台灣硨磲貝共生體之研究

Abstract

硨磲貝(Tridacna spp.)為生活於印度太平洋熱帶珊瑚礁區的大型雙殼類，在珊瑚礁生態系中扮演重要角色，被視為健康指標物種。然而，近年來硨磲貝受到水族貿易需求而遭人為過度捕撈，也因棲地破壞、海洋汙染、海平面溫度增加而有數量下降及體長減少的趨勢，多種硨磲貝被國際自然保護聯盟評估為易危中的依賴保育，華盛頓公約組織也將硨磲貝列為世界稀有海洋生物附錄二之中。前人研究發現珊瑚身上的這些共棲微生物可藉由參與固碳、固氮以及硫、磷等元素的循環來幫助珊瑚維持健康、適應環境變動，預期在硨磲貝上也會有相似的發現，但目前硨磲貝共棲微生物的研究並不多。因此，本研究採集了台灣墾丁、小琉球、澎湖、太平島、東沙、東北角、龜山島、宜蘭、花蓮、台東、綠島、蘭嶼與日本沖繩的四種硨磲貝，長硨磲貝(Tridacna maxima)、諾亞硨磲貝(T. noae)、鱗硨磲貝(T. squamosa)與圓硨磲(T. crocea)的外套膜組織(0.5cm2)，同時紀錄殼長、深度，並透過次世代定序了解共生藻及細菌組成。調查結果顯示，硨磲貝在台灣不同地區的物種分布並不平均。微生物結果顯示，菌相組成方面，地點是最主要的影響因子，其次是物種，而體型大小的影響則相對較小，生長深度則無顯著影響。優勢菌門包括Proteobacteria、Verrucomicrobia、Firmicutes、Cyanobacteria、Bacteroidetes。在共生藻方面，地點也是最主要的影響因素，其次是物種，而體型大小和生長深度對共生藻組成的影響不明顯。優勢的共生藻屬以Symbiodinium為主，其次則是Durusdinium及Cladocopium。此外，長硨磲貝、諾亞硨磲貝與鱗硨磲貝三種硨磲貝的菌相組成和共生藻組成具有明顯的地理專一性。隨著地理環境的不同而有獨特的微生物相。且隨著體型的轉變，不同地點與不同種硨磲貝的菌相與共生藻相轉變呈現不同的趨勢。

Giant clams (Tridacna spp.) are the biggest bivalves that live in tropical coral reefs in the Indo-Pacific Ocean. They play an important role in the coral reef ecosystem and are considered indicator species for ecosystem health. However, in recent years, these giant clams have faced challenges due to overexploitation driven by the aquarium trade, habitat destruction, marine pollution, and rising sea temperatures, resulting in declining populations and reduced sizes. Most of species are classified as Vulnerable on IUCN Red List of Threatened Species and protected under CITES. Previous studies have demonstrated that the coral-associated microbiota participate in carbon and nitrogen fixation as well as cycling of sulfur, phosphorus, and other elements, aiding corals in maintaining health and adapting to environmental changes. Similar findings are anticipated for the microbiota associated with giant clams, although research in this area is currently limited. Therefore, this study collected samples of mantle tissue (0.5 cm2) from four species of giant clam (Tridacna maxima, T. noae, T. squamosa, and T. crocea) across various locations in Taiwan (Kenting, Xiao Liuqiu, Penghu, Taiping Island, Dongsha, Northeast Coast, Guishan Island, Yilan, Hualien, Taitung, Lyudao, Lanyu) and Okinawa, Japan. Shell length and growth depth are also recorded. And microbiome composition, including symbiotic algae and bacteria, were analyzed using next-generation sequencing. The investigation revealed uneven distribution of giant clam species across different regions of Taiwan. Microbial analysis indicated that location was the primary factor influencing bacterial composition, followed by species, with lesser influence from shell size and growth depth had no significant impact. Dominant bacterial phyla included Proteobacteria, Verrucomicrobia, Firmicutes, Cyanobacteria, and Bacteroidetes. In terms of symbiotic algae, location was also the primary factor, followed by species, with shell size and growth depth having less discernible effects. The predominant genera of symbiotic algae were predominantly Symbiodinium, followed by Durusdinium and Cladocopium. Additionally, T. maxima, T. noae and T. squamosa, microbial composition, both bacterial and algal, exhibited clear geographic specificity, with unique microbial communities associated with different geographic environments. Furthermore, variations in shell size resulted in different trends in microbial composition among different species from different locations.