Master of Science in Environmental Management

This study aimed to determine the spatial zonation and diversity of bivalves and gastropods in mangrove forest of Casiguran, Aurora. Specifically, this aimed to provide a list of bivalve and gastropod species present in Cozo, Esteves and Tinib of Casiguran, Aurora; determine the diversity, equitability, density and abundance of bivalve and gastropod species in low, middle and high tidal zones; determine the similarity of composition between tidal zones and sampling areas in terms of bivalve and gastropod species; and determine the relationship between spatial zonation and diversity of bivalves and gastropods as influences by ecological factors such as vegetation, soil texture, soil organic matter, soil, salinity, and soil pH.
A total of 22 taxa comprising eight species of bivalves and 14 species of gastropods were identified. The bivalves belong to five families while gastropods belong to seven families. Four species of gastropods (Nerita albicilla, Nerita chameleon, Neritina turrita and Terebralia palustris) were only seen in Esteves while one species (Polinices mammilla) was only found in Cozo and one species (Muricidae sp) was found only in Tinib.
The high tidal zone of Esteves had high diversity (0.66) of bivalve species compared to other tidal zones and areas. On the other hand, the diversity of gastropods were almost equal in the middle tidal zones of Cozo (0.60), Esteves (0.57), and Tinib (0.42).
The bivalve species distribution was very unequal (0.17) and almost equal (0.59) in the low tidal zones of Cozo and Tinib while it was very equal (0.94) in the high tidal zone of Esteves. For gastropods, there were equal distributions in the middle (0.77) and high (0.67) tidal zones of Cozo, equal distribution in the middle (0.63) tidal zone of Esteves and almost equal distributions in all tidal zones (0.44, 0.48 and 0.43) of Tinib.
In terms of bivalve species’ density and abundance, Dendostrea folium was the most dense and abundant in the low tidal zone of Cozo (253,333 indha-1 or 25 ind.m2-1, 97.44%) while Gafrarium tumidum in the low tidal zones of Esteves (46,666 ind.ha-1 or 4 ind.m2-1, 73.68%) and Tinib (126,666 ind.ha-1 or 12 ind.m2-1, 50%).
In terms of gastropod species’ density and abundance, Cerithidea cingulate was the most dense and abundant in the low (666,666 ind.ha-1 or 66 ind.m2-1, 65.79%) tidal zone of Cozo and the middle (5,280,000 ind.ha-1 or 528 ind.m2-1, 61.40%) tidal zone of Tinib. It was Terebralia palustris in the low (1,263,333 ind.ha-1 or 126 ind.m2-1, 63.06%) tidal zone of Esteves.
All tidal zones within each study area had greater dissimilarities in terms of bivalves but had greater similarities in terms of gastropod species’ compositions. On the other hand, all tidal zones between study areas had greater dissimilarities in terms of bivalve and gastropod species present. Most tidal zones had Gafrarium tumidum as common bivalve species while Neritidae sp. and Cerithidea cingulate were common gastropod species. All these species were found thriving in a wide range of soil salinity, organic matter, pH and texture.
Based on the results of the study, it is concluded that the mangrove forest of Casiguran, Aurora contains variety of bivalve and gastropod species; most bivalve species belong to Veneridae family while most gastropods belong Neritidae and Potamididae families; the very low bivalve species diversity in all tidal zones of Cozo is due to the enormous density and abundance of Dendostrea folium; most bivalves in Esteves preferred the high tidal zone with very equal distribution; gastropods prefer the middle tidal zones in Cozo, Esteves and Tinib with almost equal to equal distributions; the destiny and abundance of Cerithidea cingulate and Tere balia palustris are dut to their being strict inhabitant in the mangrove forest; all tidal zones within each study area have greater dissimilarities in terms of bivalves but have greater similarities in terms of bivalves and gastropods present, and most tidal zones have Gafrarium tumidum as common bivalve species while Neritidae sp. and Cerithidea cingulate are common gastropod species that thrive in a wide range of soil salinity, organic matter, pH and texture.
It is recommended that a survey must be conducted for 12 months to cover the changes in species’ diversity, equitability, density and relative abundance during different seasons. Other ecological factors such as species of mangrove trees present, nutrient analysis of leaf litter and depth of tidal inundation must be included.