台灣南部珊瑚礁海域兩種固曲齒雀鯛食性適應性之研究 - 利用胃內含物和穩定同位素分析

Abstract

珊瑚礁魚類的食性一般具有適應性。在一個變動的環境中，攝食適應性強的魚種因為較能適應食物源的改變，所以族群受變動的影響較少，同時這些魚類也具有較大的機會入主新生的棲地。墾丁南灣核三廠入水口海域，原本以軸孔珊瑚為主要優勢種，但是近年來海葵大量的蔓生，改變魚類的棲息環境，連帶著兩種固曲齒雀鯛：迪克氏雀鯛 (Plectroglyphidodon dickii Lienard 1983) 和約島雀鯛 (P. johnstonianus Fowler and Ball 1924) 的分布有所改變。本論文將入水口海域環境分為兩個樣區：以軸孔珊瑚為主的A區以及以海葵為優勢的B區；並針對棲息在此的這兩種雀鯛，利用胃內含物分析和穩定同位素分析兩種方法，研究它們的食性變化，並探討這兩種雀鯛的食性在這不同環境中的適應能力。 胃內含物及穩定同位素分析顯示，棲息在A、B兩區的約島雀鯛，皆以珊瑚蟲為主要食物來源。而迪克氏雀鯛的胃內含物中顯示，其在A區的主要食物來源為藻類 (33.4%) 和珊瑚蟲 (22.3%)，在B區則為藻類 (35.8%) 和海葵 (28.2%)；ANCOVA分析顯示，樣區間迪克氏雀鯛在同位素δ15N值上沒有差異 (F=0.36, df=1和26, P=0.56)，而在同位素δ13C值上有差異 (F=162.07, df=1和26, P<0.01)，表示在兩樣區中的個體有不同的有機碳來源，而此差異可能為樣區間食物來源差異所造成。此外，雖然藻類是迪克氏雀鯛胃內含物中的主要項目，但其與迪克氏雀鯛無論是在同位素δ13C或δ15N值之間的差異均超過一個食階 (δ13C值平均1∼2‰差異；δ15N值平均3.4‰差異)，顯示藻類並非其有機碳或有機氮的主要來源；相對的，除了海葵和珊瑚蟲，多毛類和星蟲可能也是迪克氏雀鯛的重要有機碳和有機氮來源。上述的結果顯示，約島雀鯛食性不受不同樣區環境的影響，皆以珊瑚蟲為主要攝食對象；迪克氏雀鯛在以海葵為主的樣區中，攝食海葵的比例增加，亦即具有較高的食性適應性，此一特質可以解釋最近迪克氏雀鯛在一新生環境中個體大量增加的現象。

The feeding habit of a coral reef fish is generally adaptable. While the environment is ever changing, the fish with a higher feeding plasticity would be less affected by changes of available foods. It is also more capable of immigrating into a newly developed habitat with suitable food substitutions. The substrate in the small embayment, the intake bay of the third nuclear power plant at Kenting, was previously dominated by Acropora corals. The recent outbreak of sea-anemones has eliminated branching corals in some areas. It also occurred that more Plectroglyphidodon dickii had immigrated into these sea-anemone dominated habitats than its cogener P. johnstonianus. In this study, the reef area in the embayment was divided into two different zones, zone A dominated by branching corals and zone B, by sea-anemone. In order to study whether the distribution pattern was underlain by the food availability and feeding habit plasticity, both stomach content and stable isotope analyses were used to delineate feeding habits of these two damselfishes at two different zones. Both analyses showed that coral polyps were the major food source for P. johnstonianus from both zones (70.0%). The major food sources for P. dickii from zone A were algae (33.4%) and coral polyps (22.3%) while in zone B it mainly took algae (35.8%) and sea anemones (28.2%). ANCOVA analysis showed that, for P. dickii, difference between sample sites was significant in δ13C values, but not in δ15N values, indicating that P. dickii might have accessed different sources of organic carbon. This is consistent with the finding from stomach content analysis that the fish had taken different foods in the two zones. Meanwhile, differences in both δ13C and δ15N values between P. dickii and algae are higher than per-trophic level increase, indicating that algae might not be as important as the stomach content showed. Instead, in addition to coral polyps and sea-anemones, polychaete and sipuncula are possibly its major food source. Overall, P. dickii used sea-anemones to substitute coral polyps in the diet in the sea-anemone dominated habitat, thus showing a higher feeding plasticity than P. johnstonianus. This may help explain the recent dramatic occurrence of P. dickii in the newly developed habitat.