從種內差異探討珊瑚食性策略

大多數石珊瑚為兼養性(mixotrophy)，可利用異營行為來補償光合作用所需的碳源。然而這項從自營轉換成異營的能力似乎存在種別特異性(species-specific)，並被認為在天擇中可讓物種更具有競爭力。為了能深入了解珊瑚營養策略，我們測量珊瑚細胞與其共生藻的碳、氮穩定性同位素。希望能藉由此同位素值來量化不同種珊瑚的營養棲位 (trophic niche)，且能更進一步的探討造成種內變異性(intraspecific variability)的原因。我們認為同一物種內會存在著不同的營養行為，用以面對外在環境的變動。根據營養行為的差異，可將物種區分成廣食性(generalist)與專一食性(specialist)。在本篇研究中，我們採集了以下五種不同種的珊瑚: Isopora palifera (IP)、 Porites lutea (PL)、 Psammocora profundacella (PP)、Stylophora pistillata (SP) 、和Tubastrea cf. coccinea (TC)，並位於兩種緯度 (北台灣、綠島) 下，結合兩種深度 (水下10米、40米) 和兩種不一樣的季節 (溫暖、寒冷) 進行採樣。為了消除環境造成的同位素基準值差異，分別得到珊瑚細胞與其共生藻的同位素值後，我們將珊瑚細胞的同位素值先經過共生藻同位素值的校正再進行統計分析。透過無監督學習演算法 (k-means clustering)，將珊瑚個體根據其同位素值分配到不同群聚中。依照同一種珊瑚被分配到的群聚多寡，我們可以將廣食性與專一食性物種區分開來。結果顯示，營養棲位大小與位置的不同，可證明不同珊瑚之間具有不同程度的營養可塑性 (trophic plasticity)。SP與TC分別具有最小及最大的的營養棲位，其餘物種則分布此範圍內。另外，除了PP和IP的碳同位素值和營養棲位不受影響外，我們發現大部分的種內變異性是受到環境影響。經由分群結果，全部珊瑚個體可以被分配到五個群聚中。PP擁有四種不同群聚，可以說明此物種在行養行為上可被歸類為廣食性者。另一方面，只有兩種群聚的SP和TC則被劃分為專一食性者。廣食性者被視為具有較高的環境耐受性；而專一食性者則是在特定的環境下才會比較具有競爭力。整體來說，本研究提供了將營養可塑性整合進珊瑚適應策略中的初步貢獻，並能讓我們更加了解珊瑚對於氣候變遷的反應。

關鍵字

珊瑚食性；同位素；種內變異性；營養可塑性；營養棲位；食性策略；適應性策略

並列摘要

Most scleractinian corals are mixotrophic and can complement photosynthetically acquired carbon with heterotrophy. However, the abilities to shift of diet during stress period seem to be species-specific and have been hypothesized to offer a competitive advantage for species to be naturally selected. Using carbon and nitrogen stable isotopes on both the coral host and its symbiotic algae, we propose here to characterize isotopic niches of coral species (i.e. a proxy of trophic niches) and scrutinize the causes of intraspecific variability in order to provide some insights on their trophic strategies. We hypothesized a species may exhibit various trophic behaviors with changes in environmental conditions, and that specialist and generalist species could be distinguished. In this study, we targeted five coral species: Isopora palifera (IP), Porites lutea (PL), Psammocora profundacella (PP), Stylophora pistillata (SP), and Tubastrea cf. coccinea (TC). Coral samples were collected from contrasted environmental conditions confounded within the effects of latitude (Northern Taiwan and Green Island), depth (10 m and 40 m), and seasons (warming and cooling). Stable isotopic compositions of host tissues and symbionts were analyzed, and then values of host tissues were adjusted using baseline values from the symbionts in order to be comparable between each species and condition. Colonies were assigned to clusters based on isotopic composition by using an unsupervised learning algorithm (k-means clustering). Difference between “generalists” and “specialists” was defined according to the number of clusters exhibited by coral species. Trophic niche size and position differed among coral species, which illustrate various degree of plasticity. SP and TC had the smallest and largest niche size respectively, while other species resided in between. Environmental conditions could explain most of the variation observed within species, except in PP and IP where the carbon sources and niche positions did not change. Five clusters were distinguished among colonies. PP performed in four clusters, which could indicate that this species tends to be more generalist in term of trophic behaviors. To contrast, SP and TC were more specialized by exhibiting only two clusters. The generalists are hypothesized to have higher environmental tolerance than specialists performing particularly well in a narrow range of environmental conditions. Overall, this study represents a preliminary step toward an integration of trophic plasticity into a definition of coral adaptive strategies, which allow us to better understand the current responses of corals to climate change.

並列關鍵字

Coral diet ； Stable isotope ； Intraspecific variability ； Trophic plasticity ； Trophic niche ； Trophic strategy ； Adaptive strategy

參考文獻

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國際替代計量

從種內差異探討珊瑚食性策略

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