Coastal Biodiversity in Ranong, Thailand
Coastal Biodiversity in Ranong
The Seagrass Beds
Seagrass Cymadocea spp.
Seagrasses are flowering plants that have adapted to living in the sea. These plants are frequently found growing in dense beds or meadows. Seagrasses are found mostly on soft sediment in estuaries and shallow coastal waters. Seagrasses have extensive underground roots and rhizomes which anchor the plant in the muddy sands. These roots absorb nutrients but, unlike their land-living relatives, they do not take up water. The stems and leaves of the plant grow out from the underground roots. The leaves are thin, which allows for efficient capture of sunlight, absorption of nutrients and exchange of gases.
Seagrass species
Seagrass Halophila ovalis.
Worldwide there are approximately 55 species, the greatest numbers of which are to be found in tropical seas. The shape of the leaves and flowers, together with the growth form of the plant, are used to distinguish different species. For example, Cymadocea serrulata (above) has long strap-like leaves while those of Halophila ovalis (left) are small and oval. Seagrasses are found from the lower intertidal zone down to depths of 12 metres or more depending on the clarity of the water. Different species are found at different depths.
Seagrasses in Thailand
A seagrass bed.
Along the Andaman Sea there are several important seagrass localities. The best studied is at Trang in the Had Chao Mai National Park. Other important seagrass beds are found in Phangnga and Khura Buri which lies within the Coastal Biodiversity in Ranong Project's study area. Mapping and assessing the communities associated with different seagrass beds is an important objective for the project.
Seagrass communities
Seastar Pentaceraster regularis
Archaster typicus
Seagrass beds are productive habitats supporting a diverse flora and fauna. The blades and stems of the plant provide suitable habitats for algae, foraminiferal protozoans, hydroids and sea-squirts. While in the sediment around the plants, there are many species of invertebrates: polychaete worms; crustaceans like crabs, tanaids and isopods, together with many species of bivalve and gastropod molluscs. The beds are also inhabited by large surface dwelling species like starfish, Archaster typicus and oreasterids such as Pentaceraster regularis.
Dark holothurians, or sea-cucumbers, like Holothuria atra, feed on the sediment surface. Like mangroves, seagrass beds provide a refuge and nursery for the larvae and juveniles of prawns and fish. There is evidence that many species of crustaceans and fish commute between seagrass beds and mangroves. Larger visitors to seagrass habitats are turtles and dugong which come to feed on the seagrasses. Dugong have been sited at a number of localities along the Andaman coast, especially from a Trang Province. Fishermen report dugong from Tung Nang Dam, Khura Buri district, and the seagrass beds there show signs of dugong grazing.
The role of seagrass
The importance of these beds cannot be overstated. They play a key role in stabilising soft sediments, anchoring the sand and mud with their root systems, slowing erosion. The beds are nursery grounds for commercially important species of prawn and fish. Seagrass beds are productive habitats. While seagrasses are grazed directly by dugong and invertebrates such as sea-urchins, much of the nutrient gained by other species of the community is in the form of detritus from the breakdown of the leaves and stems of the plants. These species are in turn preyed on by larger species
Find out more about seagrass beds:
Ocean
Rescue 2000
University
of Hawaii seagrass page
EPA
site
Seagrass
grey literature
Dugong
Dugong
conservation
Australian
Goverment dugong conservation
Haribon
Foundation
Dugong
web links
Rothauscher
dugong page