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Perspectives on Marine Environment Change in Hong Kong and Southern China, 1977-2001 (ed . B. Morton). Proceedings o f a n Internationa l Worksho p Reunio n Conference, Hon g Kong 21-26 Octobe r 2001. Hong Kong : Hong Kong University Press, 2003. HONG KONG' S SUBTIDA L OYSTER S (BIVALVIA : OSTREIDAE AN D GRYPHAEIDAE ) Katherine La m an d Bria n Morto n The Swire Institute of Marine Science and Department o f Ecology an d Biodiversity , The University o f Hong Kong, Hong Kon g ABSTRACT Between 23-27 April 2001, a trawl survey was conducted in the coastal waters of Hong Kong fo r th e collection o f benthic marin e fauna. O f th e 29 station s surveyed , oyster s (Bivalvia: Ostreidae and Gryphaeidae) were collected from ten only. These were located in Tolo Harbour, Mirs Bay and the southern waters of Hong Kong. The specimens were sorted and identified a s three species. These are Pianostrea pestigris, Dendostrea folium and Hyotissa imbricata. Descriptions of shell characters and illustrations of these subtidal oysters ar e given. Multivariate analysis of the abundance dataset of the oyster species using the BrayCurtis similarity matrix and non-metric muti-dimensional scaling (MDS) ordination has grouped the ten oyster sites into two, i.e.; (1), Beaufort Island, Long Harbour, East Ping Chau and Middle Mirs Bay and (2), Inner and Outer Tolo Harbour and North Ping Chau. Comparison of previous and present data shows the local abundance of subtidal oysters is sensitive to human disturbance, suc h as reclamation an d dredging . INTRODUCTION Trawl survey s o f th e epibentho s i n Hon g Kong' s water s hav e bee n carrie d ou t approximately ever y three years sinc e 1977 . Since oysters wer e not a dominant grou p among the specimens collected, they have usually been overlooked a s members of the epibenthos (Horikoshi and Thompson 1980 ; Thompson and Horikoshi 1982 ; Shin 1985, 1990). Only Alectryonella haliotidaea (Lamarck, 1819) , Ostrea pestigris (Hanle y 1846), Crassostrea sp . (Leung and Morton 1997 , 2000) and Ostrea denselamellosa (Lischke , 1869)(Thompson et al. 1982 ) hav e bee n recorde d i n previou s traw l surveys . A preliminary guid e to Hong Kong oysters by Morris (1985) identified Hyotissa sinensis 312 K. LAM & B. MORTON (Gmelin, 1791) , Ostrea pestigris, Alectryonella haliotidaea an d Dendostrea glaucina (Lamarck 1819) , and was based on specimens obtained by Taylor (1980) in trawls along Tolo Channel and in Mirs Bay. Scott (1994) described Alectryonella haliotidaea, Ostrea pestigris an d Hyotissa sinensis as occurring in the southern waters of Hong Kong, again from trawl samples. While these researches provide habitat data and basic shell characters for Hon g Kong' s subtida l oysters , identificatio n i s stil l problemati c becaus e o f thei r inherent plasticity in shell form and a fast-changing nomenclatur e for the Ostreidae and Gryphaeidae (Stenzel 1970; Harry 1985 ; Morris 1985; Torigoe 1985; Bernard etal. 1993 ; Coan et al. 2000). Some species may also have different colou r morphs (Morris 1985) . This stud y attempt s to provide a n updated taxonomy fo r th e common subtida l oyster s of Hon g Kon g an d t o provid e informatio n i n change s i n specie s abundanc e ove r th e years sinc e 199 0 in relation to environmental change . MATERIALS AN D METHOD S Between 23-27 April 2001, a trawl survey of 29 stations in the inshore waters of Hong Kong wa s conducte d fo r th e collectio n o f epibenthi c marin e organisms . Tw o bea m trawls, each o f 250 cm width wit h a 2.5 cm cod end mesh, were taken wit h a total of five minutes trawl time at each station. The stations were located in Tolo Harbour, Mirs Bay, and the eastern and southern waters of Hong Kong (Fig., IB). The specimens were sorted, counted and identified based on the shell characters described by Stenzel (1970), Harry (1985), Morris (1985), Torigoe (1985) and Scott (1994) and are herein described. Abundance data , expresse d a s numbers o f individual s km-2 , wer e analyse d t o identif y distribution pattern s i f any . Non-parametric multivariat e technique s wer e als o applie d to the standardise d abundanc e dat a set . A similarity matri x wa...

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