Brown Seaweed Sargassum spp.
Seaweeds are often miscalled as seagrass due to their similar morphology. However, seaweeds are not plants, but they are primitive organisms possessing differentiated cell types. They belong to marine algae for which they lack roots, vascular bundle tissues and do not produce flowers, fruits and seeds. Instead, it anchors to substratum by structure called holdfast without the ability to gather nutrients. Gaseous exchange and absorption of nutrients take place across all of its tissues via diffusion. Photosynthesis in most plants take place only in their leaves, however seaweed can carry out photosynthesis in all tissues. It possesses numerous tiny air-filled vesicles which keep the whole alga buoyant and maintain up-right position in the water column.
Over 200 species of marine macrobenthic algae have been reported in Hong Kong waters. Among all macroalgae flourishing in the Hong Kong eastern waters in winter, brown seaweedof the genus Sargassum is the dominant type among the standing crop especially in areas exposing to strong or moderately strong waves. They exhibit a strong seasonality in their abundance, during which they grow rapidly in winter months from October to May, and disappear (die-back) completely in summer from June to September. Sexual reproduction takes place in Sargassum mainly in winter which involves the fusion of sperm and ovum. Fertilized egg in its early stage of germination falls down on some solid substratum and develops into a typical thallus. Asexual reproduction can also take place in seaweeds through fragmentation or division.
Beds of Sargassum which can often grow up to 2 – 3 m in length are often found to be important nursery habitats for various juvenile fish and invertebrates e.g. molluscs, echinoderms. Thus, it provides shelter and protection to juveniles that are vulnerable and susceptible to predators.
Name in Chinese: 馬尾藻
Genus/ Species: Sargassum spp.
Site of discovery: Tung Ping Chau, Yan Chau Tong, Hoi Ha Wan Marine ParksStatus: Very Common in winter