臺灣南部海域藍綠光鰓雀鯛(Chromis viridis)生殖時的能量分配

Abstract

位於南灣附近的核三廠入水口(21°57'N, 120°45'E)為一小海灣，在此棲息著很大族群的藍綠光鰓雀鯛 (Chromis viridis Cuvier, 1830)。本種雀鯛主要以浮游生物為食，灣內穩定的海流為其帶來食物。這裡的藍綠光鰓雀鯛有週年生殖的現象，為了瞭解在此一環境中，魚類個體在生殖時其體內能量如何分配，乃於2011年1月至2012年2月之間，每個月到此採集藍綠光鰓雀鯛標本，分別測量雌、雄個體之白肉、肝臟及生殖腺組織的C/N值與穩定同位素δ13C、δ15N，以檢視生殖時期魚體內部組織間能量轉換的情形。 結果顯示雌魚生殖腺組織的C/N值與GSI (gonadosomatic index)間呈現正相關，在雄魚則反之。由於組織之δ13C值會受到C/N值所影響，因此做進一步比較前，先經脂質校正，並以δ13C'表示之。 雄魚生殖腺和肝臟δ13C'之間的差值(亦即生殖腺的δ13C'－肝臟的δ13C')，呈現出正負相間的現象，並且其分布與GSI間，未具相關性，顯示雄魚生殖腺中的有機碳或非直接從食物經肝臟轉移過來。雄魚生殖腺與肝臟的δ15N差值大部分為負值，其分布亦不受GSI所影響，顯示有機氮可能是由肝臟傳遞至生殖腺，但不受到生殖腺大小或成熟度所影響。 雌魚生殖腺與肝臟間同位素的δ13C'差值，當GSI≦3時，變化範圍高達3‰；當GSI＞3時，負值增加，惟變化範圍變小。造成此現象的原因可能為未發育及排卵後的個體均呈現低GSI，外觀型態及重量相似，但是內部的有機碳來源卻有所不同：未發育的個體有較重的有機碳，且尚未從肝臟轉入新的有機碳，因此其差值為正值；排卵後的個體生殖腺內仍含有未排出的卵，含較輕的有機碳，而此碳源為發育期間直接源自於肝臟，故差值為負。至於GSI＞3時負值增加，顯示食物內的有機碳會經肝臟直接輸送至生殖腺。在δ15N方面，雌性個體生殖線與肝臟的差異，以正值居多，表示雌魚的生殖腺在發育的過程中，其蛋白質並非直接源自肝臟，或者肝臟並未提供其新得自於食物的蛋白質。 藍綠光鰓雀鯛因應生殖時所需的能量最終可能多係源自於肝臟，亦即得自於所攝食的食物，惟在雌、雄之間，體內能量的分配方式有所不同，導致此種變化的機制尚未清楚。本魚種在研究地點終年皆出現生殖現象，是否在上述能量分配方式下，較能有效利用環境中的營養來源，達到生殖的目的，則仍有待進一步的探討。

A large population of the blue-green damsel (Chromis viridis Cuvier, 1830) inhabits the small embayment encompassing the water inlet of the Third Nuclear Power Plant at Nanwan, southern Taiwan (21°57'N, 120°45'E). This damselfish mainly feeds on zooplankton. Continuous water flow in the embayment brings in its food. The fish spawns yearly round in this area. To study the energy allocation for reproduction in the blue-green damsel in this environment, monthly specimen collections were made during January 2011 and February 2012, and data on the C/N ratio, stable isotopes of δ13C, δ15N in white muscle, liver and gonad were gathered from both sexes for analyses. The C/N ratio in female’s gonad was positively related with the gonadal somatic index (GSI), while the relationship was negative in the male. Due to that the δ13C tends to be affected by the C/N ratio in the tissue, calibration of the δ13C was made (denoted as δ13C') according to the corresponding C/N value for further comparisons. In the male, both positive and negative values occurred in the difference of δ13C' between gonad and liver (viz., δ13C' in gonad minuses δ13C' in liver) and its distribution seemed not affected by the GSI, showing that the organic carbon in the gonad might not have directly come from the food through the transfer of liver. Meanwhile, negative values occurred in the majority of the difference of δ15N, and the distribution again was not affected by the GSI, showing that the organic nitrogen was transferred from liver to gonad for development and the transfer was not governed by the size or the developmental stage of the gonad. In the female, the difference of δ13C' between gonad and liver showed that: for GSI≦3, both negative and positive values occurred, where they centered in zero but were distributed in the range of about 3‰; for GSI＞3, more negative values occurred and distributional range was smaller. This might be due to that: despite that both undeveloped gonad and discharged ovary bore low GSI, their organic carbons might have stemmed from different origins. The undeveloped gonad might bear organic carbons with heavier (hence positive values) δ13C' signatures. These organic carbons were not newly transferred from the liver. By contrast, the discharged gonads contained remains of the developed ovary that bore lighter (hence negative values) signatures because the organic carbons mainly originated from the liver. The latter case also explained sufficiently the situation of GSI＞3. In the difference of δ15N, positive values prevailed, indicating either that the organic nitrogen for the ovary development might not come from the liver, or that the liver did not provided the gonad with the nutrients newly obtained from the food. In the blue-green damsel, the energy demanded for reproduction might have eventually come from foods through the transfer of the liver. However, the way of energy allocation seemed to differ between sexes. Mechanisms underlying the difference are yet to know. At the study area, spawning of this damselfish was observed throughout the year. Whether the fish may enhance its reproductive gains with the above way of energy allocation remains to be studied.