硫磺怪方蟹對硫化氫的耐受度與解毒機制

Abstract

本研究針對龜山島附近的淺海熱泉生物進行研究，自2005年8月起至2006年8月，一共完成8次的潛水調查與採樣，目前共紀錄包含4門6種大型底棲生物，包括兩種刺胞動物（Cnidaria）：圓管星珊瑚Tubastraea aurea及一種未鑑定的海葵；兩種軟體動物（Mollusca）：一種小型螺類Nassarius sp.及一種石鼈；環節動物（Annelida）一種：龍介蟲科（Serpulidae）的管蟲，與節肢動物（Athropod）一種：硫磺怪方蟹（Xenograpsus testudinatus）。其中硫磺怪方蟹數量十分龐大，並且主要在噴口中心區域活動，其牠物種均只出現在週圍區域。 硫化氫對水生生物來說是一種劇毒，它能滲入水族體內，阻抑電子傳遞鏈中細胞色素-c的正常功能，造成細胞死亡。由龜山島海域的水樣分析顯示，熱泉噴口中心區的海水硫化氫濃度高達2405.5 µM。將硫磺怪方蟹暴露在1000 µM及2000 µM的硫化氫中，半致死時間（LT50）分別為136及102小時，證實了怪方蟹對硫化氫具有很高的耐受度。分析暴露後的怪方蟹體內，發現大量的硫代硫酸根（S2O32-）累積在各部位的組織之中，藉著氧化作用，將硫化氫轉化為不具毒性的硫代硫酸根，再逐漸排出體外，可以克服硫化氫的毒性。淺海熱泉中硫化氫的毒性，不但排除了棲地中可能出現的競爭者與掠食者，也殺死潮水團中的浮游動物，讓浮游動物掉落，成為此處源源不絕的食物來源，硫磺怪方蟹慿著對硫化氫毒性的適應，得以在龜山島淺海熱泉區建立龐大的族群，成為最優勢的物種。

The study aims to investigate the marine macro fauna associated with the shallow-water hydrothermal vents off Kueishan Island. Six species representing 4 phyla were recorded, including a hexacoral Tubastraea aurea, an unidentified sea anemone, two species of mollusks (a snail Nassarius sp., an unidentified chiton), one serpulid polychaete, and the vent crab Xenograpsus testudinatus. The crab is the dominant species and often swarms in enormous populations at venting areas, while other species occur at the peripheral areas of the hydrothermal vents. Sulfide is highly toxic to aquatic animals due to the inhibition of cytochrome-c oxidase, a critical enzyme for mitochondrial respiration. Seawater samples collected near the vents showed the highest concentration of sulfide as 2405.5 µM. The median lethal time（LT50）of X. testudinatus exposed to 1000 µM and 2000 µM of sulfide were 136 h and 102 h, respectively. Elevated levels of thiosulfate（S2O3-2）in the tissues of the crabs after exposure of sulfide showed that oxidation of sulfide to non-toxic thiosulfate is possibly the major detoxification mechanism. This mechanism enables X. testudinatus to tolerate to the high sulfide concentration in the environment. The hydrothermal fluid not only provides abundant food supply for the crabs by killing zooplankton and descent as ‘marine snow’ to the bottom, but also precludes the possible competitors and predators. The special adaptation of X. testudinatus to the toxicity of sulfide enables them to colonize successfully and become dominate in the shallow-water hydrothermal vent ecosystem off Kueishan Island.