by Terence Ching-wai Fong
Seagrasses are the only angiosperms surviving wholly submerged in shallow marine waters. Seagrasses possess erect leaves and shoots and creeping stems or rhizomes. Their flowers are pollinated underwater and develop into fruits and seeds. Seagrass meadows occur in most shallow, sheltered, soft-bottomed marine coastlines and estuaries of the world (Den Hartog, 1970; Phillips and Meñez, 1988). These meadows may be either monospecific or multispecific, in that up to 12 species may co-exist in the communities (Kirkman, 1985). Globally, about 58 seagrass species belonging to two orders (Hydrocharitales and Najadales), four families (Hydrocharitaceae, Posidoniaceae, Cymodoceaceae and Zosteraceae), and 12 genera (Enhalus, Thalassia, Halophila, Posidonia, Syringodium, Halodule, Cymodocea, Amphibolis, Thalassodendron, Zostera, Heterozostera and Phyllospadix) are recognized (Kuo and McComb, 1989).
Three species of seagrass namely, Zostera japonica Aschers. & Graebn. (=Z. nana Roth.), Halophila ovata Gaud. and Halophila beccarii Aschers. have been recorded from Hong Kong (Fong, 1998). A fourth species of marine monocotyledonous angiosperm, Ruppia maritima L. S. L., has also been recorded (Hodgkiss and Morton, 1978a, b; Melville and Chan, 1992). All the Hong Kong seagrasses share the same characteristic of co-occurring with mangroves in the low to mid-intertidal region of sand and mud flat areas.
Ruppia maritima, the Widgeon grass, was once recorded from Shek O Lagoon (Dunn and Tutcher, 1912) and has been recently reported from shallow brackish water 'gei wai' (shrimp ponds) at the Mai Po Marshes Nature Reserve (Melville and Chan, 1992). R. maritima usually grows in gei wai channels at between 0.6 to 1.2m depth.
Halophili beccarii, the Spoon grass, is widely distributed in the South China Sea and the Bay of Bengal (Den Hartog, 1970; Phillips and Meñez, 1988). In Hong Kong, the plant was first recorded from Tsim Bei Tsui as dense mats amongst pioneer mangroves (Hodgkiss and Morton, 1978b). The recent distribution of H. beccarii suggests that the plant is extending its range on the intertidal mud flat along the southern fringe of Deep Bay, from Sha Kiu Tsuen to Ha Pak Nai (Fong, 1998; Fig. 1). The largest bed of H. beccarii, also the largest seagrass bed in Hong Kong, occurs at Ha Pak Nai and occupies a total area of ~4 hectares. Juvenile horseshoe crabs are commonly found feeding inside H. beccarii beds in Deep Bay. H. beccarii seems to be an important nursery and feeding ground of horseshoe crabs. Besides Deep Bay, H. beccarii can also be found in Tai Ho in Lantau (Frew, pers. comm.) and occasionally in Starling Inlet, especially Luk Keng (Wong, 1998)
Halophila ovata has a wide distribution. The centre of its geographical range is the South China Sea and the seas of the Sunda shelf, near Indonesia ([Den Hartog, 1970). Its northernmost occurrence is in Hong Kong. It used to be widely distributed on the sheltered flats at Tai Tam, Three Fathoms Cove and Hoi Sing Wan in Tolo Harbour and in Deep Bay (Morton and Morton, 1983) H. ovata is now recorded from four locations in Hong Kong, namely, San Tau in Lantau, Sheung Sze Wan in Sai Kung and Lai Chi Wo in Crooked Harbour (Fong, 1998; Fig 1). H. ovata is a short-lived annual plant with r-strategy. The extent of the intertidal H. ovata population at San Tau, close to the new international airport at Chek Lap Kok, was reduced significantly during the commencement of the construction and reclamation works in 1992. The plant almost disappeared in 1995 but recovered gradually after the completion of the reclamation work for the new airport. At Lai Chi WO, H. ovata previously occurred in low intertidal and extended to shallow sub-tidal areas and seemed to be the second largest seagrass bed in Hong Kong ( ~2 ha), however, this population disappeared completely in 1998. These two examples suggest that H. ovata is a pioneer r-strategy plant that can colonize some areas in a short time but disappears quickly under unfavourable conditions, e.g., typhoon or strong water current.
Zostera japonica, the Eelgrass, occurs from Sakhalin and Kamchatka (north of Japan) to Vietnam in East Asia, and along the Pacific coasts of North America (Den Hartog, 1970; Phillips and Meñez, 1988). Unlike Halophila ovata, Z. japonica is perennial in Hong Kong (Fong, 1998). Z. japonica Aschers. & Graebn. was first recorded from North America in 1957 but was named Z. nana Roth (Hitchcock et al, 1969). The plant was later renamed Z. americana Den Hartog (Den Hartog, 1970). Harrison (1976) suggested that Z. japonica and Z. americana were probably synonymous after comparing specimens of Z. americana with Den Hartog's drawings of Z. japonica. Bigley (1981) further used principal components analysis to confirm Harrison's suggestion.
Zostera japonica Aschers. & Graebn. Was first reported as Z. nana Roth. from Lai Chi WO, Hong Kong, by Hodgkiss and Morton (1978a). Z. nana Roth is a junior synonym of Z. japonica Aschers. & Graebn (Phillips and Meñez 1988) and the identity of the local Z. japonica was confirmed by Lee (1994).
Zostera japonica has been recorded from several locations in Hong Kong, namely, Sheung Sze Wan in Sai Kung, San Tau in Lantau Island, Lai Chi WO, So Lo Pun and Pak Kok Wan in Crooked Harbour and Sam A Chung in Double Haven, north-east N T. (Fong, 1998; Fig. 1). The first record as well as the largest Zostera bed was from Lai Chi WO (Hodgkiss and Morton, 1978a). The populations of Z. japonica at So Lo Pun (~10m2), Pak Kok Wan (~25m2) and Sam A Chung (~40m2) are small and are probably pioneer populations. Z. japonica at So Lo Pun, Pak Kok Wan and Sam A Chung may have originated from the population at Lai Chi WO in view of the proximity of the three locations. The small population of Z. japonica at Sheung Sze Wan, bed area less than 10m2, may also be originated from the population at Lai Chi WO, even though it is quite far away. Due to the development of the Chek Lap Kok airport, Z. japonica at San Tau, as with Halophila ovata, is threatened by development. The major threat influencing the growth and survival of Z. japonica at San Tau is increased sedimentation from the construction and reclamation works of the new airport development (Lee, 1994; 1997). Following the completion of the airport and, therefore, the disappearance of anthropogenic stressors, the population of Z. japonica at San Tau seems to have stopped declining and is now recovering.
Seagrass dispersal depends on two ways, vegetative and reproductive. Vegetative propagation can only allow the plants to colonize adjacent unvegetated area but not far away. Seed reproduction can help seagrass to disperse farther. The seeds of seagrasses, however, are too heavy to travel a long distance by water current alone (Orth et al., 1994). The most probable means of seagrass colonizing nearby shores is through the transportation of seeds with floating flowering shoots (Fong, 1998). The flowering shoots may have been dislodged by disturbances, such as storms or the burrowing activities of crustaceans, and may stay afloat for a long distance. New colonies may be formed if the seeds released from the detached flowering shoots encounter a suitable environment during drift. Dispersal agents such as birds are required for long distance (>40 km) dispersal, e.g., Zostera japonica from Lai Chi WO to San Tau or Sheung Sze Wan. Egrets usually feed on coastal sand/mud shore and may become the seagrass dispersal means. The seagrass seeds, accidentally eaten by egrets, can travel a long distance when the birds fly away and colonize a new area, and then defecate. An analysis of gene flow among the seagrass populations in different areas can help to explain the dispersal ability of this species and the connectivity between the locally fragmented seagrass habitats.
References
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Fong, C W. 1998. Some aspects of ecology of seagrass Zostera japonica in Hong Kong. M. Sc. Thesis, Department of Ecology and Biodiversity, University of Hong Kong, Hong Kong. 202pp.
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