Species composition and distribution of polychaete (polychaeta) in mangrove eco-system, Da Loc commune, Hau Loc district, Thanh Hoa province

Abstract. A study on species composition and density of polychaete (Polychaeta) was investigated in mangrove eco-system, Da Loc commune, Hau Loc District, Thanh Hoa province. Samples were collected in April, July, and November, 2010, from cross-sections of Len estuary and mangrove forests. A total of 15 polychaete species, belonging to 11 genera, 9 families and two groups of settlers (Sedentaria) and mobile (Errantia), were recorded in the study areas. Species composition and individual density increased correlatively with salinity. Muddy substrate (12 species, 95.40 ind/m2) was more suitable for many polychaete species than any other types, i.e., muddy sand (8 species, 56.20 ind/m2); sandy mud bottom (8 species, 135 ind/m2); and organic mud substrate (7 species, 54.10 ind/m2). In sandy mud bottom, only 8 species were recorded but the average density is highest (135 ind/m2), because two species (Nephthys oligobranchia and Nephthys polybranchia) were very high in density (114 ind/m2) in this habitat. In other substrata, the highest average density of one species did not exceed 25% of the total density of polychaete. The number of species and average density of polychaete in brackish salt (19‰ - 30‰) were highest, with 15 species and average density of 100.9 ind/m2; 9 species were found in saline brackish (5‰ - 18‰), with an average density of 47.1 ind/m2. The number of species varied with different pH levels of water, the highest at 6.6 to 7.5 (14 species), followed by 5.5 to 6.5 (9 species) and lowest at 7.6 to 8.5 (5 species).

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JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0065 Natural Sci. 2016, Vol. 61, No. 9, pp. 132-139 This paper is available online at 132 SPECIES COMPOSITION AND DISTRIBUTION OF POLYCHAETE (POLYCHAETA) IN MANGROVE ECO-SYSTEM, DA LOC COMMUNE, HAU LOC DISTRICT, THANH HOA PROVINCE Tran Thi Thanh Binh¹, Hoang Ngoc Khac², Nguyen Thi Ha³ and Vu Quang Manh³ ¹Hanoi National University of Education, ²Hanoi University of Natural Resources and Environment ³Center for Bio-diversity Resources Education and Development, HNUE Abstract. A study on species composition and density of polychaete (Polychaeta) was investigated in mangrove eco-system, Da Loc commune, Hau Loc District, Thanh Hoa province. Samples were collected in April, July, and November, 2010, from cross-sections of Len estuary and mangrove forests. A total of 15 polychaete species, belonging to 11 genera, 9 families and two groups of settlers (Sedentaria) and mobile (Errantia), were recorded in the study areas. Species composition and individual density increased correlatively with salinity. Muddy substrate (12 species, 95.40 ind/m 2 ) was more suitable for many polychaete species than any other types, i.e., muddy sand (8 species, 56.20 ind/m 2 ); sandy mud bottom (8 species, 135 ind/m 2 ); and organic mud substrate (7 species, 54.10 ind/m 2 ). In sandy mud bottom, only 8 species were recorded but the average density is highest (135 ind/m 2 ), because two species (Nephthys oligobranchia and Nephthys polybranchia) were very high in density (114 ind/m 2 ) in this habitat. In other substrata, the highest average density of one species did not exceed 25% of the total density of polychaete. The number of species and average density of polychaete in brackish salt (19‰ - 30‰) were highest, with 15 species and average density of 100.9 ind/m 2; 9 species were found in saline brackish (5‰ - 18‰), with an average density of 47.1 ind/m 2 . The number of species varied with different pH levels of water, the highest at 6.6 to 7.5 (14 species), followed by 5.5 to 6.5 (9 species) and lowest at 7.6 to 8.5 (5 species). Keyword: Polychaete, mangrove eco-system, Da Loc commune, Thanh Hoa province. 1. Introduction Vietnam has a coastline of about 3,260 kilometers long, with a dense network of rivers, and a tropical monsoon climate resulting in various types of eco-systems such as coast, estuary and lagoon, inter-tidal wetlands and mangrove eco-systems. Most of bottom substrata in estuaries and are mud or sediment where organic matters are accumulated creating a huge potential food source for organisms, as well as a typical bio-diversity. In these environments, polychaete (Polychaeta) of the phylum annelid (Annelida) dominate and play an important role in the system. Polychaete, which is a link between food chain and food web, Received September 16, 2016. Accepted November 29, 2016. Contact Tran Thi Thanh Binh, e-mail address: binhttt@hnue.edu.vn Species composition and distribution of polychaete (Polychaeta) in mangrove eco-system 133 participate in the process of de-composition of organic matters. They are also indicator organisms in assessing the quality of water and bottom substrate environment. Besides, some species, such as palolo worm (Tylorrhynchus heterochaetus), containing 84% of water; 11.33% of nitrogen; 3.2% of fat, are of great economic values. Hence, palolo worm is processed into delicious dishes, such as palolo sauce, palolo spring rolls [1-3]. Da Loc commune (Hau Loc district, Thanh Hoa province) is enriched with an abundant bio-diversity. In terms of flora, there are many species adapted to saline conditions, such as Rhizophora, Kandelia, Aegyceras, Bruguiera, and holy mangrove. The Len river is a branch of the Ma River in Thanh Hoa, from which a huge amount of sediment transferred with the inter-tidal substrates. In order to explore the diversity of polychaete species, as well as their distribution in Da Loc (Hau Loc, Thanh Hoa), this study aims to focus on these data from different forms of sediments, changes of salinity and pH in the estuary. 2. Content 2.1. Materials and methods Samples were collected in April, July, and November, 2010, from cross junctions of estuary and mangrove forests at sections from sea dyke to water edge. At each section,samples were taken from 3 positions of difference either depth levels of the river mouth or ages of the mangrove forest (1, 3s and 7 years) (see Fig.1); Three substrate samples of the size of 25 cm × 25 cm (1/16 m 2 ) large, and 20 cm deep/each were taken at each position; Substrate samples from river-bed and wetlands were taken in standard buckets of the size of 25 × 25 cm; Qualitative samples were also collected within a quantitative location to avoid an omission of species. Polychaete samples were separated by sieving on board with 0.5 - 1 mm mesh size. Samples were cleaned, anesthetized and then soaked in 70% ethanol. All samples are stored at the Department of Zoology, Faculty of Biology, Hanoi National University of Education. Salinity and pH at the sampling locations were measured by portable refractometers with an accuracy of 0.001. At low tidal level, we dug small holes, next waited for a full water-extrusion to measure needed indicators. Finally, we determined data of mechanical properties by observing bottom sediment pro and after samples separation conducting. Substrate bottom in this study area is divided into 4 types: mud bottom (composed of 30 - 50% of particles less than 0.01 mm in size), sandy mud bottom (with mud ratio higher than sand), muddy sand bottom (with sand ratio higher than mud - main component), and organic muddy bottom (mainly found in mangrove forests having an accumulation of dead organisms). In addition to small particles, it also contains certain organic ingredients [4]. Polychaete (Polychaeta) samples were identified according to Dang Ngoc Thanh [5] and Pierre Fauvel [6]. Tran Thi Thanh Binh, Hoang Ngoc Khac, Nguyen Thi Ha and Vu Quang Manh 134 Figure 1. Map of Len River mouth, with sampling stations (1. Yen Dong, 2. Yen Loc, 3. Do Ganh, 4. 7 years forest, 5. 3 years forest, 6. 1 year forest) 2.2. Results and discussion 2.2.1. Species composition of Polychaete in the research areas The results of identification of polychaete samples collected in the research area are shown in Table 1. Table 1. Species composition of polychaete in the study areas ≠ Taxons Locations Yen Dong Yen Loc Do Ganh Mangrove forest 7 years 3 years 1 years ERRANTIA 1. Nereidae 1 Nereis sp. + + ++ + + 2 Nereis trifasciata + + + + 3 Tylonereis fauveli + Species composition and distribution of polychaete (Polychaeta) in mangrove eco-system 135 4 Tylorrhynchus heterochaeatus + + + 2. Nephthydidae 5 Nephthys polybranchia + ++ +++ ++ + + 6 Nephthys oligobranchia + +++ ++ + ++ ++ 3. Glyceridae 7 Glycera tesselata + 8 Glycinde oligodon + 4. Amphinomidae 9 Pseudeurythoë sp. + SEDENTARIA 5. Sabellidae 10 Hypsicomus phaeotaenia + + + 11 Sabella porifera +++ + + ++ + 6. Amphictenidae 12 Pectinaria sp. + 7. Capitellidae 13 Capitellethus sp. + + + + + 8. Stersaspidae 14 Sterspis scutata + ++ +++ 9. Spionidae 15 Polydora sp. + Total of species 7 7 8 6 6 12 Density (ind/m 2 ) 56.00 138.07 92.50 24.30 86.50 95.40 Note: +: Few (1-20 individuals); ++: Medium (21-50 individuals); +++: Many (> 50 individuals) The results showed that: The composition of polychaete species studied includes 15 species, belonging to 11 genera, 9 families of 2 groups (i.e., settled group (Sedentaria) and mobile group (Errantia). Among these, 9 species of mobile group belong to 4 families (Nereidae, Nephthydidae, Glyceridae and Amphinomidae), while 6 species of residing groups belong to 5 families (Sabellidae, Amphictenidae, Capitellidae, Spionidae and Stersaspidae). The dominant species, Nephthys oligobranchia, was recorded at all sampling locations, ranging from moderate to high density. Several other lower dominant species, such as Nephthys polybranchia, Sabella porifera, Nereis sp, etc. were recorded at 5 or 6 sampling points, with a similar density. On the other hand, Pseudeurythoe sp. (in mobile group) and Polydora sp. Tran Thi Thanh Binh, Hoang Ngoc Khac, Nguyen Thi Ha and Vu Quang Manh 136 (settlement group) were distributed in only one habitat, with only a single individual collected. They could be two narrow ecological adaptation species. There were 12 species of polychaete collected in 1-year-old mangrove forest. This is the largest species habitat in which Sterspis scutata was the dominant one. In 3-year-old forest, there were 6 species in which Nephthys oligobranchia was the dominant one. Six species of polychaete were recorded in 7-year-old forest in which Nephthys polybranchia was the dominant species. In the mangrove areas, species composition and density of polychaete decreased gradually from 1-year-old forest (12 species, 95.40 ind/m 2 ), 3 year-old-forest (6 species, 86.50 ind/m 2 ), and to 7-year-old forest (6 species, 23.40 ind/m 2 ). It means that the number of species and individual density increases from the sea dyke to the water edge. Since the 1-year-old forests are planted on the periphery of the mangroves where bottom sediment is mud with an alluvium layer of about 40 cm thick, besides, salinity is relatively high, relating to the existence of the largest number of polychaete species. In the estuary, the number of species among study sites were not very different (7 species in Yen Dong, 7 species in Yen Loc, and 8 species in Do Ganh), while their composition is found with a significant difference. In the innermost inland site - Yen Dong, there were 7 species, of which two was not found in others sites. Yen Dong and Yen Loc shared 4 species, but have 6 different species. Besides, 5 similar species and 5 different ones were recorded in Yen Dong and Do Ganh sites, respectively, but 7 similar species and only 1 different ones were recorded in Yen Loc and Do Ganh, respectively. Thus, species composition in Yen Loc and Do Ganh was of the most similarity between Yen Loc and Do Ganh, and that was different with Yen Dong site. This difference in species composition was probably because of the location of the third site, where the salinity is lower than outer sites. In mangrove areas (including 1-year-old forest, 3 and 7-year-old forest), there were 13 species, with a higher diversity than that in Len estuary (10 species). Due to salinity and substrate quality in mangrove forests, the difference in species composition between the two habitats are more relevant to the polychaete species than that in other sites. Estuarine areas are directly affected by the un-favorable conditions of living (including their flow, involvements of human- beings) of the polychaete, resulting in a decrease of number of species. 2.2.2. The distribution of the polychaete in different substrata Substrate properties play an important role in the distribution of polychaete worms. In all types of bottom substrate, muddy bottom was the most suitable for polychaete with 12 various species living at an average density of 95.40 ind/m 2 (Table 2), and Sterspis scutata was the dominant one. In the muddy sand bottom, there were 8 species obtained with an average density of 56.20 ind/m 2 , with Nephthys oligobranchia as the dominant one; in sandy mud bottom, 8 species were recorded with an average density of 135 ind/m 2 with Nephthys oligobranchia as the dominant one (In sandy mud bottom, only 8 species were recorded but the average density is highest (135 ind/m 2 ), because two species (Nephthys oligobranchia and Nephthys polybranchia) were very high density (114 ind/m 2 ) in this habitat. In other substrata, the highest average density of one species did not exceed 25% of the total density of polychaete); There are fewer species and lowest individual density in the organic mud bottom with 7 species at54.10 ind/m 2 , of which Species composition and distribution of polychaete (Polychaeta) in mangrove eco-system 137 Sabella porifera is the dominant one. A few polychaete live in organic mud bottom due to its containing of mainly detritus. Once organic matters were de-composed, the gas of CO2 and other substances were produced, probably leading to negative effect on polychaete’s life. Table 2. Distribution of polychaete in different substrata from the study area Types of bottom substrata Muddy Sandy mud Muddy sand Organic mud Total of species 12 8 8 7 Density (ind/m 2 ) 95.40 56.20 135.00 54.10 2.2.3. The distribution of polychaete in salinity and pH of the water Salinity affects directly the distribution of the species, and the average density of the polychaete. In the study area, 15 species of polychaete recorded were distributed in brackish water (19‰ - 30‰) at an average density of 100.9 ind/m2, while only 9 species were found in saline brackish 5‰ -18‰ with an average density of 47.1 ind/m2 (Table 3). The number of polychaete species increases correlatively with either the increasing of salinity levels or the direction from the Len estuary toward mangroves forests. Twelve species were recorded in 1-year-old forest located nearest to the sea, with the highest salinity, while only 7 polychaete species were found in Yen Dong - the innermost inland sites with the lowest salinity (Table 1). Table 3. Distribution of polychaete in salinity and pH condition Environmental condition Index Salinity levels (‰) pH Saline brackish (5 - 18) Brackish salt (19 - 3 0) 5.5 - 6.5 6.6 - 7.5 7.6 - 8.5 Total of species 9 15 9 14 5 Density (ind/m 2 ) 47.10 100.90 93.80 83.10 56.00 The largest number of species (14) were recorded with a density of 83.1 ind/m 2 , in water neutral pH level from 6.6 to 7.5, which created favorable conditions for all activities of the polychaete. Nine species were recorded with an average density of 93.8 ind/m 2 , at pH water level from 5.5 to 6.5, which were measured mainly in mangroves. At high tides, flooding organisms will be oxidized and transferred to H2S acid, causing a decrease of pH level of sea-water. Acid pH environment is disadvantageous for polychaete species. At alkaline environment (pH 7.6 to 8.5), 5 polychaete species were recorded with an average density of 56 ind/m 2 , which was an inappropriate environment for the living activities of the polychaete, especially for the settled group. This pH was measured mainly in the estuary at high tides, when freshwater was waveded against the inland, leaving sea-water with a majority of salt and an increase of pH level. Tran Thi Thanh Binh, Hoang Ngoc Khac, Nguyen Thi Ha and Vu Quang Manh 138 2.2.4. Comparing diversity of species composition in the study area with some others on the Northern coast of Vietnam In this study (within estuary and mangrove forests of Da Loc Commune), species diversity was greater than that in Cua Luc, Quang Ninh and there has a higher number of families, genera and species than Thai Thuy, Thai Binh (Table 4). This result proves that the diversity of polychaete in mangroves eco-system of Da Loc Commune is relatively high compared to that of other Northern coastal regions. Table 4. Comparing the diversity of polychaete species composition in the study area with those of some other Northern coastal regions Areas Number of families Number of genera Number of species Dominant families Cua Luc - Quang Ninh (*) 12 12 14 Nephthydidae (2 species) Glyceridae (2 species) Thai Thuy - Thai Binh (*) 4 5 5 Nereidae (2 species) Nghia Hung - Nam Dinh (***) 13 20 23 Glyceridae (3 species) Nereidae (3 species) Giao Thuy - Nam Dinh (**) 11 20 33 Nereidae (7 species) Study area (Da Loc commune) 9 11 15 Nereidae (4 species) Nephthydidae (2 species) Glyceridae (2 species) Source: * Do Van Nhuong, Pham Dinh Trong [2, 7] and Tran Huu Huy ** [8] 3. Conclusion This study showed that at the time of the investigation, 15 polychaete species were found, belonging to 11 genera, 9 families and two groups of settlers (Sedentaria) and mobiles (Errantia). Amongst four different substrata (mud, muddy sand, sandy mud and organic mud) in the research area, muddy substrate had the most diversified with the number of 12 species. However, sandy mud had the highest density, with 135 ind/m 2 . Species composition and density increased relatively with salinity; the highest in the range of salinity from 19 ‰ to 30 ‰. Number of polychaete was highest at pH ranging from 6.6 to 7.5. Acknowledgements: This study was financially supported by research subjects in scientific and technological of the Ministry of Education and Training, Code: B2016- SPH-24. Species composition and distribution of polychaete (Polychaeta) in mangrove eco-system 139 REFERENCES [1] Chung N. V., 1994. Living marine resources and marine ecosystems. Vietnam Sea monograph. 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