日本鰻耳石的錳元素上升原因之探討

The dramatic elevation of manganese concentrations in the otolith of Japanese eel Anguilla japonica

江俊億(Chun-I Chiang)

指導教授 ： 曾萬年

共同指導教授 ： 韓玉山(Yu-San Han)

國立臺灣大學/生命科學院/漁業科學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.00679

摘要

耳石微量元素已經廣泛地被用來當作魚類環境生活史及族群判別研究之工具。而在利用耳石微量元素進行研究之前，釐清耳石微量元素變化與外在環境、耳石結晶型態以及魚類生理之關係是非常重要的工作。錳元素(Mn)屬於耳石微量元素之一，近年的研究陸續發現魚類耳石的核心中錳元素濃度有不尋常的上升現象，但上升原因及其調控因子至今仍不甚了解。為了暸解魚類耳石中錳元素變化的時機、原因及其生物意義，我們利用雷射感應偶合電漿質譜儀(LA-ICPMS)測量了48尾來自高屏溪及大鵬灣的野生日本鰻的耳石微量元素之時序列變化，並在掃描式電子顯微鏡(SEM)下觀察其耳石的微細構造，然後以拉曼光譜儀測量耳石的結晶型態。此外，以日本鰻鰻線為對象，進行為期六個月的高錳酸鉀室內控制實驗，以期了解環境中錳元素濃度變化與耳石錳元素之關係。
　　結果發現，鰻魚耳石的Mn/Ca比可區分成三種型態：(1)初期發育階段上升型：亦即耳石核心周圍(約距核心300μm以內)之平均Mn/Ca比顯著高於黃鰻階段 (p<0.001-0.05)，佔82.15%；(2)黃鰻階段上升型：耳石在黃鰻階段之平均Mn/Ca比顯著較高(p<0.05)，佔8.33%；(3)無上升型：核心至邊緣之Mn/Ca比皆無明顯的差異，佔10.42%。這些結果顯示，大部分日本鰻的耳石在初期發育階段皆有不尋常之Mn/Ca比升高現象，暗示日本鰻在初期發育階段會有大量吸收錳元素的現象。利用耳石Sr/Ca比與Ba/Ca比將所測定的日本鰻在黃鰻階段之棲地利用方式分為三種類型(海水型、淡水型及半淡鹹水型)，結果發現黃鰻階段的耳石Mn/Ca比在不同棲地利用類型間並無顯著差異。控制實驗結果也顯示，鰻線階段之後的耳石內錳元素濃度在長期添加1ppm的高錳酸鉀與控制組之間並無明顯差異。這些結果進一步證明耳石內的錳元素不易受到外在環境鹽度及錳元素濃度所影響。此外，拉曼光譜的測定結果也顯示，無論是哪一種Mn/Ca比的時序列變化型，耳石的核心周圍或邊緣之結晶型態都是屬於霰石結晶(aragonite)，表示錳元素濃度的升高現象並不是因為結晶型態的不同所造成的。我們將耳石Mn/Ca比的時序列變化與耳石微細構造及鍶鈣比(Sr/Ca)的時序列變化對照之後，發現初期發育階段上升型的個體，其耳石核心周圍的Mn/Ca比會隨著Sr/Ca比急遽下降而上升，Mn/Ca比最大值出現在Sr:Ca比降至低於4 ‰之後，亦即鰻線進入河口時。這顯示耳石錳元素在鰻魚初期發育階段的上升現象與柳葉魚變態為玻璃鰻之後至鰻線進入河口的過程有關，這段期間鰻魚可能有特殊的生理需求而從外界大量攝入錳元素。此現象同時也在美洲鰻與歐洲鰻的耳石中發現。因此錳元素的上升應為鰻魚的共同生理現象。綜合以上結果，鰻魚初期發育階段耳石內的錳元素上升現象可能是生理需求而非環境中的錳元素濃度或是耳石結晶型態之影響。

關鍵字

日本鰻 ； 耳石 ； 錳鈣比 ； ICP-MS ； 魚類生理與生態

並列摘要

Trace elements in otolith have been widely applied to reconstruct migratory environmental history of the fish and to discriminate the population structure as well. However, the mechanism of deposition of trace elements in the otolith is still not completely understood. The deposition is a complex biogeochemical process was influenced by the environmental factors, the physiology and ontogeny of the fish, as well as the crystal structure of otolith. It is important to understand the mechanism of the deposition of elements in otolith before otolith microchemistry is used for ecological implication. Manganese (Mn) is one of the 31 important trace elements in the fish otolith. The phenomenon of elevated Mn concentration in the otolith core has been found in many fishes, but the factors affecting the concentrations of Mn in otolith are not well understood. The elemental composition in otoliths of 48 wild yellow-stage Japanese eels was examined by laser ablation-inductively coupled plasma mass spectrometry to understand the change of manganese (Mn) concentration in otolith in relation to the life history events of the eel. The otolith microstructure and crystal structure of CaCO3 were also examined by scanning electron microscopy and Raman spectroscopy. In addition, to understand the effects of ambient Mn concentrations on the otolith Mn concentration, the elvers of Japanese eel were reared for 6 months in the laboratory.
Temporal change of Mn/Ca ratios in the otoliths of wild eels can be classified into 3 types: 1) the Mn/Ca ratios in otolith elevated at early life stage of the eel (81.25 %), the mean Mn/Ca ratios at core region of the otolith in the early stage were significantly greater than those at yellow-stage of the eel (p <0.001-0.05, respectively). 2) Mn/Ca ratios elevated in the yellow eel stage but not in the early stage (8.33%), the mean Mn/Ca ratios at the yellow-stage were significantly greater than that the early stage ( p <0.05). 3) no significant elevation (10.42%). The elevated Mn/Ca ratios in the otolith core region were found in most of the eels examined, suggesting that in the Mn was abundantly uptaken from ambient water at early stage of the eel. The Mn/Ca ratios were not significantly different among 3 types of habitat use of the eel which was classified by Sr/Ca and Ba/Ca ratios in otolith of the eel at yellow stage. In addition, Mn concentration in otolith of reared eels were not significantly different between treatments of KMnO4. This further indicated that the elevated Mn concentration in otolith was not related to the salinity and the ambient Mn concentration. The Raman effect also indicated that the elevated Mn concentration in the otolith was not due to different crystalline structures of CaCO3 because all of the otolith were aragonite, not vaterite. The peak Mn/Ca ratios occurred as Sr/Ca ratios drastically decreased to less than 4 ‰ while arriving at the estuary as elver. The peak Mn/Ca ratios in the otolith of elver stage may reflect its physiological and requirement in the upstream migration. The elevated Mn concentration in the core region of the otolith of Japanese eel was also found in the other temperate eels, A. anguilla and A. rostrata, which seems a common phenomenon of the angullid eel. In conclusion, the elevated Mn concentrations in otolith of the eel at early life stage might be due to the effect of physiology rather than environmental effect or crystalline structure of otolith.

並列關鍵字

Japanese eel ； otolith ； Mn/Ca ratios ； ICP-MS ； fish physiology and ecology

參考文獻

Adams LA (1940) Some characteristic otoliths of American Ostariophysi. J Morphol 66: 497-527.

Arai N, Sakamoto W, Maeda K (1995) Analysis of trace elements in otoliths of red sea bream Pagrus major. Fish Sci 61: 43-47.

Arai N, Sakamoto W, Maeda K (1996) Correlation between ambient seawater temperature and strontium-calcium concentration ratios in otolith of red sea bream Pagrus major. Fish Sci 62: 652-653.

Arai T, Otake T, Tsukamoto K (1997) Drastic changes in otolith microstructure and microchemistry accompanying the onset of metamorphosis in the Japanese eel Anguilla japonica. Mar Ecol Prog Ser 161: 17-22.

Arai T, Limbong D, Otake T, Tsukamoto K (1999) Metamorphosis and inshore migration of tropical eels Anguilla spp. In the Indo-Pacific. Mar Ecol Prog Ser 182: 283-293.