利用耳石有機物穩定碳氮同位素探討石首魚科個體發育之營養位階

Abstract

研究海洋生物在食物網中的營養位階可以瞭解物種和個體之間的食性關係以及生態系統中能量的流動。利用分析組織中的有機物穩定同位素值，即δ13C和δ15N值，為評估生物之間營養位階關係和潛在食物鏈關聯的常用且有效方法。然而大多數生物體組織的有機物代謝是非常活躍的，因此只記錄到約周至月的短期飲食訊息，不太可能完整揭示個體的生活史和飲食轉變。在過去的十年中已經開發出一種先進的技術來測定魚類耳石有機物中所記錄的δ13C和δ15N值，方法可以得知個體整個發育過程中的飲食訊息，因為耳石內有機物記錄了生物體終身的同位素信號，並能夠重建個體營養歷史。本研究目的為透過逐層溶解魚種左邊耳石，分析每層耳石所萃取出有機物δ13C和δ15N值，以元素分析儀連結穩定性同位素質譜儀(Elemental Analyzer-Isotope Ratio Mass Spectrometry)進行分析，重建9種石首魚科的營養位階，而石首魚科的耳石尺寸相較於其他物種來的大許多，因此使我們能夠在此物種的多個生命階段中獲得更多飲食訊息。透過逐層溶解耳石，並在逐步溶解耳石的過程中進行三維掃描耳石的型態，以評估每個溶解步驟之間耳石體積和形狀的變化，利用形態學分析，透過比較左耳石每層有機物萃取之同位素訊號與右耳石生長輪，能夠估算耳石中每一層有機物同位素訊號所相對應年齡區間。結果顯示，9種石首魚科於種間及種內皆存在著營養位階差異，本研究所使用之實驗方法強調耳石有機物之穩定同位素分析，能夠更全面了解個體飲食歷史，且具有可以更進一步擴展和應用於生物族群研究之潛力。

Studying trophic levels in marine ecosystems is crucial for understanding food web dynamics and it can be achieved by analysing stable carbon and nitrogen isotopes (δ13C and δ15N) in animal tissues. However, isotope analyses on metabolically active tissues only provide short-term dietary information in a captured individual fish. A cutting-edge technique analysing δ13C and δ15N values in otolith organic matter has emerged, allowing the reconstruction of an individual''s trophic history. In this study, we reconstructed the trophic history of nine species of Sciaenidae fishes by sequentially dissolving otolith layer by layer and analysing the δ13C and δ15N values of extracted organic matter in each layer. Moreover, we used 3D scanning to monitor volume and shape changes after each step of otolith dissolution, and based on the morphological analysis, we estimated the corresponding age of every isotope signal by comparing the growth ring width in another paired otolith. The results showed inter- and intraspecific trophic level variations among nine Sciaenidae fish species. This study demonstrates a well-established approach using stable isotope analysis on otolith organic matter that provides a more comprehensive understanding of an individual''s dietary history, with potential applications at the population level.