Abundance of commercial bivalves

1. ABUNDANCE OF COMMERCIAL BIVALVES COLLECTED AT
ZONE 6, BRGY. BULA
GENERAL SANTOS CITY, PHILIPPINES
2009

2. INTRODUCTION
Bivalves are mostly marine and chiefly sedentary bottom dwellers. They are most common from the tide lines into shallow waters, but some occur down to 5000m. (17,000ft.).A few creep slowly on the bottom, but most bivalve species burrow in the sand or mud.
According to Ward and Shumway, bivalves are one of the important factors in coastal productivity especially in the seagrass community. They perform a significant role in the cycle of nutrients in the coastal waters. These animals are generally filter feeders which feed on organic particles and microorganisms suspended in the water. Their waste materials are deposited in the substrate which will then be used by marine plants like seagrasses. Bivalves are also prey for food fishes like flounder and cod.
Several species of bivalves are consumed by humans as food since they are a good source of protein. Their shells are also used for many purposes including the making of buttons and other ornaments. Coastal residents do gleaning to get food and even to generate income. A large percentage of the shellfish industry is provided by bivalves. The culture of oysters is a major industry in many parts of the world, not only for food but also for pearl production (Ghiselin, 2006).
The presence and abundance of bivalves usually indicate a good and favorable area for their growth and survival. Changes in the water quality can adversely affect the presence of bivalves since they are filter feeders and depend only on the surrounding waters. Their abundance and distribution also signifies whether harvest of bivalves in the area is at sustainable rate or not.

3. Considering that Zone 6, Brgy. Bula supplies bivalves in the nearby residential areas and in some shellfish stands in GenSan public market, it is necessary that the temporal occurence of bivalves in the area is determined. In this way, bivalve production in seagrass community is assessed and monitored. This will tell whether the physico- chemical factors and human activities especially gleaning are detrimental or tolerable.
Objective of the Study
The study generally aims:
1. To determine the abundance index of the commercially valuable bivalve species collected in Zone 6 Brgy. Bula, General Santos City
2. To identify the commercial bivalve species present in the area
3. To determine the physico-chemical properties of the water – temperature, salinity, and total suspended solids (TSS) - along Zone 6, Brgy. Bula, General Santos City.
4. To assess seagrass abundance, frequency and dominance
5. To determine whether there is a significant difference in the number of bivalves collected with respect to date of collection and whether the number of bivalves is equal for every species using two-way ANOVA.

4. Significance of the Study
The study provides baseline information on the abundace of bivalves in the seagrass community fronting the mangrove area of Zone 6, Brgy. Bula, General Santos City. Thus, this provides an overall view of the present population of the commercial bivalves in this area where gleaning activities are at a high rate and happens daily. This can serve as a basis for enforcing management and regulation policies on the side of the LGU and the direct stakeholders. Finally, this study serves as a reference for future related studies.
Scope and Limitation of the Study
This study is limited only on the determination of the abundance of commercially valuable bivalves in Zone 6, Brgy. Bula, General Santos City. Moreover, the study included identification of bivalves collected in the area; and determination of physico- chemical parameters such as water salinity and temperature and also totals suspended solids (TSS) as limiting factors and determination of percentage cover of seagrass. This study was done from January 23 to February 11, 2009. For 20 days, the number of commercial bivalves collected by resident gleaners was monitored. Seagrass assessment was done once. Water sampling for physico- chemical parameters was done three times; during full moon and new moon. For statistical analysis of the data on bivalves collected, two-way ANOVA was used.

5. REVIEW OF RELATED LITERATURE
General Characteristics of Molluscs
The animals in the phylum Mollusca have soft bodies with bilateral symmetry, are unsegmented and many of them possess a shell of calcium carbonate material. Snails, clams, slugs, and squids do not appear to have much in common at first sight, but a closer examination reveals several characteristics that are possessed by all. One of these is an organ called the muscular foot- which the snail uses for creeping over surfaces; the clam uses for burrowing on soft substrates; and the squid uses it for seizing prey. In several animals, there is a space called the mantle cavity between the main body and the mantle. (Engemann and Hegner, 1981)
The mollusks are divided into seven (7) classes according to their symmetry, and the characteristics of the foot, shell, mantle, gills and nervous system. (Storer et. al., 1979)
Characteristics of Bivalves
Bivalves are bilaterally symmetrical and laterally compressed, with soft body enclosed in rigid shell of two parts called valves- hence, they are called bivalves. The shell of a bivalve is a firm exoskeleton that protects the body and provides attachment for the muscles. The valves are hinged dorsally by an elastic ligament. They do not possess head. The muscular foot is usually wedge-shaped (hence, the earlier name of the class is Pelecypoda), and the gills are thin and plate-like (hence, another earlier name is Lamellibranchia).

6. According to Storer et. al., bivalves make up the second largest molluscan class with over 20,000 species which are widely distributed in both freshwater and marine environments.
Marine bivalves are mostly sessile or sedentary burrowing animals. The soft- substratum dwellers (infauna) include cockles, soft-shell clam and bent-nosed clam. Some bivalves are epifauna which live on top of the substratum and creep slowly on the surface like hard-shell clam. There are also species that are sessile or attached to solid substrates like mussels and oysters. Some bivalves are highly specialized like shipworms that burrow in wood of ships and wharves; the scallop is attached when young but later swims freely by clapping its two shells.
All members of this class differ from the other mollusks in that they lack radular feeding mechanism. Bivalves are mainly herbivorous, feeding either by drawing in seawater containing plankton and other particulate nutrients or by sucking up detritus. The mantle in most bivalves has evolved into siphons to maintain the contact with the water column while burrowing. The fine food particles are drawn in through the siphon and passed over to the gills where they are trapped in ciliary mucus. Thousands of cilia beat the food and mucus towards the mouth. Food is digested in the stomach and absorbed in the intestine. (Abbott, 1982)
Reproduction and Growth of Bivalves
The sexes in bivalves are separate. Most bivalves set their eggs and sperm free. Fertilization occurs in the water and the zygote develops into trochophore larva, the into a veliger larva. The pelagic larva lives in water for about two weeks and then sinks to

7. the bottom to grow as a miniature bivalve. Since the hazards of this reproduction are great, bivalves produce vast number of eggs from about 16 million to 60 million may issue from an oyster. In the case of clams and cockles, it takes about one (1) year for the seed (miniature bivalve) to grow into a young and another 2-3 years to attain marketable size of about 2cm.-3.5cm. (Nie, 1982).
Ecological Roles of Bivalves
Along the coastal waters, particle-feeding molluscs are found in large populations that profoundly affect the pelagic and benthic processes. Bivalves play a part in the nutrient assimilation and cycling. These animals exert great control over the particle supply and distribution in many coastal ecosystems and therefore can influence the biology and ecology of other marine organisms. Through suspension- and deposit-feeding activities, bivalves can cycle large amounts of particulate matter within the environment, converting some of it into flesh and gametes, depositing varying amounts to the benthos and cycling complex molecules into inorganic forms (Ward and Shumway, 2004). Suspension feeders play such a key role in water filtration that regulates light availability and seagrass growth, thus, contributing substantially to maintaining water clarity and seagrass density. High turbidity that reduces light reaching the plant leaves would lead to the decline in seagrass population.
The Effects of Temperature and Salinity on Bivalves
Temperature and salinity are two major environmental factors controlling marine species distribution and influencing physiological processes of marine and estuarine

8. organisms, such as feeding, respiration, growth and reproduction. For marine bivalves especially those inhabiting coastal and estuarine waters, seasonal changes in temperature and salinity would influence metabolic activities. Effects of temperature and salinity on physiology and energetics of filter-feeding bivalves, especially those of commercially important ones such as mussels, oysters and clams, had been well documented (Tang, 2005).
Some bivalves have the ability to acclimate and adapt changes in water temperature to survive and reproduce, such as M. edulis and O. edulis. The ability to compensate physiological functions allows intertidal organisms to conserve energy, which would otherwise be expended by increasing metabolic costs incurred with increasing environment temperature, and potentially allows for the reallocation of this conserved energy to maintain production, growth and reproduction at higher temperature.
Clams tolerate temperature range of 280 to 340C (Young and Serna, 1982). Green mussels, Perna viridis grows to the optimum in temperatures between 260 to 280 C (Nie, 1982). According to Tang et. el., the best metabolic salinity range for Meretrix meretrix is between 26 ppt and 36 ppt. Tapes literatus can tolerate salinities between 15 to 34 ppt. P. viridis survives in salinity ranging from 26 to 36 ppt.
Relationship of Total Suspended Solids and Bivalves
Aquatic particle feeders are confronted with wide range of living and non-living materials suspended in the water column. These particulate matters include: silt, mineral grains of various sizes; mixed with a variety of organic matter derived from plankton, microalgae, feces, detritus, protozoa and bacteria industrial wastes, and sewage. These materials

9. constitute the Total Suspended Solids (TSS) in the water. Therefore, in studying the distribution of bivalves, it is necessary to measure the TSS in the water sample. High TSS in a water body can often mean higher concentrations of bacteria and nutrients (Murphy, 1998). This is beneficial to the bivalves which are feeding on these matters. Although, high concentrations of suspended solids can cause many problems for stream health and aquatic life. High TSS can also cause an increase in surface water temperature, because the suspended particles absorb heat from sunlight. According to Mitchell and Stapp (1992) as cited by Murphy, this can cause dissolved oxygen levels to fall even further (because warmer waters can hold less DO), and can harm aquatic life in many other ways.
There are various factors affecting total suspended solids. The flow rate of the water body is a primary factor in TSS concentrations. A change in flow rate can affect TSS; if the speed or direction of the water current increases, particulate matter from bottom sediments may be resuspended. Soil erosion also affects TSS level. Eroded soil particles can be carried by storm water to surface water and increase TSS. Urban and industrial runoffs can also affect TSS levels in the seawater.
Shellfish Industry
Bivalves such as oysters, mussels, clams and cockles are widely distributed throughout tropical waters. In many countries of Asia and the Pacific, it is a tradition to collect the naturally occurring mollusks as a cheap source of food. In the review of bivalve exploitation in 13 countries in Asia and the Pacific indicated that in some countries, resources are not being exploited, while in others, an industry involving millions of

10. dollars exists. Exploitation ranges from collection of natural stocks to major culture operations (Davy and Graham, 1982). High production of oysters, mussels, cockles and clams were reported from many countries including Philippines where the export earnings from shell production are 850 million pesos. (Young and Serna, 1982)
There are 22 commercially important bivalve species in the Philippines. But of the species marketed domestically, only green mussel (Perna viridis) and slipper oyster
(Crassostrea iredalei) are farmed commercially. Most other bivalves are collected from the wild. (Young and Serna, 1982)

11. METHODOLOGY
Study Area
The study was conducted in Zone 6, Brgy. Bula, General Santos City specifically in the intertidal zone fronting the mangrove area. It is located at 6o05’56.31”N 125o12’08.23”E. The site has a total area of about 15,000 m2. Approximately, the seagrass meadow has about 300 m stretch along the coast. During lowest tide, the intertidal zone is exposed up to 300m. away from the mangrove area. The mangrove species found in the area are Ceriops sp., Rhizophora sp., Sonneratia sp. and Avicennia sp. Mangrove rehabilitation is still implemented by the CENRO and Brgy. Bula. Adjacent aquaculture ponds that grow Chanos chanos are present in the area. There is a residential area adjacent to the site. Gleaners are remarkably abundant in the area.
Figure 1. The Study Area

12. Figure 2. Map of the Study Area
S E W N N Legend Fish pond Mangrove Seagrass The Study Site

13. Methods for Collecting Bivalves
Most bivalve species collected in the area are infaunal which means that they hide in burrows of a few centimeters deep into the substrate especially during low tide.
Resident gleaners usually collect shells during low tide. They gather bivalves with bare hands or simple digging blades or “bolo”. Others simply feel the shell with bare feet especially in muddy substrate. But the most famous method is through looking for the “smoke” or actually the silt that forms smoke-like when bivalves spurt out water from their burrows. The “aso” indicates the presence of bivalve; the gleaners then clinch the substrate or dig that portion and collect the bivalve. During high tide, when the intertidal zone is submerged, gleaners still collect bivalves by diving. At this time, many bivalves go into the surface of the substrate. Gleaners use gaggles in looking for bivalves and simply pick them. Gleaners transfer from one portion to another in each day to assure that they could collect maximum numbers. There were around 20 gleaners in the area.
Figure 3. Gleaners in the area

14. Monitoring the bivalves collected per day
Each species of bivalve was identified in local terms. The first hand buyer, who gathers all the bivalves collected by every gleaners recorded the bivalve catch from January 23 to February 11, 2009.
Figure 4. Bivalves collected by gleaners
Sampling Method for Seagrass
The method to be used for assessment of seagrass was adopted from English et. al. (1994). The bearing was determined first using a compass. A 50m. transect line was used. Samples will be taken at 10m. interval along the transect line with at least two (2) quadrats of 50cm. X 50cm. All shoots of every species inside the quadrat was counted. Every species were identified. The abundance, frequency and dominance indices were determined after.

15. Figure 5. Seagrass assessment
PHYSICO- CHEMICAL ANALYSIS
Salinity was determined using a hand held refractometer. The viewing chamber was rinsed first with distilled water and wiped with a tissue paper. A drop of sea water was placed on the chamber and while holding the refractometer, salinity was recorded. Seawater temperature was measured with the use of a mercury thermometer in degree Celcius (0C). This was done by direct immersion of mercury thermometer in the water. Weather conditions, wind and sea condition were also observed and recorded using the modified Beaufort scale (Appendix 3).

16. Figure 6. Measuring the temperature
Figure 7. Collecting water sample

17. Figure 8. Measuring TSS
DATA ANALYSIS
For Bivalve
The statistical analysis for bivalve distribution was computed using the formula:
Abundance index = Total # of particular species X 100
Total # of all species
For Seagrass
Abundance index = Total # of particular species X 100
Total # of all species
Frequency index = # of quadrat where a particular species is found X 100
Total # of quadrats
Dominance index = Abundance index + Frequency index

18. STATISTICAL ANALYSIS
The data on the number of bivalves collected in each day was subjected to two-factor Anaylsis of Variance to determine whether number of bivalves collected differ signicantly in relation to time or date of collection and the kind of bivalve.
Two-way ANOVA basically employs two factors or independent variables. This involves the study of simultaneous effects of two factors on a dependent variable. In this case, the effect of date of collection and the species of bivalve on the actual number of bivalves collected was studied.

19. RESULTS AND DISCUSSIONS
This chapter presents the data gathered during the conduct of the study. Furthermore, it discusses and analyzes the information gathered.
I. Species Composition
There were 8 species of commercial bivalves belonging to 6 families identified in the area.
Table 1. The species composition of commercial bivalves at Zone 6, Brgy. Bula, GSC.
FAMILY
SPECIES
COMMON NAME
LOCAL NAME
Arcidae
Anadara maculosa
Blood Cockle
Litub,Bakan
Cardiidae
Trachycardium flavum
Common cockle
Aso-aso
Glycemeridae
Glycemeris ambionensis
Dog cockle
Paypay
Lucinidae
Codakia tigerina
Pacific tiger lucine
Talam-isan
Mytilidae
Perna viridis
Green Mussel
Tahong,Amahong
Veneridae
Meretrix meretix
Asiatic hard clam
Punaw
Tapes literatus
Lettered venus clam
Kandiis
Gafrarium pectinatum
Comb venus clam
Bug-atan
The commercial bivalves in the area are characterized as follows:
Family Arcidae
Anadara maculosa Reeve, 1844
The shell is solid, equivalve and globose, postero-ventral part extended. Surface with 37-39 regular radial ribs which are supported by furrows. Valves are entirely white with

20. velvety periostracum. The shells remarkably have many epiphytes. The size ranges from 4-6 cm.Very common in muddy sand substrates in intertidal area, where they form their burrows around seagrasses like Thalassia hemprichii and Cymodocea rotundata.
Common in the markets throughout the year and commonly cooked as soup.
Figure 9. Blood cockle (Anadara maculosa Reeve, 1844)
Local name: Litub,Bakan
Class: Bivalvia
Order: Veneroidea
Family: Arcridae
Genus: Anadara
Species: maculosa

21. Family Cardiidae
Trachycardium flavum Linne, 1758
Shell ovate and rather thick, inflated, slightly higher than broad. Surface with 27- 30 radial ribs which are partly scaly. Outside is ivory-yellowish, periostracum is yellowish brown. Inside is white but sometimes is tinged with purple. Size is between 3-5 cm. Common in protected bays, like the rest of the bivalves in the area and inhabitat sand or sandy mud substrates where the often form beds. Sold in Gensan Public Market. The meat is eaten.
Figure 10. Common cockle (Trachycardium flavum Linne, 1758)
Local name: Aso-aso
Class: Bivalvia
Order: Veneroidea
Family: Cardiidae
Genus: Trachycardium
Species: flavum

22. Family Glycemeridae
Glycemeris ambionensis
Shell circular and laterally compressed, with 24-25 broad radial ribs. Hinge line broad and strongly curved. Outside is whitish and mottled or spotted with brown flecks or chevron-shaped lines. Interior is bluish, anterior end and hinge line white. Periostracum hardly visible. Sizes range from 3-5 cm. Common in coral sand or sand patches within shallow water below tide marks and seldom found within seagrass beds. Sold in the market and meat is consumed as food.
Figure 11. Dog cockle (Glycemeris ambionensis Gmelin, 1791)
Local name: Paypay
Class: Bivalvia
Order: Veneroidea
Family: Glycemeridae
Genus: Glycemeris
Species: ambionensis

23. Family Lucinidae
Codakia tigerina Linne, 1758
Shell large, circular, thick and compressed with narrow radial ribs and concentric cords, thus appearing like decussate sculpture. Conspicuous nodules at the points of intersection. Outside white, inside yellow, margins with a red flush. Hinge line broad.
The largest among the bivalves in the area that ranges between 5-12 cm. Found in coral sand and sand patches near the coral reef usually in deeper parts. Sold in public market. Meat is cooked for food.
Figure 12. Pacific tiger lucine (Codakia tigerina Linne, 1758)
Local name: Talam-isan
Class: Bivalvia
Order: Veneroidea
Family: Lucinidae
Genus: Codakia
Species: tigerina

24. Family Mytilidae
Perna viridis Linne, 1758
Shell rather elongated and compressed, umbones bluntly pointed. Valves outside whitish with brownish spots; inside nacreous white. Shells often with bluish edges. Sizes range from 4-10 cm. Found on hard substrate in shallow water to low tide mark. They do not burrow or epifaunal. Very abundant in the public market but only very few come from this area. Served as soup,“baked Tahong” or eaten raw with vinegar or sauce.
Figure 13. Green Mussel (Perna viridis Linne, 1758)
Local name: Tahong, amahong
Class: Bivalvia
Order: Veneroidea
Family: Mytilidae
Genus: Perna
Species: viridis

25. Family Veneridae
Meretrix meretrix Linne, 1758
Shell roughly triangular, thick and inflated. Ventral margin gently curved. Outside, yellowish gray to brownish. Inside white, occasionally at the ends brownish to bluish. The size is between 5-6 cm. but the usually size of the area is only 3-4 cm. They inhabit the sandy or sandy-muddy substrates from the low tide mark to shallow water. Very common in markets and the meat is served as food.
Figure 14. Asiatic hard clam (Meretrix meretix Linne, 1758)
Local name: Punaw
Class: Bivalvia
Order: Veneroidea
Family: Veneridae
Genus: Meretrix
Species: meretix

26. Tapes literatus Linne, 1758
Shell large, relatively thin, rhomboidal-ovate, compressed with umbones in the anterior
third of the shell. Surface with fine regular concentric ribs. Outside light brownish to white, with indistinct rays of light brownish, variously marked with dark brown zigzag lines, spots or mottles. Inside white, with yellowish to light orange flush in the umbonal area, sometimes extending to the ventral pallial line. The size is from 5-9 cm. Found in fine sandy and sandy-muddy substrate in the tidal flats and in shallow water. They are completely buried in an upright position at a depth of 3-5 cm. A circular hole is formed by them on surface of the substrate and they spurt out water out of the hole. They are sold in the public market in just a small amount and eaten as food.

27. Figure 15. Lettered venus clam (Tapes literatus Linne, 1758)
Local name: Kandiis
Class: Bivalvia
Order: Veneroidea
Family: Veneridae
Genus: Tapes
Species: literatus

28. Gafrarium pectinatum Linne, 1758
Shell ovate, very variable, more or less compressed, thick and solid with more or less broad noduluos radial ribs, which bifurcate at the anterior and posterior end. Hinge strong and hinge plate with 3 cardinals and one anterior lateral tooth in each valve. The ventral margin is crenulate. Pallial sinus is very small and hardly visible. Outside is whitish with brown mottles and brownish to purplish brown streaks on the posterior slopes. Inside white with hinge line is purplish brown. Sizes range from 3-4 cm. They are found in sandy or sandy-muddy bottom in mangrove areas between tide marks and in shallow water. They do not inhabit the seagrass beds. They are the only species not found in the seagrass. Buried in an upright position. Lives more or less on the same vertical level or substratum.

29. Figure 16. Comb venus clam (Gafrarium pectinatum Linne, 1758)
Local name: Bug-atan
Class: Bivalvia
Order: Veneroidea
Family: Veneridae
Genus: Gafrarium
Species: pectinatum

30. II. Bivalve Population and Abundance
Table 2. Total Population and Abundance index per Species of Bivalves
Species
Total number
Abundance index (%)
Trachycardium flavum
14824
92.04
Anadara maculosa
923
5.73
Codakia tigerina
117
0.73
Tapes literatus
114
0.71
Glycemeris ambionensis
63
0.39
Gafrarium pectinatum
18
0.11
Meretrix meretrix
35
0.22
Perna viridis
12
0.07
The most abundant species of commercial bivalves collected was Trachycardium flavum at 92.04% roughly followed by Anadara maculosa (5.73%). The species Codakia tigerina (0.73%), Tapes literatus (0.71%), Glycemeris ambionensis (0.39%), Meretrix meretrix (0.22%) each had abundance index of less than 1%. Gafrarium pectinatum (0.22%) was the second to the last while Perna viridis (0.07%) was the least abundant species.
Figure 17. Abundance Index per Bivalve Species
Chart Title
T. flavum
A. maculosa
C. tigerina
T. literatus
G. ambionensis
G. pectinatum
M. meretrix
P. viridis

31. The data in Appendix1. represents the number of bivalves collected at Zone 6, Brgy. Bula, GSC from January 23 to February 11, 2009.
We use a 0.05 level of significance to test the hypothesis that
(a) the number of bivalves of every species collected from January 23 to February 11, 2009 do not vary significantly
(b) the number of bivalves collected are the same in all kinds of species
Table 3. Analysis of Variance for Two-way Classification of the number of commercial bivalves collected in Zone 6, Brgy. Bula, GSC
Sources of Variation
Sum of Squares
Degrees of Freedom
Mean Square
Computed f
Row Means
250,443.775
19
13,181.250
f1= 0.0098
Column Means
9,410,492.375
7
1,344,356.054
f2= 118.098
Error
1,513,989.630
133
11,383.380
total
11,174,925.750
159
Since the computed F- ratio of 0.0098 in the row means is lesser than 2.0096 at 0.05 level of significance, then, the first hypothesis (a) is not rejected. It means that the total number of bivalves collected in a day do not vary significantly with respect to the date of collection. Thus, from January 29 to February 11, 2009, the number of bivalves collected is almost uniform. This result could be attributed to the gleaning strategies of the people. According to them, they transfer from one portion to the other in each day. They do not return to where they gathered for few days, so they can still collect as much bivalves as before. If gleaners would have a permanent area to collect bivalves, it is expected that their catch would decrease day after day. Also, there are a little fluctuation on the temperature and salinity in the area that can greatly affect bivalve distribution.

32. Since the computed F-ratio of 118.098 in the column means is greater than 1.58968 at 0.05 level of significance, then, the second hypothesis (b) is rejected. It means that the
number of bivalves are not equal in every species. This result was greatly affected by the very far difference in number of T. flavum from the rest of the species.
Blood cockles or locally known as “aso-aso” was the most abundant species of commercial bivalve in the area. This can be attributed to its habitat preference which is the sand or sandy mud, as in the area. It was also said that they often form beds in quiet bays especially in the intertidal zone or shallow waters. According to (Springsteen and Leobrera, 1986), T. flavum have low vulnerability.
Table 4. Physico- chemical parameters of water and weather conditions along Zone 6, Brgy. Bula, GSC
PARAMETERS
1st Sampling
2nd Sampling
3rd Sampling
Temperature(0C)
34.3
34
35
Salinity (ppt)
35
36
34
TSS (mg/L)
13
84.5
8
Weather Condition
Sunny
Fair
Sunny
Wind*
2
5
0
Sea condition*
2
4
0
Note: * based on the modified Beaufort Scale (Appendix 3)
Based on the physico-chemical parameters of water along Zone 6, Brgy. Bula, the area is good for the growth and survival of some few species especially T. flavum while it poorly supports the other species. The salinity of water in the area ranges from 34ppt to 36ppt which T. flavum, the most abundant species, can tolerate. M. meretrix and T. literatus can tolerate salinities between 15 to 34 ppt. P. viridis survives in salinity ranging from 26 to 36 ppt.

33. The temperature of the coastal water in Zone 6, Brgy. Bula ranges from 34 o to 35 oC. It must be noted that the temperatures were measured during low tide where the temperature is much higher since the substratum also absorbs heat. This temperature range is highly tolerable to T. flavum and A. maculosa since they have hard and thicker shells with radial ribs that prevent direct penetration of heat from sunlight. Clams like P. amabilis and T. literatus tolerates temperature range of 28o to 34oC. On the other hand, P. viridis can only tolerate temperatures ranging from 26-28oC and this species could not go into burrows for they attached on hard substrates. They can not stand being exposed to direct sunlight because they survive in 3-5m of water.
The total suspended solids (TSS) levels ranges from 8 mg/L and reaches as high as 84.5 mg/L. TSS concentrations were almost the same during low tides except on the 2nd sampling where it reached the highest at 84.5 mg/L. This can be due to the weather condition, wind and sea condition during the sampling. The second sampling was during the lowest tide reaching about 300 meters seaward. The water was very shallow and turbid and strong wave and wind action was observed. This can be explained by the fact that one of the greatest factor affecting suspended solids in the water is the water flow. During the sampling, the water was highly dynamic compared to the rest of the samplings.
Based on the classification of coastal water as cited by Rivera (2003), the water in the area should be class SA. This class is suitable for propagation, survival and harvesting of shellfish. The standard TSS for this class is 25 mg/L. Results showed that TSS level in the area which is between 8-84.5 mg/L and it means that the area is favorable for bivalve propagation and survival.

34. Table 5. Abundance index, Frequency index and Dominance in of seagrass at Zone 6, Brgy. Bula GSC
Seagrass species
Abundance index (%)
Frequency index(%)
Dominance index(%)
Halodule pinifolia
52.92
100
152.92
Cymodocea rotundata
11.60
90
101.60
Thalassia hemprichii
31.99
100
131.99
Halophila ovalis
2.18
40
42.18
Enhalus acoroides
1.31
20
21.31
The seagrass assessment done at Zone 6, Brgy. Bula revealed that the most abundant seagrass species was Halodule pinifolia at 52.92% followed by Thalassia hemprichii (31.99 %) while the least abundant species was Enhalus acoroides at 1.31%. In terms of frequency, H. pinifolia and T. hemprichii was the highest at 100%, next was C. rotundata (90%), H. ovalis (40%) and the least frequent species was E. acoroides at 20%. The most dominant species was H. pinifolia (152.92%), followed by T. hemprichii (131.99%), C. rotundata (101.60%), H. ovalis (42.18%) and E. acoroides was the least dominant at 21.31%.
Figure 18. Abundance index, Frequency index and Dominance index of seagrass at Zone 6, Brgy. Bula GSC
Abundance index (%)
Frequency index(%)
Dominance index(%)
LEGEND:

35. Overall, Halodule pinifolia has the highest abundance index (52.92 %), frequency index (100%) and dominance index (152.92%) among the five seagrass species present. This is followed by Thalassia hemprichii while the least abundant, least frequent and least dominant species was Enhalus acoroides. Therefore, seagrass condition in the area is healthy as shown in their abundance, frequency and dominance indices. The seagrass condition could indirectly affirm that bivalve abundance in the area is also in a good condition. This is due to the fact that bivalves play a key role in water filtration that regulates light availability and consequently, seagrass growth. They maintain water clarity and seagrass density. Highly turbid waters reduce light reaching the plants’ leaves and would lead to decline in their population. But the area remarkably has large population of seagrasses and TSS is also low which means the water has low turbidity.

36. CONCLUSION AND RECOMMENDATION
Zone 6, Brgy. Bula, though it is densely populated especially in the coastal areas, can still support an array of marine life like mangroves, seagrasses and macroinvertebrates. The area can still produce coastal resources for human consumption. One of these is bivalves. The study revealed that there are 8 species of commercially valuable bivalve species in the area and these are under 6 families namely: Anadara maculosa (Arcidae), Trachycardium flavum (Cardiidae), Glycemeris ambionensis (Glycemeridae), Codakia tigerina (Lucinidae), Perna viridis (Mytilidae) and Meretrix meretrix, Tapes literatus, Gafrarium pectinatum (Veneridae).
Of all the commercial bivalves collected and monitored, it turned out that Trachycardium flavum or blood cockle is the most abundant species at 92.07%, followed by Anadara maculosa (5.73%) and the least abundant species were Gafrarium pectinatum (0.11%) and Perna viridis (0.07%).
Statistical analysis showed that there is no significant difference in the number of bivalves collected in each day. The number of bivalves harvested from January 23 to February 11, 2009 was almost uniform. Gleaners do not have a permanent site to gather bivalves and so prevents a decreasing catch each day.
Physico-chemical parameters show very small fluctuations as observed in the temperature and salinity that falls between 34oC to 35oC and 34ppt to 36ppt. respectively. Total suspended solids (TSS) levels range between 8 mg/L to 84.5 mg/L. Based on the statistical analysis, these minimal fluctuations in the physico- chemical parameters does not significantly change the amount of bivalves harvested each day.

37. Therefore, the area is good for the growth and survival of bivalve species like Trachycardium flavum. However Zone 6, Brgy. Bula is not very suitable for the growth and survival of other bivalve species since they were found in a very small number and abundance.
For future studies, it is highly recommended that the zonations of bivalves should be studied. There were studies suggesting that in the present time, there happens to be a bathymetric shift of bivalve species into the deeper parts of the intertidal and subtidal as affected by increasing temperature in the shallow waters. Physico-chemical parameters like turbidity, current speed and direction should be considered. Since bivalves mainly feeds on phytoplankton suspended in the water column, it is necessary that the phytoplankton biomass will be measured because food supply greatly affects growth and survival of organisms.
There must also be a regular monitoring of bivalves species collected in the area. Long term monitoring of commercial bivalves catch could reveal the effect of daily gleaning to their population and productivity.
The nearby coastal community should practice proper waste disposal because almost all their wastes go directly to the coastal waters. Knowing that bivalves are eaten, it must be secured that these organisms’ surrounding waters are clean and as much as possible free from pollutants.
Lastly, intensive community organizing should be done in the area to teach the residents to be responsible of their area and empower them to do conservation and protection of the nearby resources on their own.

38. REFERENCES
Books
Abbott, Tucker. 1982. Kingdom of the Seashells. Bonanza Books.
Cosel, Rudo Van and Filipina Sotto. 1982. Some Commercial Bivalves of Cebu Philippines. The Philippine Scientist. San Carlos Publications. University of San Carlos. Cebu City, Philippines.
Davy, Brian and Michael Graham, eds. 1982. Bivalve Culture in Asia and the Pacific.
Engemann, J.G. and R.W. Hegner. 1981. Invertebrate Zoology. New York: Macmillan
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English, S. et.al.1994. Survey Manual for Tropical Marine Resources. Australian Institute of Marine Science. Townsville.
Nie Zhong-Qing.1982.Country Reports: China. Bivalve Culture in Asia and The Pacific.Davy, Brian and Michael Graham (eds.) pp: 21-28
Rivera, Paul C. 2003. Manual On Water Quality Monitoring And Modeling Studies.
Storer, Tracy I. et. al. 1979. General Zoology. Philippines: McGraw-Hill Inc.
Young, Adam and Evelyn Serna. 1982. Country Reports: Philippines. Bivalve Culture in Asia and the Pacific. Davy, Brian and Michael Graham (eds.).pp: 89-97
Internet Articles
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Koch, Evamaria et.al. 2003. Modeling the Influence of Filtration by Restored Bivalves Stocks on Seagrass Growth via Sediment Dynamics and Turbidity in Shallow Waters. Accessed at http://www.cdphe.state.co.us/cdphereg.asp#wqreg) last February 7, 2009

39. Koch, Evamaria et. al. 2003.Understanding the relationship between bivalve mollusks and seagrass habitat:new tools for restoration planning. Accessed at http://www.cdphe.state.co.us/cdphereg.asp#wqreg) last February 7, 2009
Murphy, Sheila. 2001. General Information on Solids. Accessed at http://www.ecs.umass.edu. Last February 18, 2009
Student Watershed Research Project: A Manual of Field and Lab Procedures.
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http://www.ecs.umass.edu. Last February 18, 2009
Springsteen, F.J. and F.M. Leobrera. 1986. Shells of thePhilippines. Accessed at
http://www.sealifebase.org. last March 1, 2009
Tang, Baojun et. al. 2005. Oxygen consumption and ammonia-N excretion of Meretrix meretrix in different temperature and salinity. Accessed at http://findarticles.com last January 30, 2009
Ward, Evan J., and Sandra E. Shumway. 2004. Separating the grain from the chaff: particle selection in suspension- and deposit-feeding bivalves. Accessed at
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Website
http://www.googleearth.com