形態與分子證據闡述陸寄居蟹屬（Coenobita）之階段性陸棲演化與物種分類鑑定

Abstract

在分類學上，陸寄居蟹屬於陸寄居蟹科（Coenobitidae），共包含兩屬，亦即椰子蟹屬（Birgus）和陸寄居蟹屬（Coenobita）。椰子蟹（Birgus）是世界上最大的陸棲節肢動物，外觀形態和同為十足目的螃蟹頗為相似，擁有一身堅硬的外甲殼。而平時背著貝殼的陸寄居蟹（Coenobita）則必須仰賴身上所背負的貝殼來提供防禦掠食者之用。陸寄居蟹的不對稱腹部與背殼行為，一直以來都被視為演化自海寄居蟹的直接證據。陸寄居蟹為了保護柔軟的腹部，必須將腹部旋入貝殼之中，也因此對陸寄居蟹的形態演化產生了影響。本論文主要之重點目標為以下兩項：一、藉由形態與分子證據，對陸寄居蟹之種間分類進行再次的檢驗與深入的探討；二、日趨成熟的系統發生學理論與分析軟體，有助於解決物種間之親緣關係與演化之謎，本文亦藉此強大的工具對陸寄居蟹進行全面的剖析。在分析方法上，本研究採用形態特徵與分子序列的綜合比較分析，此方法的優點是：提供更高的解析力，以及更佳的正確性。本研究陸續取得了來自世界各地的陸寄居蟹mtDNA序列，包含COI和16S rDNA兩個片段；其中有九個物種是世界首次被定序，以作為本次研究之用（C. brevimanus, C. cavipes, C. clypeatus, C. hilgendorfi, C. perlatus, C. pseudorugosus, C. purpureus, C. rugosus, C. violascens）。分子序列分析結果顯示，在陸寄居蟹屬（Coenobita）之中，形態特徵與演化適應之間有明顯而直接的相關性。另一方面，陸寄居蟹的相近物種之間，一直存在著分類與鑑種的難題。本論文提供分子遺傳距離之數據分析，以重新檢驗並明確定義適合用來分類的形態特徵，以供未來其他相關研究之用。在陸寄居蟹演化之議題上，本研究之分析結果異於過去甲殼類學家所堅信之「由海濱到內陸」的演化方向；陸寄居蟹之陸棲適應演化另闢一條極其特殊之「由內陸至海濱」路徑。

Terrestrial hermit crabs are represented by the Coenobitidae, which includes only two genera, Birgus and Coenobita. Coconut crabs (Birgus) are among the world’s largest terrestrial arthropods, having a crab-like morphology and a strongly calcified exoskeleton. The shell-carrying terrestrial hermit crabs (Coenobita), in contrast, have depended on gastropod shells for protection and as an aid in respiration. The asymmetrical pleons of terrestrial hermit crabs have been considered an evidence of marine hermit crab ancestry. Shell-carrying has constrained the morphological evolution of hermit crabs by requiring a decalcified asymmetrical pleon capable of coiling into gastropod shells. The target purpose of this work is to explore the species identification, phylogenetic relationships and evolutionary processes of terrestrial hermit crabs based on the morphological and molecular information. Comparative analysis of morphological characters and molecular sequences in a phylogenetic effort efficiently improves the sensitivity and accuracy of evolutionary inference, producing more precise results than single dataset analyses can provide. The partial sequences from the mtDNA COI and 16S rDNA genes of Coenobita species, nine of which are presented here for the first time (C. brevimanus, C. cavipes, C. clypeatus, C. hilgendorfi, C. perlatus, C. pseudorugosus, C. purpureus, C. rugosus, C. violascens), demonstrate how morphological patterns can illuminate the processes of evolutionary transition at the genus scale. Besides, there have been several taxonomic ambiguities among these Coenobita species. We reexamined and reidentified reliable morphological characters for species identification by referring to the patristic distances between each species. And, most important of all, we present evidences of molecular and morphological studies that the evolutionary route of terrestrial adaptation in the Coenobita phylogeny was from the inland to the shore, and not, as some carcinologists would have believed, gradually from the shore to the inland.