耳石的微細構造及微化學在美洲鰻及歐洲鰻的初期生活史上之應用研究

Abstract

美洲鰻(Anguilla rostrata)與歐洲鰻(Anguilla anguilla)在大西洋藻海(Sargasso Sea)附近產卵，孵化後的柳葉魚經由洋流輸送，分抵大西洋的兩岸。至今其柳葉魚在海上漂遊的時間及種間分離洄游機制尚未明瞭。本研究分別採集北美東岸及海地六個河口的美洲鰻鰻線，以及歐洲沿岸五個河口之歐洲鰻鰻線。以電子顯微鏡及電子微探儀分析鰻線耳石的微細構造及微量元素，來研判變態輪位置及計算日週輪數，藉此瞭解兩種鰻的柳葉魚的變態日齡以及鰻線抵岸時的日齡。並比較成長率的種間差異。同時由日齡回推其孵化日期。 結果發現，離核心大約90-100μm的地方有一明顯的變態輪。變態輪附近，耳石的結晶形態由同心圓轉變成放射狀，且輪寬急速變寬，Sr/Ca比值由1.4-2.0×10-2降至1.0×10-2的低水準。美洲鰻的變態日齡及抵岸日齡分別為189-214天及220-284天，歐洲鰻為319-397天及420-468天。美洲鰻的體成長率為0.213mm/d，而歐洲鰻只有0.149mm/d。很顯然，美洲鰻抵達美洲沿岸時，歐洲鰻還在海上漂游的柳葉魚階段。其分離洄游機制可能與兩種鰻的變態日齡及成長率的差異有關。另外，美洲鰻鰻線抵岸時的體長有明顯的南短北長的地理傾斜現象，其原因可能是變態之後在沿岸停留天數不同所造成的。美洲鰻的孵化日期約在八至九月，歐洲鰻約在十二至一月，但是中美洲海地的美洲鰻及南歐葡萄牙的歐洲鰻孵化日期都與其他採樣點不同，表示這兩種鰻魚似乎都非單一產卵群。

American eel (Anguilla rostrata) and European eel (Anguilla anguilla), spawn in the Sargasso Sea. Their larvae, leptochphali, are transported by Gulf Stream, metamorphose to glass eel and migrate towards eastern and western coasts of the Atlantic. However, the larval duration and the migration mechanism have not been well understood. This study collected American eel elvers from six estuaries in the eastern coast of North America and Haiti, and European eel elvers from five estuaries in European. The daily ages of the leptocephali at metamorphosis and of the elvers at arrival to the estuaries were determined by the analyses of otolith microstructure and microchemistry. The growth rates and birth dates were also calculated. A metamorphosis check was found on the otoliths approximately 90-100.tm from primordium where microstructure pattern showed a drastic change, increment widths became wide, and Sr/Ca ratios decreased from 1.4-2.0x10-2 to a low level of 1.0x10-2 Daily ages at metamorphosis and at arrival to the coasts were, respectively, 319-397 and 420-468 days after hatching for European eel, which were much longer than 189-214 and 220-284 days for American eel. Also, the growth rate of elvers was 0.213 mm/d for American eel, which was much faster than 0.149 mm/d for European eel. The difference in the growth rate might be the principal cause of the separating in migration of these two species of eels. The geographic decline in total length of the American eel elvers was related to the duration of glass eels in coastal waters and estuaries after metamorphosis. The hatching dates was estimated to be August to September for American eel and December to January for European eel. However, American eel elvers from Haiti and European eel elvers from Portugal had different hatching date from those mentioned above. This indicating that these eels may belong to different stocks and perhaps have different spawning grounds.