烏魚仔稚魚期耳石微細構造與初期生活史事件的關係之研究

Abstract

耳石可以提供魚類的年齡、成長、變態、棲地轉換或是洄游環境史等生活史事件之資訊。為了闡明烏魚（Mugil cephalus）耳石微細構造、微化學與其初期生活史事件之關係，本研究於西元1996年至2004年在台灣東北至西南沿岸的四個河口域進行烏魚稚魚的採樣，然後利用光學及電子顯微鏡、電子微探儀（EPMA）及雷射熔蝕-感應偶合電漿質譜儀（LA-ICPMS）分析稚魚耳石的日週輪、鍶鈣比及微量元素組成之變化。結果發現在每個稚魚的耳石大約在孵化後14天，距離耳石核心約186μm處皆有一個過渡記號（transition check, TC），這個形成時的平均日齡並無採樣地點、月份及年間的差異，但會隨著個體的成長率增快而變小。此外，在烏魚稚魚耳石的邊緣也發現一個針狀碳酸鈣的結晶之特異結構區，稱為第二成長區（secondary growth zones, SGZ），此成長區的起點為一副核心（accessory primordium, AP）。SGZ大約發生在孵化後第28天，距離耳石的核心約300μm。耳石鍶鈣比在核心處大約為11×10-3，到了第二成長區則降為3×10-3。微量元素測得的結果也發現核心區域與第二成長區之鍶鈣比有顯著性差異。與人工孵化的烏魚苗之發育階段比對結果，耳石中的TC可以對應到烏魚仔魚期至稚魚期的轉變時期，而耳石中的SGZ則是烏魚稚魚期自高鹽度之外海進入低鹽度之河口域地區所形成的記號。利用這些耳石上的特殊記號，可以回推烏魚初期生活史的成長史及洄游過程。

Fish otoliths can provide life history information such as age, growth, metamorphosis, habitat use and migratory environmental history. To elucidate the relationship between otolith microstructure and microchemistry and the life history events of the grey mullet Mugil cephalus in the early stage, the daily growth increments, growth check, Sr/Ca ratios and trace elements in otoliths of the juvenile mullets collected from the four estuaries from NE to SW Taiwan coast from 1996 to 2004 were examined by using the light and electron microscopes, electron probe microanalyzer (EPMA) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). A transition check (TC) was found in otoliths of all mullets. The check was characterized by a subsequent convergence of several daily growth increments at a distance of ca 186μm from primordium, which was deposited at ca 14d posthatching irrespective of sampling months, locations and years. In addition, secondary growth zones (SGZ) in otolith edge of the juvenile were discernable. And there is a accessory primordium(AP) in the pinnacle of the SGZ. SGZ was characterized by a needle-like calcium carbonate crystallization, which was deposited at a distance ca 300μm from primordium (ca 28d posthatching) and the timing of occurrence was the same. The Sr/Ca ratios transect revealed a decreasing trend from the otolith core (ca 11×10-3) to SGZ (ca 3×10-3). The Sr/Ca ratios are different between the primordium region and SGZ. Comparison with the developmental stage of artificially propagated larval and juvenile of the mullet, the TC in the otolith was proposed to be corresponded to the larval-juvenile transitional stage. In addition, as the grey mullet juvenile dispersed from the high saline offshore to the low saline estuarine nursery ground, a corresponding SGZ was deposited and a decreasing Sr/Ca ratios were detected in otoliths of the fish. The marks we found in the otolith provide the feasibility in studying the early life history events of the mullet.