mangrove in sai keng

Contents

I. Aims and objective of study...........................................Page 1

II. Introduction....................................................................Page 3

III.General information of the field trip..........................Page 4

IV. Method of study......................................................... Page 5

V. Abiotic factors in Sai Keng........................................Page 9

VI.Analysis of the results obtained................................Page 12

VII. Problems of life in mangrove....................................0

VIII. Adaptations of plants to mangrove ...........................Page 22

IX.Adaptations of animals to mangrove ......................Page 25

X. Biotic factors in Sai Keng........................................Page 27

XI.Food web and dichotomous key of mangrove.........Page 29

XII. Man’s impact on mangrove habitat ..........................Page 30

XIII. Conservation of Mangroves in Hong Kong...............Page

31

XIV. Safety precaution......................................................Page 32

XV. Comment & conclusion.............................................Page 33

Biology Field Trip Report──The Mangrove Habitat 2000 / 01

Title

A study of the mangrove ecosystem (Sai Keng)

I. Aims

1. To understand the structure and functioning of a mangrove ecosystem.

2. To learn and practice basic ecological techniques.

3. To manipulate simple field equipment so as to measure environmental factors.

4. To identify the common mangrove organisms.

5. To identify and interpret adaptive features pertain to the mangrove plants and animals.

Objectives

1. To identify animals and plants with the aid of keys and field guides.

2. To measure climatic factors by means of an environmental comparator with a light probe a whirling hygrometer, a hand-held wind meter, and a thermometer.

3. To find out the aspect of the habit by means of a compass.

4. To determine the slope of the beach by means of the levelling technique making use of two ranging poles, a piece of cotton thread and a spirit level.

5. To collect soil and soil water samples.

6. To measure pH of the soil water on the spot.

7. To capture burrowing animals in the mud.

8. To sample the mangrove animals and plants by means of a transect line and a quadrat.

9. To carry ort routine soil analysis in the laboratory.

10. To construct a profile chart of the habitat along the transect line showing changes in altitude and slope on the beach, soil composition, soil water content and pH, and distribution of plant and animal species.

11. To infer zonation of the animal and plant species on the beach.

12. To relate the zonation pattern to the abiotic factors on the beach.

13. To identify the adaptive features of the animals and plants species.

14. To interpret the ecological significance of these adaptive features.

Page 1 I. Aims and Objective of Study


15. To construct a food web for the mangrove ecosystem.

16. To evaluate the developmental status of the mangrove ecosystem.

17. To write an ecological field trip report based on the data collected on the field trip on the mangrove habitat.

Apparatus and materials

For animal capturing and sampling:*aquarium net 1 piece*dip net 1 piece*enamel tray 1 piece*forceps 1 pair *plastic bags some*quadrat 1 piece*trowel 1 piece*vials, different sizes 10 pieces

For measuring physical factors:*alcohol-in-glass-thermometer 1 piece*compass 1 piece*nylon string 1 piece*environmental comparator with light probe 1 set*hand-held wind meter 1 set *meter rule 1 piece*pH meter 1 set*ranging poles 2 pieces*spirit level 1 piece*whirling hygrometer 1 piece

For laboratory work:*electronic balance 1 set*beaker, 100 ml 2 pieces*oven 1 set*aluminium foil 1 piece*soil sieves 1 set*razor blade 1 piece*petri dish 1 set*brush 1 piece*glass slides & coverslips some*glycerine some*methylene blue some*microscope 1 set

Page 2 I. Aims and Objective of Study


II. Introduction

A. What are mangroves?

Mangrove is an inter-tidal wetland ecosystem, which marks the transition between the sea or an estuary and the land. Its relatively gentle gradient makes it affected by tides and characterized by high temperature, fluctuated salinity, alternating aerobic and anaerobic conditions, periodic wet and dry soils, and unstable and shifting substratum.

B. Significance of Mangroves

Mangroves are important for healthy coastal ecosystem because they are buffers between land and sea, and serve as a link between marine and terrestrial ecosystem. It shows some of their important functions below:

1. Provide food for gastropods and animals e.g. fruits and leaves of Avicennia marina.

2. As Chinese medicine, e.g. the bark of Bruguiera gymnorrhiza to treat diarrhea and smoke generated from burning the wood of Excoecaria agallocha can treat leprosy and their leaves can treat epilepsy.

3. Provide beautiful scenery, which can promote tourism.

4. Protect the shorelines from erosion due to current, wave, wind and storm.

5. Paradise for aquatic animals especially for mollusks and crustaceans, due to the rich sediments brought by the rivers and high deposits of silt and detritus in the mangrove environment.

6. Retain nutrients, e.g. nitrogen, and toxic substances such as heavy metal ions from wastewater, thus it can serve as a natural water and wastewater treatment plant.

7. Prime nesting and migratory sites for hundreds of bird species and wildlife, so they are important in maintaining biodiversity.

Page 3 II. Introduction


III. General information of the field trip

Date of study: 23 rd May, 2001 (Wednesday)

Time of study: 2:24 p.m.

Time of highest tide: 2.2 m at 09:07 and 1.6 m at 22:26

Time of lowest tide: 1.0 m at 02:32 and 0.4 m at 15:51

Weather conditions: It is a sunny day with fine weather, there is a little cloud and water speed is slow.

Location of the study: The venue for the study is the mangrove along the coast near Sai Keng. It is next to the Kei Ling Ha Hoi. The fresh water outlet to it may run from the underground water of Shap Sz Heung and Ma On Shan.

A map showing the location of Sai Keng

Sai Keng consists of 13 species, dominated by Kandelia candel and Aegiceras corniculatum. A total of 44 species of benthic macro fauna has been identified and they are dominated by crabs and gastropods. Before the building of houses in this area and the construction work, Sai Keng is much less polluted. Its high ecological value gives an opportunity for academic study and research.

Page 4 III. General Information of the Field Trip


IV. Method of study

A. Field work

i. Vegetation analysis

1. Lay out a line marked by a string across the zone and be perpendicular to the coastline in the mangrove community.

2. Take the end of the transect line in contact with the back of shore landmark as zero point. Move along the line and note down the presence of plants which touch, overlie or underlie the transect line.

3. Identify each plant with the aid of keys or field guides.

4. Identity the adaptive features of the plants and note down the relevant features of the immediate surroundings of the plant.

5. Remove a leaf from each plant species and bring it back to the laboratory for microscopic study.

ii.Measurement of physical factors

1. With the aid of a compass, note and record the aspect of the beach at every tenth-meter point along the transect line. Make sure that all readings are identical.

2. Record the change in height and gradient of the beach surface by the following steps:

a. Place one ranging pole at 0m (point A in the diagram) by the transect line and the other at 10m (point B in the diagram) along the line.

b. Tie the nylon string to each ranging pole to make a horizontal line at suitable level above the ground. Use the spirit level to check that whether the line is horizontal or not.

c. Measure the height of the nylon string above the ground at 0m and at 10m and record the difference. If the ground increases in height from the 0-10m positions, record the difference as ‘+’; if the ground decreases in height, record the difference as ‘-‘.

Page IV. Method of Study5


For simplicity, the set-up employed in the surveying is shown below:

3. At 0m, 10m, 20m, 30m, 40m point of the transect line:a. Use a thermometer to record the temperature in the air and under the canopy

of plants.b. Record relative humidity by whirling the hygrometer. Record the wet and

dry bulb temperatures. Calculate the difference and check against the scale to get the relative humidity.

c. Record the speed of wind by holding hand-held wind meter against the wind. Note also the direction of the wind.

d. With the aid of a trowel, dig a sample of soil.e. Measure the soil pH by pH meter, or simply just using pH paper.f. Record light intensity by means of an environmental comparator with light

probe.Record the data on table 6.1.

iii. Animal sampling

1. Place a quadrat on 0m, 10m, 20m, 30m and 40m of the transect line.

2. Search for animals on surface of mud and under stones. Capture all the animals within the quadrat and place them in a plastic bag. Identify the animals.

3. By means of a spade dig up the top 5cm (if hard soil) or 10cm (if soft mud) of mud within the quadrat.



Quadrat on the stony soil. Various organismscan be found within thequadrat ►

A lot of live analysis has to be done in the field trip ►



B. Laboratory work

i. Soil analysis

1. Determine the soil water content by the following steps:a. Weigh an aluminium foil tray.b. Weigh about 10g of the soil sample into the tray.c. Place the tray and its content in the oven at 105oC overnight.d. When the soil is dry, transfer it to a desiccator and allow cooling, then

weigh the tray and its soil content on an electronic balance.2. Analyses the composition of the soil by the following steps:

a. Place the dried soil in a clean mortar.b. Use a pestle to break up the soil aggregates particles ready for sieving.c. Assemble the sieves in order of decreasing mesh size, the coarsest at the

top. d. Place the soil in the top sieve and shake the sieve vigorously but with

care from side to side.e. Weigh the soil particles on each sieve and record as a percentage of the

total weigh of soil sieves.

ii. Microscopic study

1. Carry out free hand sectioning practice for each leaf.

2. Stain the leaf sections with methylene blue.

3. Select the best slice of each leaf, and prepare a temporary mount slide. Cover with a cover slip on a glycerine mountant. Clean the slide.

4. Observe under microscope.

◄ A slide of a leaf of Kandelia candel stained with methylene blue is shown



5. If necessary, observe the adaptive features under microscope.



V. Abiotic factors in Sai Keng A. Live analysis

Quadrant number

Distance (m)Animals

found in the quadratSoil texture

1 0shrimp,

Cerithidea spp. (b),Terebralia sulcata Born (b)

mainly muddy,with slightly sandy

2 10sesarmid crabs (ss),

Terebralia sulcata Born (b)sandy & stony

3 20sesarmid crabs (us),

Terebralia sulcata Born (ss)sandy & stony

4 30 Clithon spp. (ss) mainly sandy,

with slightly muddy

5 40

fiddler crabs (us),monk crabs (ss),

Gafrarium tumidum (ss),Mudskipper (ss)

muddy and waterlogged

Table showing the animals found in the quadrat along the transect line:

The area covered by different species in 5 quadrats:

0m 10m 20m 30m 40m

m = mosses g = gastropods b = bivalves

Page 9 V. Abiotic Factors in Sai Keng

*Key: b = base of mangrove trunks ss = on soil surface us = under soil


Table 6.1 shows the abiotic factors along the transect line in Sai Keng:

Quadrat number

Distance(m)

SlopeSoil temp

(℃)Soil pH

Wind speed& direction

(km/h)

Relativehumidity

(%)

Light intensity

(kJm-2min-1)1 0 -0.0201 25 8.27 10 (S) 32 462 10 -0.0124 26 8.42 15 (W) 30 503 20 0 26 8.40 12 (NW) 29 484 30 -0.0083 27 8.45 20 (W) 27 605 40 -0.0127 29 8.50 28 (SW) 28 60

*Remark: Slope at ‘0 m’ means that the slope is measured from 0 to 10 m, ’10 m’ means that it’s measured from 10 m to 20 m, etc.

Slope profile of the mangrove habitat along the transect line:

Page V. Abiotic Factors in Sai Keng

*Key:

= Kandelia candel

= Aegiceras corniculatum

10


B. Laboratory analysis

Table 6.2 shows the soil water content at 10m of the transect line:

Soil before drying (A) 7.62 gSoil after drying (B) 4.29 g

Soil water content (× 100 %) 43.7 %

Table 6.3 shows the composition of soil at 10m of the transect

line:Composition

Weight

Gravel(＞ 2 mm)

Coarse sand(＞ 0.85 mm)

Silt(＞ 0.15 mm)

Clay(＜ 0.15mm)

Weight (g) 2.38 0.83 0.96 0.12Weight (%) 55.48 19.35 22.38 2.80

Page V. Abiotic Factors in Sai Keng11


VI. Analysis of the results obtained

A. Zonation pattern of the plants along transect line

Along the transect line, there are altogether three types of plant found along the transect line. They are mosses, Aegiceras corniculatum and Kandelia candel. Note that both Kandelia candel and Aegiceras corniculatum are quite short (they always below 1.3m), since the substratum in Sai Keng is hard (the soil is composed of nearly 56 % gravel). Here are some of the description and their distribution.

i. Mosses:

It is found mainly in the 0m to 10m along the transect line. As the slope gradient is descending along the transect line, the stone will be covered with seawater in most of the time. This high salinity is not suitable for the growth of mosses. Moreover, the habitat at a higher attitude ensure that it can be explored to air for sufficient oxygen to gaseous exchange. However, it cannot be survived at a higher zone without water as it will died of desiccation and unavailable of the mobility of antheridium.

Mosses are generallyfound on the stone at a higher attitude ►

ii. Kandelia candel

Family: Rhizophoraceae

General features: The tree varies in height, from 1.5 to 5 m with stilt prop root system, pale brown knee joints can be found around the tree. The bark is smooth, greyish or reddish brown.

Leaves: Green, simple, opposite. It usually has 6 to 13 cm long by 2.5 to 6 cm broad, petiole 1 to 1.5 cm long.

Page VI. Analysis of the Results Obtained12


Flowers: White coloured and in cluster, 1.5 to 2 cm long with numerous stamens and 5 or 6 thin petals. Flower begins from April to August.

Fruits: The fruit is a typical mangrove dropper, green to yellowish green; smooth, viviparous. Seedling hypocotyl is pointed, 1 to 1.5 cm in diameter and up to 30 cm long at maturity. Fruit 1 to 2 cm long with persistent reflexed sepals, peduncle elongating.

The flowers of Kandelia candel. Note that they are white and have five petals. ►

Kandelia candel occupies almost all transect line. It is highly adaptive to the mangrove habitat and occurs at the back of the mangrove communities or along the banks of estuarine rivers.

The whole view of Kandelia candel►



Here are some of its structural adaptations to the mangrove habitat:

1. Pneumatophores and knee joints, both with aerenchyma tissue (i.e. a specialized tissue with large intercellular spaces) and lenticels for rapid gaseous exchange in anaerobic condition during tide period.

2. Prop roots to provide stability on unstable substratum and against tide.

3. Viviparous reproduction with pointed apical droppers. This can be inserted in the muddy soil for dispersion or float on water and develop adventitious roots when they touch the soil.

4. Leaves with sunken stomata and thick waxy cuticle to prevent desiccation.

Dropper of Kandelia candel. Adventitious roots are developed for anchorage and absorption. ►

Owing to the high adaptability of the seed of Kandelia candel, it is not difficult to find that Kandelia candel is the dominant species in Sai Keng that is abundant along 0m to 30m of the transect line.

iii. Aegiceras corniculatum

Family: Myrsinaceae

General features: Evergreen shrub with height of 1.5m to 4m. The bark is smooth, dark grey or reddish brown.

Leaves: Simple, leathery, often notched at the tip, 5 to 7cm long by 3 to 4cm board, it has salt glands to remove excess salt absorbed, thus can survive in seawater.

Flowers: White and small, appear in dense clusters, with 5 partly fused petals, pentamerous in shape, pointed in bud with 1 to 2 cm long.

Fruits: 4 to 6cm long, small, curved and sharply pointed, green to reddish in colour. Viviparous with pointed apical droppers.



Dropper of Aegiceras corniculatum, note that they are curved and pointed ►

The structural adaptation of Aegiceras corniculatum is the similar to that of Kandelia candel. However, the leaves of Aegiceras corniculatum have salt glands to secrete absorbed salts, which is not found in Kandelia candel.

I suppose Aegiceras corniculatum grows better in the muddy soil which is at the lower gradient, since they seldom found on the stony ground from 0m to 30m and they are getting denser beyond 30m of the transect line (i.e. at the seaward).

B. Zonation pattern of the animals along transect line

Along the transect line, there are altogether eight types of animal found along the transect line. They are Fulvia, Gafrarium tumidum Roding, Lunella coronata granulata Gmelin, Clithon oualaniensis Lesson, Terebralia sulcata Born, Mictyris longicarpus, Uca vocans vocans Linnaeus, and Periophthalmus cantonensis. Here are some of the description and their distribution.

i. Clithon oualaniensis Lesson:

It is common on sandy shore and shell and hard cover where fresh water off and on the bases of mangrove trunks. Its shell length does not exceed 1cm and is very abundant in mangrove.

ii. Terebralia sulcata Born:

It is very common on mangrove swamps and on mid-tidal mudflat. It has very large thick shell (up to 6cm length) to reduce water lost. It uses mantle cavity as an air-breathing lung. That is why it can be found on the large range along the transect line.



Clithon oualaniensis Lesson (right hand side) ◢

Phylum: Mollusca Class: Gastropoda Order: Archaeogastropoda Family: Neritidae

iii. Gafrarium tumidum Roding:

It is found on mud flat and lower part of the middle shore. Its shell is thick and inflated. This facilitated it from hiding under the muddy soil.

Gafrarium tumidum Roding ►

Phylum: MolluscaClass: BivalviaOrder: VeneroidaFamily: Veneridae

iv. Lunella coronata granulata Gmelin:Page VI. Analysis of the Results Obtained16

Terebralia sulcata Born(left hand side) ►

Phylum: MolluscaClass: GastropodaOrder: MesogastropodaFamily: Potamodidae


It is found near the tide line. Its shell is flatten, with large body whorl inflated base as well as round operculum.

Lunella coronata granulata Gmelin ►

Phylum: MolluscaClass: BivalviaOrder: ArchaeogastropodaFamily: Turbinidae

(Note that there is some mosses attaches on the shell of it, which is an example for commensalism)

v. Fulvia sp.:

It is common on muddy bottom and in low or shallow waters. Its calcareous shell prevents it from desiccation and provides protection.

Fulvia sp. ►

Phylum: MolluscaClass: BivalviaOrder: VeneroidaFamily: Cardiidae

vi. Mictyris longicarpus (monk crab / soldier crab):



It is very common on mud flat and sandy shore. Its carapace is spherical and powder blue at the back. Its legs are white and capable of digging down the sand.

Mictyris longicarpus ▼►

Phylum: ArthropodaClass: CrusaceaOrder: DecapodaFamily: Mictyridae

vii.Uca vocans vocans Linnaeus (tidal crab):



It is found on protected soft mud flats beyond the mangroves. Its large claw granulose is one of the main characteristics for distinction and its legs are dull yellow. Just like another Uca species, male adult has one enlarged chela on one side.

Uca vocans vocans Linnaeus ▲►

Phylum: ArthropodaClass: CrusaceaOrder: DecapodaFamily: Ocypodidae

viii. Periophthalmus cantonensis (mudskipper):

It is very common on mudflat of mangrove swamps and on branches or twigs of mangrove trees. Its body is grey-brown with irregular markings. Its strong pectoral fins are well muscled to enable them to hop along the mud surface. Their eyes are large and elevated well above the head to have a greater field of vision for predation and escape. Their buccal cavity can traps water and gills can absorb oxygen from it. That is why it appears on the wet muddy soil althoughit belongs to Osteichthyes.

Periophthalmus cantonensis ►

Phylum: ChordataClass: OsteichthyesOrder: PerciformesFamily: Periophthalmidae



VII. Problems of life in mangroveOne of the major differences between mangrove habitats and the others is

mangroves are covered with seawater periodically. The drastic physical factors make it a special habitat for plants and animals. Here are some of the problems they have to face with.

1. Anaerobic condition:

Anaerobic condition is one of the common physical factors with the descending of slope gradient. The muddy soil is waterlogged with poor aeration during tides. The grey or dark colour of the soil is an evidence as sulphur bacteria react with the mineral of the soil producing hydrogen sulphide in anaerobic condition. The availability of oxygen for cellular respiration is the basic survival factor organisms have to meet.

2. Unstable substratum:

The muddy soil of mangrove habitat is composed of fine silt particles, which is packed loosely. These particles are easily washed away by water currents. As a result, organisms have to face the problem of being washing away by tide and how to settle down.

3. High salinity:

The high salinity of seawater make plants has to face the osmotic problem. In the estuarine area as well as rainy days, the soil pH of about 8.4 will vary for about 2-3 pH. Hence organisms have to tolerate the fluctuation of salinity.

The muddy soil will be covered with seawater during high tide. This leads to the large variety of the mangrove habitat ►

Page VII. Problems of life in mangrove20


4. Desiccation:

During exposure at low tide and movement of water away from root tissues of plants to outside by osmosis, as well as the salty water are unavailable for metabolically uses, mangrove organisms have to cope with how to conserve water.

5. Difficulty in reproduction:

The seeds of plants can only grow under low salinity condition and develop adventitious roots when contact with soil. However, the soil is covered with seawater periodically. In order to disperse the next generation, plants have evolved unique reproductive strategies.

Page VII. Problems of life in mangrove21

Biology Field Trip Report──The Mangrove 2000 / 01

VIII. Adaptations of plants to mangrove

In order to withstand the drastic condition of mangrove habitat, plants have developed several structural adaptations which are unique to survive.

1. Adaptation to anaerobic condition:

Pneumatophores are erected aerial roots, which arise from cable roots with aerenchyma tissue (with large intracellular air space) and numerous lenticels. This allows plants to exchange gas with atmosphere and facilitate rapid transportation of gases.

Knee joints are roots that bend up like an arch, they are also rich in aerenchyma tissues and lenticels. Its function likes pneumatophores.

Both of the adaptations can be seen in Aegiceras corniculatum and Kandelia candel.

Pneumatophores of Kandelia candel. They are erected from cable root under soil as aerial roots.►

2. Adaptation to unstable substratum:

Mangroves have developed specialized root systems for anchoring. For example, the prop roots. Prop roots are branched and looping aerial roots. Since they arise from the trunk, they can broaden the base and thus provide stability. Prop roots can be found in both Aegiceras corniculatum and Kandelia candel.

Another root system is cable roots. Cable roots spread horizontally and laterally just below the soil surface and spread widely. So the plants can anchor the soil firmly in a larger surface area.

Page VIII. Adaptations of Plants to Mangrove22


Prop roots of Kandelia candel at the base of trunk. The erected structures beside prop roots are pneumatophores.►

3. Adaptation to high salinity:

To cope with salinity, mangroves have developed the following mechanisms to regulate the salt concentration in the plants:

Salt glands in leaves can concentrate and secrete the absorbed salt actively. It is presence in Aegiceras corniculatum.

Just like other halophytes, mangrove plants have to face the osmoregulatory problem. Storage of low molecular weight carbohydrates is one of the solutions. Such lower osmotic potential enable the plants to absorb water (which has a relatively high osmotic potential) from a salty environment.

Moreover, mangrove plants (e.g. Aegiceras corniculatum and Kandelia



candel) prevent salt from entering the root xylem by an active pump mechanism to maintain a relatively constant osmotic potential of their cells.

4. Adaptation to desiccation:

Besides how to absorb more water for the mangrove plants, one other solution to the shortage of water is to conserve water. Mangrove plants can achieve this by developing a special type of leaves:

For both Aegiceras corniculatum and Kandelia candel, sunken stomata and thick waxy cuticle in leaf surface can reduce water loss by evaporation.

Other than these special features of leaves, multi-layered water storage tissues are also present to store water (e.g. Kandelia candel and Aegiceras corniculatum).

5. Adaptation to difficulty in reproduction:

Due to wave action and unstable substratum, survival rate of seedlings are low. So, mangrove plants need to develop unique reproductive strategies:

The seeds of Aegiceras corniculatum and Kandelia candel germinate inside the fruit without resting stage such that they can obtain water and nutrients from parent plants. Once the viviparous development accomplished, the droppers can develop rapidly to young plants after they were detached and contacted with substrate. This method can prevent the seeds from suffocation and desiccation by seawater.

Their droppers also have elongated shape with flesh hypocotyls that they can float on the seawater for dispersal. Its pointed apical also enable them to insert into substrate after detached so that up-right position can be maintain to expand the root system rapidly. This can prevent it from washing away by tide.



A developing dropper of Kandelia candel. Note that it can grow on stony soil and four leaves are developed. ►



IX. Adaptations of animals to mangrove

1. Adaptation to loose soil:

Crabs and mollusca are able to burrow themselves in mud to prevent it from washing away by tide. Gastropods can also attach on the trunk of mangrove plants. Rock oysters cement their shells strongly to surface of rocks.

Burrows in mud. It is very common in mangrove habitat. Crabs can be found in such borrows.►

2. Adaptation to desiccation:

Crabs and gastropods have calcareous shells, which are impermeable to water so that they can reduce the risk of desiccation during low tide. Gastropods are also able to close its operculum during low tide to reduce water loss.

The ground-dwelling animals such as crabs hide in burrows in mud to conserve water and secrete mucus to keep its body surface wet.

Page IX. Adaptations of Animals to Mangrove25


3. Adaptation to feeding:

Base on the fact that mangrove habitat is rich in organic debris and suspension, many organisms are filter feeders (e.g. rock oysters and mangrove clams) and detritus feeders (e.g. crabs and worms).

4. Adaptation to movement:

Crabs have four pairs of legs for movement between muddy soil and tidal zone for feeding.

Mudskippers have strong muscled pectoral fins, which are jointed half-way down to enable them to hop along the mud surface.

5. Adaptation to gaseous exchange in poorly aerated soil:

Crabs use gills for gaseous exchange in water and have haemocyanin which can store oxygen.

Mudskippers have bony gill filaments so that they will not collapse in the air. Their buccal cavity also able to traps water, which contains oxygen, then oxygen diffuses to gills and absorbed for respiration. That is why they can temporary move to muddy soil for predation (mudskippers are carnivores).

Mudskipper on the muddy soil. ►

Page IX. Adaptations of Animals to Mangrove26


X. Biotic factors in Sai Keng

Besides abiotic factors affecting the plants and animals live in mangrove, certain biotic factors also interact between the organisms. Just like another habitat, inter-specific interaction (e.g. inter-specific competition and predation) and intra-specific interaction (e.g. intra-specific competition, commensalism and mutualism) also exist in mangrove. Here are some of the examples in which indicating the biotic factors in Sai Keng.

A. Inter-specific interaction

i. Inter-specific competition

When two species occupying the same niche, they have to compete food, water or sunlight with others. Take Kandelia candel and Aegiceras corniculatum as an example, they both are plants which grow on the muddy sand in mangrove. On the ground that the aerated space is limited, they have to compete for the development of branches and leaves for growth and photosynthesis.

As they grow so crowd, their droppers also have to face such competition. The one that grow faster with stem and leaves will mask the others, which cannot grow up successfully. This may be one of the factors that contribute to the domination of the species in that niche.

This competition also exists between animals. Cerithidea and

Page X. Biotic Factors in Sai Keng27


Clithon will compete for mosses grow on the rock as food. Mudskippers and fiddler crabs will compete for territory of burrow in mud.

Inter-specific competition ─── Kandelia candel (low left corner) and Aegiceras corniculatum (top right corner) compete with aerated space, water and nutrients. ►

i. Predation

When two distinct species occupying different trophic levels, the upper one would be predator and the lower one would be prey.

ii.Commensalism

Commensalism is a close association between two living organisms of different species, which is beneficial to one (commensal) and does not affect the other (host). Terebralia sp. attaches on the branches of Aegiceras corniculatum, which can prevent it from washing away by buffeting force in the highest tide and attachment in the lowest tide. Terebralia taking advantages from Aegiceras corniculatum without harming it is a typical example of commensalism.

Another example is the mosses attaches on Lunella sp. benefit for locomotion. (Refer to p.17)

B. Intra-specific interaction

i. Intra-specific competition

Within the same specie, once they are live in the same niche, competition for food and territory are common among them. These can be easily seen in the examples of Aegiceras corniculatum for aerated space for growth and capturing sunlight; hermit crabs for shells and burrow in mud.



Intra-specific competition ─── Two gastropods are competing for the food of mosses on the rock. ►

C. Human activity

Human activity probably attribute to the main disturbance of the mangrove organisms. As Sai Keng mangrove is beside Sai Keng village, villager may change the land use of mangrove habitat to building and contaminate the mangrove (these will be discussed in XII). Such activities will disturb the equilibrium of the ecosystem in Sai Keng mangrove.



XI. Food web and dichotomous key of mangroveA. Food web in mangrove

Birds

Mudskippers

Crabs

Bivalves Gastropods Worms

Algae Detritus & Dead animalsbacteria

A simplified food web in mangrove habitat

B. Dichotomous key of mangrove animals along the transect line

1a. With hard shell 21b. Without hard shell Periophthalmus cantonensis

2a. With legs 32b. Without legs 4

3a. With an enlarged claw Vca vocans vocans Linnaeus3b. Without an enlarged claw Mictyris longicarpus

4a. Shell with an anterior round hole 54b. Shell without an anterior round hole 7

5a. Shell length does not excess 1cm Clithon oualaniensis Lesson5b. Shell length excess 1cm 6

6a. With a conical shape shell Terebralia sulcata Born6b. Without a conical shape shell Lunella coronata granulata

Gmelin

7a. With a thick shell Gafrarium tumidum Roding7b. With a thin shell Fulvia sp.

Page XI. Food Web and Dichotomous Key of Mangrove29


XII. Man’s impact on Mangrove habitat

Indeed, the Mangrove we visited was not really far from human habitation. Little pollution there was found. We found some trash like old furniture, wood and bulky waste. In fact, the pollution was not so serious as to harm the animals in the mangrove. However, if the trash is not dealt with properly, some poisonous substance may leak out under microbial decomposition to deteriorate the soil affecting the normal growth of Mangrove plants.

As seen in the previous parts, environmental factors are exerting great influence on the ecosystem of Mangrove. For example, a change in oxygen content, as caused by a sudden increase in micro-organisms in the water as a result of an large increase in organic waste released by villagers, can cause suffocation of marine organisms in the area.

Moreover, domestic wastes (e.g. nitrogenous waste) released act as nutrients which promote the growth of root system of mangrove plants and lead to algae bloom. These abnormal vary in organisms population will disturb the equilibrium of the ecosystem in mangrove.

According to the continuous construction works in Sai Keng village recently, destruction to the mangroves in Sai Keng has brought an irreversible damage. Construction wastes are dumped beside the mangrove and the scenery of Sai Keng mangrove has been destroyed by the villa.

The villagers’ buildings

Page XII. Man’s Impact on Mangrove Habitat30


have destroyed the scenery of Sai Keng mangrove. Some boats are abandoned on the muddy soil ►

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XIII. Conservation of Mangroves in Hong KongMangroves in Hong Kong have been destroyed seriously over years due to

coastal development and reclamation. The two areas of greatest mangrove abundance in Hong Kong, Deep Bay and Tolo Harbour, have lost 85% and 42% respectively of original mangrove cover. Recently, due to the construction of the new airport at Chek Lap Kok on Lantau Island, associated port and highway developments, an additional 7 ha of mangroves on Lantau Island have been destroyed.

There is now a growing awareness that mangrove ecosystems are of great importance to Hong Kong. Mangroves are now considered to be a conservation priority in Hong Kong.

The size, species diversity, rarity and abundance, degree of pollution and naturalness are the main factors affecting the order of priority of conservation of mangroves. Indeed, Mai Po, Lai Chi Wo and Ting Kok are the extremely important mangrove stands that must be conserved immediately, which cover a total area of 127 ha.

In fact, HK government has done something so as to conserve the mangroves:

1. Enforcement of existing ordinances to protect mangrove stands, designated as restricted area (e.g. Mai Po), marine parks (e.g. Hoi Ha Wan), and country parks (e.g. Kei Ling Ha Hoi).

2. Good management of the stands (e.g. Sai Keng, Ting Kok) to prevent illegal development and damage.

3. Promote public awareness, e.g. develop educational programmes, encourage scientific research and study tours, etc.

4. Planting and replanting of mangroves. Replanting of mangroves takes place in Hong Kong throughout the year but in a small scale only. Examples include replanting by Agriculture, Fisheries and Conservation Development (AFCD) and Friends of Earth (FOE) at Kei Ling Ha Lo Wai.

Page XIII. Conservation of Mangroves in Hong Kong31


XIV. Safety Precautions

1. Do not go to the site or still work in mangroves if the weather is not good, or likely start to get worse.

2. Suitable clothing (e.g. thin long sleeve shirts for summer) and shoes (e.g. waterproof boots) are recommended, since the branches of the mangrove plants may be very sharp, you may get scratches when you walk across it.

3. As mangroves are located in inter-tidal zone, the time of the tide are important fieldwork, so it’s necessary to know the time of the extent of tidal range before go there. It’s dangerous when doing a field study during thunderstorm.

4. Take care for the wild dogs if they are present in there.

5. The substratum of mangrove stands usually varies from stony to muddy. When wandering across the mangroves, or crossing a creek, you must take care for the nature of the bottom because soft and deep mud may make you get stuck or even sink into the mud. If this occurs, do not panic and do not make violent movements, leave your boot inside the mud slowly and rest the leg on roots or just ask your team members for help.

Such muddy substratum is quite dangerous ►

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XV. Comment Exciting! Meaningful! Unforgettable! Being a F.6 student, it was our first time

to have lesson outside classroom! On 23rd May, 2001, all our biology group went to Mangrove in Sai Keng. We were asked to do all sorts of field works like soil sampling, measuring physical factors and animal capturing in Mangrove. The sun was shining high above our heads on that day and our clothes nearly got soaked through, but we did enjoy every moment. We believed that it was much more fun to have a closer contact with nature than having lesson in a boring room!

In Chinese, Mangroves are literally “red-tree forests”. They are actually trees and shrubs growing in the zone between high and low tide marks, and there are abundant animals and micro-organisms associated with such plants. Through this trip, we have learnt how plants adapt to the adverse environment. Believe it or not, some of the Mangrove plants are said to be able to cure stomachache. Isn't it marvellous?

What impressed us the most in the trip were mudskippers! They were fish-like organism being able to jump! We found it so hard to capture them─as we were about to capture them, they were so sensitive to skip “miles” away! However, we never say die! Finally, we could capture few of them for specimen! So lucky!

This was really an exciting, meaningful and unforgettable lesson. Apart from learning lots of features of Mangrove, we learnt a great deal about the value of nature. However, we hoped that time for our field work could have been prolonged because the time provided was too rush for us to do all the field work required. That’s why we grasped every second to do our research!

Environment changes, as it always does. We cannot control the changes that occur naturally, but we can minimize the detrimental effects of human impact by living in a more environmentally friendly way. Conservation proves to be a difficult task in Hong Kong since the density of population is so high and the awareness of local residents is yet to be raised. It is high time for not only the government, but we also protect the valuable habitat and resources for a better place for human to live.

We, our whole biology group, are looking forward to another visit…

XVI. Conclusion

The most noticeable thing about mangroves is the zonation of organisms. The particular species of organism in each zone may vary with different physical factors such as levels of tide and air exposure at different time intervals, but all show special adaptations to living in this area, and the basic structure is recognizable on most mangrove environments.

─The End─ XV; XVI. Comment & Conclusion

mangrove in sai keng

Documents

mangrove page

mangrove animals

set page

mangrove plants

dichotomous key of mangrove

common mangrove organisms

piece of cotton thread

adaptations of animals