SCIENTIFIC REPORT MAB YOUNG SCIENTISTS AWARDS

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1 SCIENTIFIC REPORT MAB YOUNG SCIENTISTS AWARDS Outbreak of the Coral Eating Snails (Drupella spp.) in the Cat Ba World Biosphere Reserve, Vietnam Looking at the habitat utilization and prey selection Prepared by: Nguyen Duc The Vietnam, November

2 TABLE OF CONTENTS LIST OF TABLES... 3 LIST OF FIGURES... 3 ACKNOWLEDGEMENT... 4 ABSTRACT... 5 I. INTRODUCTION... 6 II. MATERIAL AND METHODS Time of study Area of study Fieldwork Preparatory work for the survey Method of survey, field sampling Method of specimen analysis in laboratory III. RESULTS AND DISCUSSION The coverage percentage of the bottom components at surveyed sections Species composition and density of Drupella in survey areas Species composition Density of Drupella Age structure of Drupella population Spatial structure of Drupella populations Discussion on dangerous changes in choice prey of coral-eating snails (Drupella) IV. CONCLUSION REFERENCES

3 LIST OF TABLES Table 1. The geographic coordinates of the survey sites Table 2. Coverage percentage of bottom components on the reef in survey Table 3. Density of Drupella (individuals/m 2 ) at surveyed sites LIST OF FIGURES Figure 1. The map showing the study area and the location of sample collection at Cat Ba Island, Vietnam... 9 Figure 2. The equipment for fieldwork Figure 3. The placement of transects surveyed Figure 4. Sampling in the quantitative squares placed on the transect line Figure 5. Sample analysis in the laboratory Figure 6. Measure the length of shells Figure 7. External morphology of coral-eating snails (Drupella) in Cat Ba Figure 8. The density of Drupella as per surveyed area on the reef flat and the reef slope zones Figure 9. Size groups (shell length - L) of the Drupella populations Figure 10. Distribution of Drupella group on the living coral Figure 11. The correlation between the density of Drupella with the coverage of living corals on reef zone at survey locations Figure 12. Chart of Drupella density distributed on reef flat and reef slope at survey locations Figure 13. Drupella density distributed on reef flat and reef slope at closed and open areas Figure 14. The percentage of Drupella distributed on the substrate components Figure 15. The number of live coral colonies were occupied by Drupella/transect 100m Figure 16. The acroporids are being occupied by Drupella, observations in 2002 (Resource: Nguyen Văn Quan, 2002) Figure 17. The living coral are being occupied by Drupella are coral blocks, observations in

4 ACKNOWLEDGEMENT The author would like to thank especially UNESCO MAB Young Scientist Award 2015 for support. This research will be not possible also without great support of Prof. Nguyen Hoang Tri (President & Secretary General, MAB National Committee of Vietnam-Unesco). I would like to express my profound gratitude to Dr. Tran Dinh Lan and Dr. Nguyen Van Quan (Institute of Marine Environment and Resource, Vietnam) for their help and comments regarding the common approach for my project. Field work for this work was made thanks to the providing necessary facilities during field works support by The Institute of Marine Environment and Resource - Vietnam Academy of Science and Technology (VAST). Also, I am very much grateful for collaboration from the Management Board of Cat Ba Archipelago Biosphere Reserve was very important for all the help provided. I am thankful for all member of the project: Mr. Chu The Cuong, Mr. Pham Van Chien and Mr. Dang Do Hung Viet for supporting research activities and collection of samples. Contact address: Nguyen Duc The Department of Marine Biodiversity and Conservation, Institute of Marine Environment and Resource (IMER), Vietnam Academy of Science and Technology (VAST) No. 246 Da Nang Str. Hai Phong City, VIETNAM Tel: (office) Fax: thend@imer.ac.vn; ductheimer@gmail.com 4

5 ABSTRACT This project aimed to be the breakdown study about biological and ecological characteristics of coral-eating snails (Drupella spp.) on the coral reefs in the Cat Ba World Biosphere Reserve of Vietnam, that focused on observing the natural distribution-density specific, prey choice on the coral colonies to the density of coral eating snail populations. The study area is the Cat Ba reef areas, which is location have unique characteristics represent for the eastern reef of North Vietnam, unlike the other reefs in the Indo Pacific, the coral communities are very abundant in term of species diversity and reef morphology (Poritidae, Faviidae). Six reef sites were proposed and observed to collect basic information of the Drupella on the coral reefs in the two area of the Cat Ba (the open area and close area). The methods used for research were modified according to English et al. (1997), Hodgson (2000), and Cumming, RL (2009). For all reef sites, the coverage percentage of the bottom components at surveyed sections are proposed and discussed. Also, the species composition, density, size group, distribution characteristics and choice prey of coral-eating snails (Drupella) monitored at each reef sites for two transects (reef flat and reef slope). The results show that virtually the monitored points have the coverage of live coral is low (<40%), the dominant coral species on the reefs belong to genus Galaxea, Goniopora and Pavona. Three Drupella coral-eating snails distributed on coral reefs in the study area identified: Drupella Cornus, D. rugosa, D. margariticola, in which D. margariticola predominates over the rest two species. The spatial distribution of coral-eating snails are related to the abundance and coverage of living corals, and the concentration on the reef flat is higher than on the reef slopes. Drupella primarily sticks on living corals, and the coral species were occupied by snails belong to the genus Goniopora and Galaxea. This indicates Drupella have a selected range of prey larger than previously reported, this issue is a danger to risk factors for the health of coral reefs in the epidemic area and is a massive challenge for researchers working in reefs conservation of worldwide. The results of the research have provided basic information on the conservation status of corals reef by several crucial information such as the spatial distribution of coraleating snails, prey selection, the population of coral reef eating snails. Thus, the results of this study directly/indirectly contribute to the management of the coral reefs at the Cat Ba World Biosphere Reserve of Vietnam. 5

6 I. INTRODUCTION The coral reef is a typical tropical ecosystem which is widely distributed in coastal waters and many surrounding inshore and offshore islands of Vietnam. A reef is a place of high biodiversity, large primary production, and wonderful scenery. However, reef degradation caused by human activities, natural disasters, diseases, and predation are majors problem which has been warned in recent years. The horn drupe (Drupella), a marine gastropod mollusk in the family Muricidae has been identified as a dangerous predator of living coral. This species is mostly distributed around the Indian Pacific coral reefs. The activity of eating coral has an impact on the survival and growth of reef-building corals, simultaneously alter the structure as well as the ecological function of coral reefs. Three major reef regions have been the subject of many scientific reports about the increase in density of Drupella population related to the decline in the reef area in Western Australia s Ningaloo, the Red Sea (Cumming, R. L. 1999; Johnson M.S et al., 1993), Aqaba Gulf (Al-Moghrabi, S.M.,1997), Japan (Cumming, R. L., 2009; Fujioka, Y. & K. Yamazato, 1983) and Hong Kong (Cumming, R. L. & D. McCorry, 1998; Morton B., et al., 2002). Typically, in Western Australia's Ningaloo, the increase in Drupella density from individual/m 2 (in the 1970s) up to 1-2 individuals/m 2 (1997) destroyed 90% of the reef in this area (Raymundo L.J. et al., 2008). The most recent outbreak of Drupella occurred in Koh Tao Island in the Western Gulf of Thailand in 2010 destroying about 70% branches of coral (Hoeksema, B. W. Et al., 2013). Thus their ability of coral destruction can take place in any other regions in the Indian Pacific region, not excluding any potential risks to the coral reefs in Vietnam. Cat Ba Island is the famous area in cultural values, aesthetics, geology, economics, and biology for Vietnam, also to the world, that is not only for Vietnam but also for the whole world. Cat Ba National Park recognized as a UNESCO Biosphere Reserve of the World, which has identified the outstanding value and the global value of Cat Ba area. The value of biodiversity, particularly the existence of coral reefs has significantly contributed to the achievement of the above title. 6

7 Maintaining the survival and protection of coral reefs in Cat Ba is a highly important task to ensure the integrity and maximize those values. Cat Ba coral reefs demonstrate typical characteristics of ones in Northeast Vietnam, unlike others in the Indo Pacific region. They have abundant species of stony corals (Poritidae, Faviidae), especially Porites and Goniopora having strong adaption to high turbidity and resistance with good freshness and the rapid ability of recovery. This coral is only distributed to a depth of 6-8m (reef slope), most concentrated in the relatively shallow depth of 2-4m (reef flat), which results in their sensitivity to the volatility of environmental factors. In fact, the environmental conditions here are quite harsh for the survival of coral, for example, coral distributes near estuary areas and affected by tropical storms driving to the partial freshening and especially the undergoing of the cold winter here leads to decrease in sea temperatures (Latypov Yu.Ya., 1992; Nguyen Dang Ngai, 2002; Nguyen Huy Yet, 2010; Nguyen Van Quan, 2012). It is suggested from many studies that some ecological characteristics of Drupella shows major changes by geographic regions, because the specific biotic and abiotic factors of a particular reef will set up or have direct impact on Drupella s typical selection of prey and distribution over the reef space (Dirnwo ber, M. & J. Herler, 2007; Wallace, C.C., 1999). However in Vietnam up to now, the intensive studies on Drupella on coral reefs have not been published in any reports, except several scattered notices of their appearance in the thematic reports, technical reports, and projects being implemented in the region. Before the complex evolution of the phenomenon of global climate change, the coral predator like Drupella can significantly impair the natural ability to recover of reef-building corals population which have already endured the harsh conditions in Cat Ba. Experiences from other countries shows that, when policy makers, managers, people get the system information on the status, value, threats to natural resources and with the help of scientists in guiding them to do, then the resolution of the issues on the state of exploitation and protection in the political system or the application of conservation policy for local situation. The answer for this problem, if is given, should be a valuable scientific basis for monitoring the current characteristics of 7

8 coral reefs as well as constructing technical solutions to protect and restore degraded coral reefs here and a basis for conducting the next intensive studies of predators on coral reef ecosystems in Vietnam. Goals and object of the study Our project focuses on providing basic information about site specific distribution of the coral reef and suitable prey of coral-eating snails that characterize the distribution patterns of the Drupella on the coral reefs in Cat Ba island. Along with the direct contribution, this project will provide the science sound information for policy makers to be a better understanding of the natural resources and how to cut down the degradation trends of the coral reefs. Specifically, this study aims to provide in-depth knowledge of specific, distinct population of coral-eating snails in Cat Ba National Park on the following issues: (1) To assess the status of coral reefs in Cat Ba Island (2) To identify the species composition, structure of the Drupella population on some reefs in Cat Ba National Park. (3) To determine the distribution characteristics of the Drupella population on some reefs in Cat Ba National Park: - How do the water depth and coral abundance (species richness, live coral cover) influence the distribution of the Drupella? - What types of the coral genera are preferred by the Drupella regardless of their abundance? - Does the schooling behavior influence the habitat choice and prey selection in the population dynamic of Drupella? II. MATERIAL AND METHODS 1. Time of study study area: The author and the research group conducted two surveys of sampling in the - Period 1 (wet season) in October 2015, surveyed overview the distribution of Drupella and composition of coral-eating snail species at Ba Trai Dao and Van Boi. 8

9 - Period 2 (dry season) in March 2016, implemented the repeated researches on the same and other study sites. 2. Area of study The topic was studied in the territorial water of Cat Ba National Park with the specific areas as coastal islands in Cat Ba island. The map showing surveyed sites expressed below: Figure 1. The map showing the study area and the location of sample collection at Cat Ba Island, Vietnam The survey was conducted in two areas: (1) Close area, less affected by wind and waves, but more suffered from the impact of tourism activities, aquaculture, and fisheries, including coral reefs in Ba Trai Dao, Tai Keo, Van Boi; (2) Open area, under the significant impact of wind and waves, with little affected by tourism activities, aquaculture, and fisheries, including coral reefs in Ang Du, Coc Cheo, Tung Ngon. 9

10 Detail of the geographic coordinates of the survey places in the following table 1: Table 1. The geographic coordinates of the survey sites No. The name of reefs Geographic coordinates Area condition 1 Bai Trai Đao (0-8m) Close 2 Tai Keo (0-4m) Close 3 Van Boi 1 (0-4m) Close 4 Van Boi 2 (4-8m) Close 5 Ang Du (0-8m) Open 6 Coc Cheo (0-8m) Open 7 Tung Ngon (0-8m) Open 3. Fieldwork 3.1. Preparatory work for the survey Human Resources: For a group of three divers: the first diver sprayed the tape measure on the transect line, the second diver dived followed to observe current status of coral reefs such as coverage, species composition and the components of the reefs bottom. The third diver who had the duty placed the plastic quadrat on the tape measure sections for collection samples and data about Drupella with the support of the first diver after finishing tape measure spreader. Equipment used in the survey Small boat and safe equipment; Diving equipment; GPS GARMIN- 72; underwater camera Canon G9; Two tape measures (range 60m); SCUBA diving equipment; plastic quadrat (50cm x 50cm); Pencils and a plastic sheet to record the data underwater, mesh bags containing samples; refrigerated containers for keeping samples. 10

11 Figure 2. The equipment for fieldwork 3.2. Method of survey, field sampling Place transects survey: in each of the survey, sites placed the 02 transect at 02 different depths (reef flat from 0-4 m and reef slope from 4-8 m), at the Tai Keo site only at a depth of 0-3 m. Placement, placing sectional survey is shown in figure 3. Figure 3. The placement of transects surveyed 11

12 - Methodology to observe the different bottom components: The classification of corals in the field was based on the documents of Veron.J.E.N, (1986, 2000a, b). Determining the coverage of living corals and other bottom components was identified by the Reefcheck method according to the guidelines of English et al. (1997) and Hodgson (2000). All bottom components shall be recognized at every 0.5m touch point of the transect 100m as shown in figure 4. The bottom components of the form were observed at each 0.5 m point of the tape measure. The different bottom components including hard corals, soft corals, recently dead corals, fleshy seaweeds, sponges, rock, corals rubble, sand, mud or silt /clay and others will be recorded at 160 points for each transect. In addition to the component types are defined as above, the dominant species caught, coverage percentage of each component were observed at each point. - Methodology to observe the Drupella population: Drupella monitoring methods on coral reefs were adapted from methods of Cumming, RL (2009). The samples and data of Drupella were collected from 30 squares (50x50cm) per 100m transect line; the quadrat was placed at 3m points on the tape measure (default ) as shown in figure 4. Figure 4. Sampling in the quantitative squares placed on the transect line 12

13 - Preservation and storage of specimens: The entire sample received were taken photography before moving to the laboratory stored in the freezer to serve for the taxonomy identifying Method of specimen analysis in laboratory Equipment for sample analysis in laboratories: Magnifying glass (diameter 8cm) Scales electronic disk (use range: g, error: ± 0,01 g) Palmer size (range: 1-300mm, error: ± 0.01mm) Figure 5. Sample analysis in the laboratory - Measured body weight (Wg) and shell length (Lcm) of all individuals Drupella obtained as shown in figure 6. Figure 6. Measure the length of shells 13

14 - Some rare species of rare coral, can not determine right field, then take photos, collect samples and bring back to the lab. Based on morphological, structural skeletons and photographs, samples of coral species are classified to the laboratory. Classification of coral in the laboratory is based on the document's classification Veron.J.E.N, 1986, 2000a, b. - Identify taxonomic the Drupella by using morphology of shell and color body according to Johnson MS & Cumming R.L. (1995); Wilson B. (1992) Data analysis - Species richness (SR) is defined as the total number of species of snails on a surveyed section. - Using specialized software to handle data as Microsoft Excel with ANOVA statistical analysis tool. - Regression analysis was used to quantify the difference of a particular element of the reef habitat (coverage of RSH) led to a change in density of Drupella by RSH surveyed. III. RESULTS AND DISCUSSION 1. The coverage percentage of the bottom components at surveyed sections The ratio of the bottom components on corals shows specific structures of a particular reef; this characteristic is an important factor which plays a decisive role to the composition of the living beings population on that reef. The percentage of bottom components on the reef in the surveyed sections in March 2016 in Cat Ba shown in Table 2. Table 2. Coverage percentage of bottom components on the reef in survey Name of surveyed sites % cover of live coral % cover of dead coral % cover of Rock, sand, mud % cover of other Tung Ngon (reef flat) Tùng Ngon (reef slope) Coc Cheo (reef flat) Coc Cheo (reef slope)

15 Ang Du (reef flat) Ang Du (reef slope) Ba Trai Đao (reef flat) Ba Trai Đao (reef slope) Van Boi (reef flat) Van Boi (reef slope) Tai Keo (reef flat) Virtually the monitored points have the coverage of live coral is low (<40%), The reefs in the open area t as Tung Ngon, Coc Cheo, Ang-Du are living coral cover higher than the close area reefs. The coverage of rule showed changes significantly: the reef flat with the high live coral cover than reef slopes. This situation can be explained by the cause these Cat Ba marine area was influenced from the freshwater from large river mouths in regional (the Bach Dang River and the Van Uc river) that has maintained to high turbidity that limits the lighting to deep water layers. Thus, symbiotic algae in the corals at greater depths can not get enough the light source to perform photosynthesis, nutrient supply to feed the coral. 2. Species composition and density of Drupella in survey areas 2.1. Species composition The results obtained from the surveys in October 2015, and March 2016 have revealed three Drupella species of coral-eating snails distributed in Cat Ba as Drupella Cornus, D. rugosa, D. margariticola. Thus, the results of this study have added up 02 species of snails D. rugosa and D. margariticola to the list of Drupella species in Cat Ba (Nguyen Huy Yet, 2010), raised the total number of recorded species up to 4 species. However, since there is no detailed description of the species D. Concatenata in the previous reports, it is impossible to confirm whether this species is a synonym of the identified ones in this study or not. 15

16 (a) D. margariticola (Broderip, in Broderip & Sowerby, 1833) (c) Drupella cornus (Röding, 1798) (b) D. rugosa (von Born, 1778) Figure 7. External morphology of coral-eating snails (Drupella) in Cat Ba 2.2. Density of Drupella Survey results on 11 sections of several reefs in Cat Ba in March 2016 showed the average density of Drupella species as shown in Table 3 and figure 8 below: Figure 8. The density of Drupella as per surveyed area on the reef flat and the reef slope zones 16

17 Name of Surveyed sites Tung Ngon Coc Cheo Ang Du Ba Trai Đao Vạn Boi Table 3. Density of Drupella (individuals/m 2 ) at surveyed sites Distributed on the reef D. margariticola D. cornus D. rugosa Total for three species M SD M SD M SD M SD Reef flat Reef slope Reef flat Reef slope Reef flat Reef slope Reef flat Reef slope Reef flat Reef slope Tai Keo Reef flat Through table 3 and figure 8, it is shown that shows the average density of all three species on the reef surface at Coc Cheo (open reef) is the highest (13.9 ± 10.9), on the reef surface at Tai Keo (closed reef) is the lowest: 1.6±1.6, and D.margariticola is a species with high-density and holding the upper hand over the two species D. cornus, D. rugosa. The result of this study is dissimilar from the places were occurring Drupella outbreaks around the World, that attached with two species D. Cornus and D. fragum. The two places have much research on the Drupella population outbreaks are the Ningaloo Reef in Western Australia and the Northern Red Sea, the D. cornus species is main object and dominant distribution on coral reefs, the highest density of D. cornus at Ningaloo were reported in 19.4 ± 5.0 individuals/m2 (Taylor, J. D. & D. G. Reid, 1984), the D. fragum is the main species outbreak in Japan with the highest density was reported in 5.1 individuals/m2 (Fujioka, Y. & K. Yamazato, 1983) Age structure of Drupella population The proportion of age groups of each generation has an important signification in the study of ecology, and in fact, the age structure is also expressed through the 17

18 body size groups. The structure of size groups (shell length - L mm) of the coraleating snail populations Drupella in the two surveyed regions in Cat Ba is shown in figure 9. Figure 9. Size groups (shell length - L) of the Drupella populations Remark: Group of shell length (L) N - number of samples observed Group 1 - shell length L <5mm Group 3 - shell length of mm Group 5 - shell length of 30-40mm Group 7 - shell length of 50-60mm Group 2 - shell length of 5-10mm Group 4 - shell length of 20-30mm Group 6 - shell length of 40-50mm Group 8 - shell length L> 60mm In ecology the development stage of the organism divided into three stages: immature (before spawning), mature (proliferating) and after spawning (Spight T.M., 1979). Body size indicator to distinguish between mature individuals for D. Cornus will have a shell length L> 2 cm, smaller than this size are considered immature individual (Turner, SJ, 1994b; Spight T.M., 1982). The result of this study showed that all immature snail individuals obtained occupy with low rate respectively species are D. margariticola (12,65%), D.cornus 18

19 (8,33), and D. rugosa (6,67%). Also, To compared the size group structure of the three species found the small size group of D. margariticola is a greater proportion of the two other species, and the proportion of immature group in the reef flat is higher than in reef slope. The ecological rule used to explain the survey results of this study. The low proportion of an immature group of Drupella population related to the reef ecosystem degradation has been going on here. The degradation of coral reefs was led to the decline in a diversity of coral composition species and the coverage of living corals are increasingly diminishing. The difference in the proportion of immature group at the reef flat is higher on the reef slope is also be explained by the difference in coverage the living coral at the reef flat and the reef slope. However, the differ according to the depth of reefs should be further studied. The difference in the structure of the mature group and immature group in Drupella population will lead to different temporary effects to the current status of coral reefs. The mature Drupella group bitten a small area tissue of coral then move to the individual new colony. Thus, the mature occurred on the living coral with a small scar. Therefore, the tissue of corals occupied by mature will be lost in the short term but can be restored, exceptions may have several species of the genus Acropora corals (Schoepf V., et al., 2010). In contrast, the coral colonies occupied by immature will be lost tissue completely or inability to recover (Schoepf V., et al., 2010). Thus, the normal density of mature Drupella will affect coral reefs in the large area, However, the rate of coral colony completely destroyed is not many. In addition to the indicators of Drupella density, the size structure of the Drupella populations on a particular reef will be important biological indicators to assess the impact of Drupella on corals at present and the following time. The size structure of Drupella populations should be included in the group of biomarkers to assess the health of coral reefs, to support the management and use of a sustainable coral reef ecosystem. 19

20 2.4. Spatial structure of Drupella populations Distribution of the populations Results from the survey of 330 quantitative squares at 11 sections in the study area, only 3 Drupella groups were caught with the number of 6-9 individuals/stone of coral, the remaining snails obtained were distributed individually or from 2-3 ones together. Image of Drupella with single distribution, gathering into small or large groups on the reefs in Cat Ba is defined as shown in figure 10. Single Small group Big groups Figure 10. Distribution of Drupella group on the living coral Results from the survey found that Drupella tends to occupy on the damaged coral colonies, this signal is due to the hurt positions of coral facilitate the opportunity for Drupella easily perform bite the coral tissue. Drupella focuses many of individuals in the same place was make wounds tissue area will be rapid larger, then created a safe zone for Drupella occupants to avoid contact with the tentacles of coral, reduced secretion of toxic reactions from the tentacles of coral. The relationship between the density of Drupella with the coverage of living corals in this study are also considered as shown in figure 11 below. 20

21 Figure 11. The correlation between the density of Drupella with the coverage of living corals on reef zone at survey locations At the points with high coverage of living corals, it also shows a higher density of Drupella and some living corals occupied by snails, since most coral reefs in Cat Ba are atypical coastal borders (mostly with leopard-like spots) with many groups living in separation. The survey results of the live coral cover on reefs flat at sites respectively: Tung Ngon 40%, Coc Cheo 44%, Ang Du 30%, Ba Trai Dao 40%, Van Boi 8% and Tai Keo 2 %, The result of this study showed that live coral coverage is related to the density of Drupella. At the transect had high abundant live coral, simultaneous the number of occupied coral colonies are also high-density. The most of the coral reefs in Cat Ba coastal fringing reef type of atypical (mostly mosaic) with separate colonies. This result explains the reason for the distribution of the single Drupella individual occurred with high proportion. The results of this study showed that the difference with previous reports of relationship between increase in population density of Drupella and decline in the area of coral reefs in Ningaloo Reef in Western Australia, the Red Sea, the Gulf of Aqaba, Japan and Hong Kong, all recorded the Drupella snails occupied congregate in large numbers up to hundreds of individuals on a coral colony. (Al-Moghrabi, S.M., 1997; Cole AJ et al, 2008; Cumming, R. L. & D. McCorry, 1998; Cumming, R. L., 2009; Fujioka, Y. & K. Yamazato, 1983; Johnson M.S, 1993; Zuschin, M. et al., 2001). 21

22 Thus, along with indicators of the density of Drupella, size structure of these Drupella populations, characteristics gathered are also important biological indicator for assessing the reef health Distribution by depth Results from the survey of the coral reefs in Tung Ngon, Coc Cheo, Anh Du, Ba Trai Dao, Van Boi and Tai Keo only discovered that Drupella distributed to the depth of 6m above sea level concentrated in the depth of 1-3m. If these reefs are divided as per zone, the density of Drupella on each survey section of reef surface (0-4m) is much higher than the sections of reef slope (4-8m). This difference is reflected in the chart of Drupella density distributed on reef surface and reef slope at survey locations (Figure 12). Figure 12. Chart of Drupella density distributed on reef flat and reef slope at survey locations The distribution of Drupella density with the depth of reef in Cat Ba also result in similar studies in Japan, North of the Red Sea and Ningaloo Reef in Western Australia. The distribution of Drupella on the space of reefs associated with the composition and structure of coral reefs and depth. Drupella density decreases with depth, the study of other authors have shown that the highest density at the reef crest (the reef flat) with a depth of 1 m and rarely less than 20 m deep distribution (Al- 22

23 Moghrabi, S.M., 1997; Schoepf V. et al, 2010; Schuhmacher, H. 1992; Turner, S.J., 1994a; Zuschin, M., J. et al., 2001). Coral reefs in Cat Ba has typical characteristics of coral reefs in Northeast Vietnam, unlike other coral reefs of the Asia Pacific. Coral distributed only to a depth of 6-7 m below 0m of tide charts, most focus areas 0-4 m, the coral reef is atypical of coastal and only a narrow strip (width wide approximately 2-3m) (Nguyen Chu Hoi et al., 1988; Latypov Yu.Ya., 1992; Nguyen Dang Ngai, 2002). These conditions have explained that Drupella in Cat Ba distributed to a depth lower than other areas of the Asia - Pacific. Additionally, coral in the shallow are easily vulnerable to human activities such as fishing, tourism activities or by storm, this will attract Drupella to focus in the position colonies suffer trade (Morton B. Et al, 2002; Nguyen Van Quan, 2012) Distribution by reef location Result from the survey of the density of Drupella distribution at three reefs of closed area (less influenced by waves, wind) as Ba Trai Dao, Van Boi, Tai Keo and three reefs of open area (more influenced by waves, wind) as Tung Ngon, Coc Cheo, Anh Du is shown in figure 13. Figure 13. Drupella density distributed on reef flat and reef slope at closed and open areas 23

24 It can be seen that the density of Drupella distributed on reef slope at survey points between the two regions shows no difference. The density of Drupella distributed on reef surface of open area is higher, but the difference is not clear. According to the discussion above, the abundance of Drupella s food sources will be a determinative factor in this distribution Distribution of substrate components The results from a survey on the distribution characteristics of Drupella on the substrate components (Living coral, dead coral, muddy sandstone and other components) in figure 14. Figure 14. The percentage of Drupella distributed on the substrate components The results indicated that 71.8% D. margariticola, 65.4% D. rugosa and 47.1% D. Cornus distributed on living corals and a small percentage on dead corals or stone, gravel and other substrate components, only D. Cornus on dead corals with the highest rate up to over 52.9%. All immature Drupella spp. individuals were collected on living corals, whereas the mature ones were distributed on the different types of substrate components. Almost immature individuals were found at the locations which are hard to find such as on minor groove of coral blocks or on the underside of coral plates. While mature individuals were found in all locations of coral blocks, even on the stone grooves, which may reveal the higher demand on self-defence against enemies or waves of immature individuals (Schuhmacher, H., 1992). 24

25 Figure 15. The number of live coral colonies were occupied by Drupella/transect 100m The results of this study are also consistent with the findings of other studies which refers that the preferred environment of mature Drupella and its offspring is on living corals. However, Drupella mature still occupied cling on rocks, gravel or pieces of dead coral, while juveniles only are found on the living coral blocks (Al- Moghrabi, S.M., 1997; Cumming, R. L., 1999; Taylor, J. D., 1984). Compared to reports in other areas in the world, the result of this study shows that there is a significant proportion of Drupella non-distributed on the live corals. According to research by Taylor, J. D. & D. G. Reid (1984) and Al-Moghrabi (1997) indicate that still found mature individuals cling to rocks, gravel or pieces of dead coral and over another complement of the bottom. In addition, the distribution of the Drupella related to abundance and coverage of live coral. Drupella adults in the substandard reef tend to move from each coral colonies on the reef to search new prey, so explained that Drupella is not occupied on the living corals more than the other regions were announced. 3. Discussion on dangerous changes in choice prey of coral-eating snails (Drupella) Few data are available on the dangerous changes in choice prey of coral-eating snails (Drupella) on the coral reef Cat Ba. Coral Reefs in Cat Ba is the unique 25

26 characteristics of the typical reef northeast Vietnam, unlike the other reefs of the Pacific Asia, they are plentiful coral blocks (Poritidae, Faviidae), especially Porites and Goniopora varieties well adapted to high turbidity, opposed to the chemical with freshwater, the ability to recover quickly (Nguyen Huy Yet, 2010; Nguyen Van Quan, 2012). Actually, coral-eating snails Drupella group known as agents to destroy coral reefs occur around coral reefs of the Indian - the Pacific Ocean, their potential can destroy large areas of the reef Cat Ba. Figure 16. The acroporids are being occupied by Drupella, observations in 2002 (Resource: Nguyen Văn Quan, 2002) Figure 17. The living coral are being occupied by Drupella are coral blocks, observations in

27 Observations on Drupella appear on the reef Cat Ba in 2002, to surprise had the 3-50 individuals Drupella occupation on Acropora coral branches (Figure 16), hardly any found Drupella on the coral blocks (Nguyen Van Quan, 2006). The differences the 2016 survey at Cat Ba has identified the whole of living coral are being occupied by Drupella are coral blocks of genus Goniopora and Galaxea. The Drupella density on transects with highest is 13.9 ± 10.9 Drupella/m2, lowest 1.1 ± 1.1 Drupella/m 2 (Figure 17). The most common type of multiple studies on preferred prey Drupella mainly coral branches and Pocillopridae Acroporidae which priority is the same Acropora (Fujioka & Yamazato, 1983; Taylor & Reid, 1984; Boucher, 1986; Turner, 1994b; Al-Moghrabi, 1997; McClanahan, 1997; Cumming, 1999; Zuschin et al., 2001; Morton et al., 2002; Shafir et al., 2008; Morton & Blackmore, 2009). However, the fact that Drupella in Cat Ba is the same as in Hong Kong and Thailand. Over time, the structure of reef has been changed fundamentally with the coral branches replaced by the big chunks of coral (Galaxea, Goniopora and Porites) (Nguyen Huy Yet, 2010; Nguyen Van Quan, 2012). To survive, the Drupella forced to adapt when switching to eating coral which is not the preferred prey of them previously (Cumming, RL & D. McCorry, 1998; B. Morton et al., 2002; Hoeksema, BW et al., 2013). 27

28 IV. CONCLUSION - The current reefs in Cat Ba are only a small and narrow range, between 0m to 4m deep above 0m sea chart; the dominant coral species on the reefs are Galaxea, Goniopora and Pavona. - Three Drupella coral-eating snails distributed on coral reefs in the study area are identified: Drupella Cornus, D. rugosa, D. margariticola, in which D. margariticola predominates over the rest two species. - Distribution characteristics of Drupella are related to the abundance and coverage of living corals, at the locations with low coverage will encounter a large number of single individual, and the density of Drupella is high at the locations with a higher living coral coverage, and the number of living corals were occupied by snails. Also, the density of Drupella distributed on the reef surfaces is higher than on the reef slopes. - Drupella primarily sticks on living corals, all of the living corals occupied by Drupella are stony corals, in the genus Goniopora and Galaxea. Results of this study demonstrated that selected range of prey of Drupella is large than previously reported, this issue is a danger to risk factors for the health of coral reefs in the epidemic area. 28

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