangroves are now threatened by
urbanization and pollution. Conservation of
mangroves in HK involves setting up marine parks(
e.g. Hoi Ha Wan) to protect mangrove stands and
also replanting some mangrove seedlings replace
any lost.
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Ways of Protection

• Protect mangrove stands in Restricted Area,
  Marine Park or Marine Reserve - e.g. Mangroves
  in Hoi Ha Wan Marine Park.

Hoi Ha Wan
Mai Po Marshes
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• Minimize the impact of urbanization through
  landuse planning and environmental assessment
  process.

• Promote public awareness on the conservation of
  mangroves through education, mass media and some
  non government organizations, such as World Wide
  Fund.

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• Designation of important mangrove stands as Site
  of Special Scientific Interest - e.g. Ting Kok
  mangroves

Ting Kok mangroves
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• Mangrove planting

Kandelia candel is the most common species
planted in mangrove replantations because they
have relatively larger droppers. To enhance the
replantation site to be more resemble to the
natural habitat, however, multi-species
replantations should be conducted which include
other common mangrove plant species such as
Bruguiera gymnorrhiza and Aegiceras corniculatum.
Kandelia candel
Bruguiera gymnorrhiza
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Be that as it may, there are actually
some differences in species composition between
the natural and replanted mangrove.
Plants in replanted mangroves site are often
regularly arranged and the number of species is
usually lower than the natural mangroves, as time
is needed for organisms to colonize a new
habitat. Although replanting mangroves attempts
to replace the mangroves lost in the past, the
biological assemblages of replanted mangrove
stands will take a long time to resemble the true
natural habitat.
Replanted mangroves
Mangrove planting
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The End
by 7S Cheng Tsz Lui
Leung Hoi Ying