盤古蟾蜍蝌蚪在不同水流狀態下的表型可塑性之探討

Abstract

許多物種能因應棲地的環境條件來調整表型特徵，此能力稱為表型可塑性（phenotypic plasticity），可以提升個體在該棲地中的適存度。有些兩棲類的蝌蚪能夠生活在不同的水流環境之中，但針對水流這項環境因子的表型可塑性研究卻極度缺乏。本研究結合形態、生理、行為與生活史等面向，以實驗檢測盤古蟾蜍蝌蚪在靜止與流動水域環境下的表型可塑性。我們將野外帶回的Gosner Stage 27 期（G27）蝌蚪隨機分成兩群，分別飼養於人工模擬的靜止與流動水域之中，待蝌蚪發育至G35時測量各項形態形值；隨後進行游泳耐力實驗和尾部肌肉的紅肌層數分析；最後測量變態（G42）時的體重。結果顯示：飼養在流動水域下的蝌蚪會擁有相對較小的體寬和體高，但具較長尾長和較寬尾肌，且尾肌的紅肌層數也較多，游泳耐力表現也比較好，但蝌蚪期較長且會以較輕的體重變態。此外，耐力游泳表現和相對體長（＋）、體寬（－）、體高（－）、尾長（＋）和尾肌寬（＋）組成之PCA軸（PC1）呈顯著正相關，顯示擁有相對流線型的身體、較長的尾長及較寬尾肌的個體，在耐力游泳上會有較好的表現。因此，生活在流動水域下的盤古蟾蜍蝌蚪可藉由表型可塑性來降低水中阻力，並提升尾部肌耐力以回應水流環境中持續游泳的需求。此一可塑性或許有提高覓食效率與避敵能力等優點，卻也必須因此付出代價：亦即較慢變態且變態時的體重較輕。另外，本研究也發現來自池塘與溪流的蝌蚪在尾長和肌肉層數上，對於水流的反應不同，這暗示著不同棲地類型之間可能有族群分化，但這需要從卵開始實驗研究才能給予有力的支持。

Phenotypic plasticity allows individuals respond to environmental challenges timely, thus generally promotes fitness. Tadpoles of some anuran species inhabit aquatic habitats with different flow regimes. However, the effects of flow regime on tadpoles are unclear due to the dearth of studies. In this study, I investigated the phenotypic plasticity of morphological, physiological, behavioral and life history traits of Bufo bankorensis tadpoles under different flow regimes. In the laboratory, I randomly assigned tadpoles at G27 stage to two treatments: static vs. flowing water. The results showed that tadpoles reared in flowing water had relative smaller body width and body height, but the values of relative tail length, tail muscle width and red muscle layers were larger than those reared in static water. Tadpoles living in flowing water also had better sustained swimming performance than those from static water groups. Furthermore, the regression analysis demonstrated a significant positive relationship between sustained swimming performance and PC1 composed by relative body length(＋), body width (－), body height (－), tail length (＋) and tail muscle width (＋). It showed that individuals with a relatively narrow and shallow body, and had a long tail with thick tail muscle performed better in sustained swimming. Although tadpoles with such characteristics may gain benefits in foraging efficiency and predator avoidance during larvae stage, they pay the costs of entering metamorphosis late and at a small size. Moreover, I also found that the responses of tail length and muscle layers to treatments were distinct between pond and stream populations, which suggested population divergence between habitats. Further research is needed to provide sound evidence of population differentiation.