重新審視極向遷移:東亞軸孔珊瑚的隱蔽譜系與生物地理結構

Abstract

物種向高緯度遷移被視為海洋生態系對氣候變遷的重要反應，然而此概念多以物種為單一演化單位作為前提。本論文檢驗此假設，使用三種在日本已有向高緯度遷移紀錄的優勢造礁軸孔珊瑚為研究對象，於台灣進行系統調查，鑑定其隱蔽種組成，並解析形塑現今空間分布的可能因素。本研究整合形態、分子與族群歷史分析，結果顯示三個名義物種皆由形態上難以區分但遺傳上高度分化的隱蔽種所構成，而且這些隱蔽種的分化程度已經達到屬於不同物種的程度。在台灣，隱蔽種分布與生物地理界線陳氏線高度一致，然而於更廣泛的東亞尺度，隱蔽種則呈現彼此不同的空間分佈模式，且分佈模式無法僅以當代環境梯度解釋。族群歷史推論顯示，目前的分布格局可能受到過去歷史氣候事件的影響，而這些事件對當前分布的形成可能具有關鍵作用。結合歷史族群軌跡與現行分布顯示，即使目前看似具有遷徙潛力的物種，也未必能在不同棲地建立長期穩定的族群，而遷徙後能否持續，也可能取決於該物種是否維持足夠的遺傳多樣性與適應能力以因應新的環境條件。即使有能力跨越多種棲地的物種，也無法保證一定能往高緯度擴張，這可能來物種本身限制，或是當地非生物條件與原棲地之差異，而生物與非生物因子可能會與該隱蔽種的族群歷史交互作用，決定其在氣候變遷下是擴張、維持或衰退。綜合而言，東亞珊瑚的向高緯度遷移並非物種層級的現象，而是由隱蔽種各自的演化與族群歷史所塑造的差異化回應，且現有空間分布範圍並不能保證他們未來面對氣候變遷時皆具備向高緯度遷移的能力，因此辨識與評估這些隱蔽種對於解讀生物地理格局、預測珊瑚在氣候變遷下的反應以及制定有效的保育策略至關重要。The poleward migration of species is considered a primary response of marine ecosystems to climate change. However, this hypothesis often assumes species are single evolutionarily units. This thesis challenges this assumption by investigating three dominant reef-building Acropora corals in Taiwan that have been reported to undergo poleward range expansion, identifying their cryptic lineages composition, and examining potential factors leading to the current spatial pattern.Using an integrative framework combining morphology and genetics, this research reveals that all three nominal species are composed of morphologically cryptic but genetically divergent lineages. The depth of their genetic divergence is consistent with species-level differentiation, indicating that they should be regarded as distinct species. Within Taiwan, these lineage distributions align with a major biogeographic boundary known as Chen’s Line. At the broader East Asia scale, however, the spatial patterns of cryptic lineages become more complex, with lineage-specific distributions that differ from one another and cannot be explained by present-day environmental gradients alone. Demographic inferences suggest that historical events may be a major cause of the observed inconsistencies in distribution patterns, with scenarios such as post-glacial recolonization from refugia during the Last Glacial Maximum representing plausible explanations for these patterns.Nevertheless, demographic trajectories and current distributions suggest that apparent migration potential may not guarantee long term establishment in newly colonized habitats. Persistence after arrival may depend on whether populations retain sufficient genetic diversity and adaptive capacity to cope with novel conditions. Even lineages that can occupy a wide range of habitats within their present range may still encounter strong barriers when expanding into environments that differ substantially from their source habitats, due to biotic constraints and abiotic conditions. These combined constraints may interact with the demographic history of a lineage to determine whether it expands, maintains its range, or declines under climate change.This thesis concludes that the poleward migration of corals in East Asia is not a uniform, species-level phenomenon but a set of lineage-specific responses, each shaped by its own distinct evolutionary and demographic history. The current spatial range of a lineage may not necessarily indicate its potential for poleward migration under future climate change. Recognizing these cryptic lineages is essential for accurately interpreting biogeographic patterns, predicting coral responses to climate change, and developing effective conservation strategies. These results highlight the need to integrate traditional morphology-based species concepts with genetic and genomic data in biodiversity assessments and conservation planning for coral reefs in the Anthropocene.