Abstract
淡水鰻魚屬於鰻目鰻鱺科(Anguillidae)，共一屬15種3亞種。鰻鱺特殊的生活史從降海產卵開始，在深海中孵化的幼魚變態成柳葉型後利用洋流的飄送到達大陸棚邊緣，而後變態、溯河、洄游到淡水中成長，因此鰻鱺的遷徙擴散應同時牽涉到產卵場及棲息地的變化。所有的鰻鱺在外形上都十分類似，在傳統的系統分類中，只能依靠背鰭長短比例、體色花紋、齒痕模型等外在形質來做為分類依據，而且這些形質被認為是單一起源的祖徵。鰻鱺大部份都分布在印度太平洋，尤其被認為是種源中心的印尼群島附近最多。而與其他主要海洋隔離的大西洋則只有兩種鰻鱺，美洲鰻(Anguilla rostrata Le Sueur, 1817)及歐洲鰻(A. anguillaLinnaeus,1758)，因此大西洋鰻鱺的起源一直是個很值得探討的問題。在本研究中收集到9個種及1個亞種。針對粒線體ＤＮＡ完整的Cytochrome b片段進行定序分析及比對工作。研究結果顯示在分子類緣關係上類緣相近的種類並沒有同時顯示出類似的外型特徵。因此可以推測以往這些傳統上重要的分類依據，包括背鰭長短比例、體色花紋等，也許在鰻鱺屬的演化過程中是不穩定、或是重複出現的特徵。結合分子上的資料和化石證據，推測鰻鱺屬的起源約在二千萬年前。南北美洲大陸之間原本是分離的，兩大洋間的通道直到五百萬年前才因板塊運動而封閉。因此可以合理的假設，和南太平洋鰻親緣關係較接近的大西洋鰻的播遷路徑，並不是經由古地中海，而可能是藉由成鰻降海產卵的生態模式，穿過中美洲地峽，而後在藻海找到新的產卵場，成為現在大西洋鰻鱺的祖先。另一方面，如果鰻鱺選擇產卵場的因子和當地海底環境有高度相關，板塊運動也可能提供種化的動力。在本研究中同時提出利用限制脢切位分析，以提供有效率的鰻苗鑑定方法。Freshwater eels, the genus Anguilla Shaw of Family Anguillidae,containeighteen species (including three subspecies). Theunique catadromouseels are born in remote sea abysses and theirflat-body larvae driftingin ocean currents travel to the edgesof continents and, then, migrateinto freshwater followingmetamorphosis for further growth. Therefore,appropriatespawning places and freshwater environments for juvenilegrowthmay contribute to the distribution, speciation and dispersionofeel species. Classifying freshwater eels is rather difficultowing to their similarmorphological appearance. Major criteriafor classification consist ofproportion, coloration, dentition,and number of vertebrae. As generallyaccepted, a short dorsalfin and the absence of variegated markings areconsidered theprimary monophyletic features. A majority of theAnguillaspecies is distributed throughout the Indo-Pacificregion, particularlynear the equator, an area widely assumed tobe the center of origin offreshwater eels. In contrast, only A.rostrata (Le Sueur, 1817) andA. anguilla (Linnaeus, 1758)inhabit the Atlantic Ocean. Of relevantinterest is how theAtlantic eels migrated all the way from the centerof origin tothe Atlantic Ocean.In light of above concerns this studyexamines the phylogenetic relationshipsof freshwater eelscovering more than half of the Anguilla species andemploying theentire cytochrome b gene to ensure the genetic variation.Theclose species in the phylogenetic tree do not share a similarcolorationor the types of dorsal fin. Above characteristicsimply that thesemorphological features might be unstable or havereoccurred duringevolutionary history.By combining the molecularresults and geographical evidences, we speculatethat the originof the genus Anguilla could be deduced to twenty millionyearsago. In addition, the ancestors of Atlantic eels may nothavemigrated by the Tethys Seaway, but via the Central Americanisthmusinstead. The plate movement could provide anotherimportant factorfor eel speciation. In addition, thisinvestigation presents an efficientmethod by RFLF analysis toidentify the elvers for eel culture.