?郭魚仔魚在海水適應過程中氯離子之調控

Abstract

硬骨魚類在胚胎與仔魚初期，鰓、腎等滲透壓與離子調節器官雖尚未發育分化完備，但仍然能維持體內滲透壓與離子的恆定。相較於在成魚的研究，仔稚魚在面臨鹽度的改變時，鹽份的平衡調節機制之瞭解仍有不明之處。本研究以廣鹽性莫三比克吳郭魚（Oreochromis mossambicus）仔魚為研究材料，探討仔魚在發育過程中，面對環境鹽度變化時，氯離子的調節策略。實驗分成淡、海水長期適應與海水轉移兩部分，分別探討仔魚發育過程及海水轉移時，其體組織氯離子含量與流速（流進及流出速率）及Na+, K＋-ATPase活性變化情形。 結果顯示，淡、海水長期適應仔魚由孵化第一天到第九天，體組織氯離子含量均無顯著增加，而且適應在兩種環境中仔魚的氯離子含量，在發育過程中並無顯著差異。在氯離子的流速方面，海水適應仔魚隨發育過程逐漸增加；至於淡水適應仔魚則到發育後期才顯著增加。在發育過程，海水適應仔魚的氯離子的流進及流出速率均遠比淡水適應仔魚高。此外，海水適應仔魚在鰓及卵黃囊膜上Na+, K+-ATPase活性皆比淡水適應仔魚高。此可能是因為海水仔魚需要大量主動排除體內氯離子，及平衡海水中大量氯離子的被動流進所致。在海水的轉移實驗部分，將孵化第三天淡水仔魚急遽轉移到20?海水中，仔魚體組織氯離子含量會迅速上升，在轉移後4-8小時即達到最高，而後逐漸下降，在轉移後24小時已與淡水控制組相仿。仔魚氯離子的流進及流出速率，同樣在轉移後4小時急遽上升，在轉移後16小時達到最高，隨即逐漸降低，在轉移後24小時時與海水控制組相近。以上結果顯示，仔魚在面臨鹽度改變時，可藉由同時調節氯離子之流進及流出速率，來調節仔魚體組織氯離子含量恆定。然而仔魚鰓及卵黃囊膜上Na+, K+-ATPase活性卻在轉移後24小時才顯著高於對造組。此顯示在Na+, K+-ATPase的活性尚未增加之前，仔魚便已有排鹽的情形，顯然在轉移早期，仔魚的排鹽機制並不是藉由增加 Na+, K+-ATPase的活性，而可能透過其他機制完成。

Embryos and larvae of several teleosts, even though their gills or kidneys are not well developed and functioning, are capable of maintaining the constancy of ions and osmolarity in their body fluids. However, little is known about how developing larvae regulate their salt balance upon salinity change. In this study, chloride regulation in developing tilapia larvae was investigated. Tilapia larvae were acclimated under either freshwater (FW) and seawater (SW) condition or were directly transferred from FW to SW. Whole-body chloride content, Cl- fluxes (influx and efflux) and Na+, K+-ATPase activity were monitored. Results show that there is no significantly increase in whole-body chloride contents in FW- or SW-acclimated larvae during 1-9 days after hatching, and the values in the FW- and SW-acclimated larvae were not significant different at respective developmental stages. Cl- influx and efflux, as well as the Na+, K+-ATPase activity of gill and yolk-sac epithelia in SW-acclimated larvae were higher than those in FW-acclimated ones. This suggested that the SW-acclimated larvae have much more active driving force to excrete chloride and maintain ionic balance in body. After transfer from FW to 20? SW, whole-body chloride content of larvae increased rapidly and reached a peak at 4-8 h after the transfer, then declined to near the level of FW-acclimated larvae (control). Both Cl- influx and efflux increased rapidly at 4 h after transfer and reached a peak at 16 h, then declined to near the level of FW-acclimated larvae (control). All the results suggest that developing tilapia larvae can regulate the whole-body chloride content upon SW challenge by modulating the both Cl- influx and efflux. A significant increase in gill and yolk-sac epithelia Na+, K+-ATPase activity did not occur until 24h after transfer, suggesting that some other mechanisms other than increasing the Na+, K+-ATPase activity may be involved in the early phase of SW acclimation in developing larvae.