台灣海域茗荷藤壺之隱蔽種多樣性

Abstract

茗荷藤壺 (Lepas) 為海洋表層的生物 (epipelagic organism) ，牠們的成體 (adult) 附著於海洋漂流物上，此特性使得茗荷藤壺得以隨著洋流漂流而成為廣泛分佈於世界各個海洋的生物。近年來有研究以COI、16S及18S三個遺傳標記所構成的分子證據指出，茗荷藤壺的族群因為陸地阻礙、洋流流向與洋流表層水溫等地理隔閡效應的影響而分化成區域性的分支 (biogeographic clades) 。然而，西北太平洋海域卻因為只有5隻L. anserifera 採集於日本，而不能有代表性地代表西北太平洋地區的多樣性。本研究以台灣周圍海域的茗荷藤壺標本做分子與形態的綜合分析，以探討茗荷藤壺在台灣周圍海域的多樣性模式。從四個DNA條碼基因 (COI, 16S, 18S and H3) 所構成的親緣關係樹 (phylogenetic trees) 中，本研究發現在台灣茗荷藤壺共可分成四個物種及五個區域性分支 (biogeographic clades)。之前被認為只在美國奧勒岡州 (Oregon) 發現之 L. anatifera (Oregon clade) 也出現在台灣海域，故本論文改稱之為L. anatifera北太平洋分支 (North-Pacific clade)。本論文也鑑定出L. pectinata於西北太平洋的一個新的分支 (NW-Pacific clade) 與一個未知的物種Lepas sp.。L. anatifera的北太平洋分支與L. pectinata的西北太平洋分支可能是受到北太平洋環流 (North Pacific gyre) 與赤道逆流 (Equatorial Countercurrent) 局限而分化。L. anatifera的全球性分支 (Global-clade) 與L. anserifera 也同樣出現在台灣海域，而L. anserifera是唯一未分化的全球性物種 (cosmopolitan species) 。以Assemble Species by Automatic Partitioning (ASAP) 和Bayesian implementation of the Poisson Tree Processes Model (bPTP) 兩種物種界定方法做出的結果也顯示，各個物種內大多數的區性分支之間的差異可能足以視為不同物種，而綜合分子與形態分析後，本研究認為L. anatifera 北太平洋分支與全球性分支可被視為不同物種。此外，未知物種L. sp.在過往曾以外觀特徵被認為是L. testudinata。然而，本研究經由分子證據佐以對過往的文獻記錄做形態上的比對後，認為L. sp.與L. testudinata為不同物種。L. testudinata模式地 (type locality) 於南非，故分佈於南半球的物種為L. testudinata，而本論文採得於西北太平洋的物種可被視為一個未知的新物種L. sp.。

Gooseneck barnacles of the genus Lepas are epipelagic organisms. Their adults attach to ocean-floating objects and can be dispersed by the ocean currents. Lepas barnacles thus have worldwide distributions. Previous studies studied the diversity of Lepas using COI, 16S, and 18S genes and showed that the populations were differentiated into several biogeographic clades which are influenced by vicariance effects of continental isolation, ocean currents, and sea surface temperature (SST). However, the diversity in the Northwest Pacific was understudied because the pattern addressed was based on five L. anserifera specimens collected from Japan. In the present study, extensive Lepas specimens were collected around Taiwan and were analyzed with DNA barcoding genes (COI, 16S, 18S, and H3 genes) and morphological approaches. Compared to previous studies, our study revealed the original Oregon clade of Lepas anatifera is also present in Taiwan waters and we named it as North-Pacific clade. We have identified one more clade of L. pectinata which is distributed in the northern hemisphere of the Pacific. We also identified one unknown, probably new species of Lepas in the NW Pacific. The differentiations of L. anatifera North-Pacific clade and L. pectinata NW-Pacific clade may be caused by the limited gene flow which is affected by the North Pacific gyre and Equatorial Countercurrent. The Taiwan waters also contain the Global clade of L. anatifera and L. anserifera. Only L. anserifera is undifferentiated and is cosmopolitan in the world. The species delimitation results of Assemble Species by Automatic Partitioning (ASAP) and Bayesian Implementation of the Poisson Tree Processes Model (bPTP) also suggested most regional clades of Lepas species may be considered as separate species. According to the combination of morphological analysis and molecular analysis, the North-Pacific clade and Global clade of L. anatifera can be considered as different species. In addition, the undescribed L. sp. identified from the present study was morphologically similar to L. testudinata. However, the present study shows they are different species according to the molecular evidence and historical taxonomic records. L. testudinata was first identified in South Africa, and therefore the L. testudinata in the southern ocean represent the true L. testudinata and the species identified in the Northwest Pacific (morphologically similar to L. testudinata) can be revealed as an undescribed new species.