臺灣南部海域藍綠光鰓雀鯛(Chromis viridis)生殖時的能量分配

Abstract

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.