西太平洋變色雀鯛的族群連通性

Abstract

了解海洋生物族群間的連通性是漁業管理與保護區設立必須具有的基礎資訊，藉由比較不同地理族群間的遺傳差異，可間接估算族群間過去或正在進行的基因交流，而耳石中的化學元素濃度變化也可用於研究魚類的遷徙與幼生散佈的途徑，因此藉由此兩種方法可以了解過去至現在族群之間的動態關係，本論文即利用上述兩種方法研究西太平洋變色雀鯛族群的連通性，並探討族群新添量的來源。 過去的研究發現，日本本島與琉球群島的變色雀鯛族群的生殖特徵如卵徑大小、卵塊大小與產卵頻率，會隨著不同緯度而呈現梯度變化，這些特徵被認為可能與遺傳變異有關，本研究以粒線體基因控制區的片段，分析台灣南部至日本本島共8族群的遺傳結構，結果顯示，日本本島與琉球群島族群間的基因交流很低，而族群間分化的型式與生殖特徵的變異型式相符，意味著生殖特徵的變異很可能具有遺傳基礎；另一方面經由族群變動係數的估算與族群擴張的檢測發現，各變色雀鯛族群可能在2萬七千到七萬年前遭受過嚴重的瓶頸效應與之後的族群快速擴張，因此推測更新世的冰河期海平面變遷可能對變色雀鯛歷史族群變動有很重大的影響。 藉由估算台灣鄰近海域變色雀鯛族群間的Φst值，本研究發現台灣周圍海域的變色雀鯛族群可以分成兩大群，一群包括北台灣與澎湖族群，另一群則包括海南島、南台灣與琉球族群，此一分群的模式與過去研究台灣海域珊瑚礁生物族群結構的結果相似，本實驗進一步使用多基因座基因型和貝氏分配方法判斷新添入幼生的可能來源，結果顯示台灣南部族群新添入的幼生約有50%可能來自海南族群，且台灣南部與海南島變色雀鯛成體族群之間的基因交流非常強，此幼生散佈途徑與過去的推論並不符合，另一方面北部與澎湖的新添量組成則較為均質，對成體Φst 與幼生Fst分析中發現，台灣南部族群與澎湖—台灣北部族群間的基因交流皆很有限，因此推論在台灣周圍的變色雀鯛族群入添可能有兩個來源。 為了進一步驗證此幼生散佈路徑，分別自北部的卯澳及野柳、澎湖的青灣外灣與墾丁的眺石，每地點各採集十隻，共40隻變色雀鯛成體，分析不同地點魚體內扁平石(sagittae)的化學元素組成，結果顯示，Mg/Ca, Mn/Ca, Sr/Ca, Ba/Ca 與 δ18O等元素在不同採樣點之間皆有顯著差異(p < 0.05)，主成分分析(PCA)結果顯示，眺石族群的化學元素組成明顯可與卯澳、野柳及青灣外灣區隔，而分成兩大群。此結果與先前的族群結構分析結果相符，且眺石族群的平均δ18O的數值為四個採樣點中最高(-2.239 ± 0.104)，表示在幼生漂浮期所接觸到的水團性質為高溫低鹽，過去研究顯示南中國海表層流水團的性質是台灣鄰近海域中溫度最高鹽度最低，因此符合先前以基因型分析法所得結果，也就是南部新添入的幼生可能源自南中國海。此外，在耳石微量元素分析過程中發現，Mn與Ba元素的累積可能受到體內生理調節的影響因此無法正確反映水體元素濃度的變化，因此Mn與Ba兩種元素必須謹慎使用。 本研究結果顯示，變色雀鯛的族群結構受到更新世冰河期海水面變遷的影響甚大，而現今族群間的連通性則主要受到當今洋流系統的影響。本研究藉由了解變色雀鯛不同地理族群間的連通性，可提供未來西太平洋珊瑚礁魚類資源管理與海洋保護區設立的參考資料。

Analysis of genetic data can reveal past and ongoing connectivity among reef fish populations, and using geochemical signatures in calcified structures provide another indirect means of tracing movements and determining the population origin of reef fishes. These methods can be applied to determine which populations are connected by larval transport, and the results of these studies are crucial for effective management of fisheries and the design of marine reserves. In this dissertation, I used both methods to study the connectivity of Pomacentrus coelestis among populations in the West-Pacific. The results of genetic analysis indicated that a genetic break existed between mainland Japan and Okinawa populations, and this break was congruent with the pattern of phenotypic variations documented in previous studies. This evidence supports that the latitudinal variation of reproductive traits among P. coelestis populations is likely being genetically based. The historical demography of P. coelestis revealed by molecular data suggested that drastic changes in sea level and sea surface temperatures during the last glaciations might have resulted in dramatic reduction of their population size in the northwestern Pacific. Among populations around Taiwan, a genetic break was found between the Penghu-northern Taiwan group and the southern Taiwan group (Kenting and Hainan populations). However, the results of Bayesian assignments revealed a larval transportation pattern differed from that proposed by previous studies. The recruits in southern Taiwan were mainly from the South China Sea (approximately 50%), while the recruits in Penghu and northern Taiwan were from mixed sources. Moreover, both Φst of adult populations and Fst of recruit populations indicated limited gene flow between Penghu-northern Taiwan and southern Taiwan groups. It suggests that the recruits in southern Taiwan may have different origins comparing with those of northern Taiwan and Penghu populations. The geochemical signatures in sagittae showed that significant differences were found in Mg/Ca, Mn/Ca, Sr/Ca, Ba/Ca and δ18O values among sites in pre-settlement region of otoliths. By using the combined data of trace element ratios and stable isotopic compositions, the results of PCA (principal component analysis) showed that the otoliths collected from Tiaoshi, southern Taiwan and other three sites (Maoao, Yehliu and Chinwan Outer Bay) around Taiwan could be separated. This indicates that the population at Tiaoshi may have different source which is congruent with the results of genetic studies. Overall, it suggests P. coelestis larvae may be transported by the South China Sea Surface Water (SCSSW) from Hainan Island to southern Taiwan, and the population at Chinwan Outer Bay in Penghu and Yehliu in northern Taiwan may share the same gene pool (Fst = -0.0001, p > 0.05). In addition, the results also indicated that the incorporation of Ba/Ca and Mn/Ca element ratios in P. coelestis otolith could control by physiological regulations. Thus, caution is warranted when using these two element ratios. These results suggest that the last glaciations had profound effect on the historical demography of P. coelestis, and the recent demographic connectivity was mainly determined by local hydrodynamic regimes. Since self retention is unlikely the case in P. coelestis populations, the connectivity patterns of P. coelestis could be used as a reference for designing effective management strategies on marine organisms with low degree of self retention in the West-Pacific.