大眼海鰱栁葉魚變態過程中耳石微量元素比值與外在水體元素濃度之關係

Abstract

大眼海鰱 (Megalops cyprinoides) 與鰻鱺目魚類相同，都具有特殊柳葉魚(Leptocephalus) 發育階段的生活史。研究指出，柳葉魚仔魚轉換到稚魚的變態過程(Metamorphosis)，須經歷行為、棲地轉換與身體組織重組等個體發生(Ontogenetic) 的劇烈變化；在此過程將觀察到耳石日週輪明顯變寬、耳石Sr/Ca比因GAGs降解而急速下降等現象。另外，影響耳石微化學變化的眾多環境因子中，以水體元素濃度的影響最為明顯，其次為鹽度及溫度。為了探討變態時期耳石微化學受生理效應和外在水體元素濃度的影響程度，本研究設計一室內養殖控制實驗，來比較大眼海鰱不同發育階段耳石元素比值的變化情形。 實驗用的大眼海鰱柳葉魚標本，於2005年8月到9月間，採自於淡水公司田溪，其發育階段已進入變態期 (StageⅡ) 的第一天，隨即帶回實驗室以河口半淡鹹水馴化一天後，用四環黴素 (Tetracycline) 螢光標示耳石當作實驗開始的第一天。個別添加 SrCl2 ˙ 6H2O和Ba(NO3)2 兩種藥品，將飼育水依2×、4×、8×的比例調配成含控制組的四個濃度加兩重複添加實驗，飼育螢光標示後的柳葉魚。根據耳石日週輪的變化與發育階段的關係，得知變態期大約14天，以此將耳石劃分為柳葉魚期、變態期和稚魚期三個發育階段。水樣分析在添加Sr、Ba後的倍率變化為Sr/Ca比1×、5.3×、13.4×和29.0×，Ba/Ca比1.00×、0.99×、1.15×和1.53×非等倍率的變化關係。利用EPMA及LA-ICPMS測定結果，發現變態時期耳石Sr/Ca劇烈下降的現象只出現在Sr1×，而在Sr的四個濃度下，耳石Sr/Ca比呈現變態期1.0×、2.5×、7.4×和16.0×，稚魚期1.0×、6.5×、20.0×和47.1×的倍率關係，表示Sr/Ca比從水體進入到耳石後，具有放大的效果；同樣地，耳石Ba/Ca比在Ba的四個濃度下，倍率關係則呈現變態期1.00×、1.19×、1.15×和1.16×，稚魚期1.00×、1.37×、1.60×和1.81×，相較於Sr/Ca比，Ba/Ca比的放大效果則較不明顯，但兩者皆反應了水樣中Sr/Ca、Ba/Ca比非等倍率增加的關係。耳石Sr/Ca比在兩發育階段皆隨水體Sr濃度增加而上升，並呈現高度的正相關 (r=0.99)，且斜率檢定為 p>0.05，表示不論在變態期或稚魚期，耳石元素吸收的狀況皆是環境元素濃度的影響程度遠大於生理的效應；但Ba/Ca比在變態時期與水體Ba元素濃度之間的關係卻不顯著，而稚魚期耳石則同樣地受飼育水元素濃度增加而有正相關關係 (r=0.7)；由以上結果顯示，耳石對於水中元素的吸收除具有選擇性差異外，亦會受不同發育階段關鍵性的影響。此外，魚體長、體重及存活率也受Ba元素濃度而變化較鍶實驗組來得大。 本研究重要的發現為大眼海鰱柳葉魚在變態過程中耳石元素比值與飼育水之間的關係，水體元素濃度的效應超過過去所認為很重要的變態期生理因子的調控，且其關係在不同元素間又有很大的差異。並由分配係數得知，耳石吸收鍶的效率約為鋇的20倍，鍶鋇雖同為鹼土族元素 (Alkali metals)，在控制組(1×)和稚魚期也有著相似的變化趨勢，但進入耳石內的比例卻有很大的差別。經由本研究耳石對元素利用的發現，將有助於耳石微化學組成在魚群洄游環境及生態研究上之應用。

The Pacific tarpon Megalops cyprinoides are similar with Anguilliformes which has a willow-leaf-shaped larva called leptocephalus. Previous research indicated a drastic ontogenetic change in behavior, habitat, body shape, and recombination of tissue during metamorphosing from leptocephalus to juvenile stages. A maximum otolith increment width (OIW) and sharp decreasing in otolith Sr/Ca ratios resulting from glycosaminoglycan (GAGs) broken–down were observed during metamorphosis. In addition to such physiological factor, otolith microchemistry was affected mostly by ambient concentration rather than salinity and temperature. In order to compare with the affected degree of physiological factors and water chemistry during metamorphosis, we designed a controlled experiment with adding strontium (Sr) and barium (Ba) in rearing water to discuss the relationships between otolith chemistry and life stages in Pacific tarpon. Leptocephali (entering StageⅡ) were collected in the estuary of Gong-shy-tyan Creek, a tributary of the Tanshui river in August to September 2005. After acclimating 1 day, the leptocephali were immersed in a tetracycline solution to mark the otoliths with a fluorescent band as the onset of the experiment (Day 1) and then reared in the treatments spiked with different concentrations (2×、4×、8×) of Sr and Ba. According to the changes of otolith daily growth increments (DGIs), the leptocephalus completed metamorphosis is approximately 14d during rearing conditions. The results of Sr:Ca and Ba:Ca ratios in the rearing water were measured to be 1×、5.3×、13.4×、29.0× and 1.00×、0.99×、1.15×、1.53× respectively after spiking of Sr and Ba. The Sr:Ca ratios of the otoliths were analysed with EPMA and the period of metamorphosis (M-zone) increased times in:1.0×、2.5×、7.4×、16.0× and juvenile (J-zone) in:1.0×、6.5×、20.0×、47.1×, and the phenomenon of Sr:Ca ratios rapid decreasing only presented in control treatment (Sr1×). Other Ba:Ca ratios in the otoliths were analysed with LA-ICPMS and the concentration ratios at M-zone were: 1.00×、1.19×、1.15×、1.16× and J-zone: 1.00×、1.37×、1.60×、1.81×. Although Ba:Ca ratios has less magnification effects than Sr:Ca in the otoliths, both of them reflected the relationships of non-equal times of the element spiked in rearing water. Whether in metamorphasis or juvenile stages, the Sr:Ca ratios in the otoliths were highly related to water chemistry (r=0.99) which indicated that otolith Sr:Ca ratios were mostly influenced by ambient elemental concentrations. Besides, the relationships only appeared within juvenile (r=0.70) in Ba treatments. Sr and Ba belong to alkali metals, but mean partition coefficients DMe in Sr:Ca ratios are around 20 times more than Ba:Ca ratios. In this study the great discovery is the effects of water chemistry are more important than physical factor in metamorphosis, and selective absorption rate from ambient element to otoliths is also existed especially in different life stages.