仔稚魚的水及離子之平衡

Abstract

本文擬以狹鹽性淡水金魚(Carassius auratus)、廣鹽性吳郭魚(Orechromius mossambicus)及迴遊性的鰻魚(Anguilla japonicus)的仔稚魚為材料，進行轉移(transfer)的實驗來探討仔稚魚維持體內滲透壓及組成分之?定。由觀察轉移適應的情形，與測定水含量，體內鈉、鉀及鈣離子和滲透壓的變化，以瞭解在不同鹽度的環境下，仔稚魚體與環境間之水和離子的平衡。 孵化後第4天的金魚放入14?的海水中，1天後，大量死亡。體內水含量變化並不顯著，但滲透壓及鈉、鉀和鈣離子含量變化不斷上升，無法維持體內離子和滲透壓平衡而導致死亡。但轉移到7.5?之海水中，存活率與淡水對照組一樣，其變化皆較14?平緩，其滲透壓只略為增加之後維持平穩，且具有狹鹽性淡水成魚維持休內滲透壓較環境為高之高張特性。 淡水孵化後第3天的吳郭魚仔魚直接轉移到32%。海水中，1天時仔魚死亡率達到85%以上，魚體嚴重脫水，鈉、鉀及鈣各離子和滲透壓急速增加，且缺乏調節能力。但在略低的25?的海水中，和淡水對照組一樣的沒有顯著死亡，其滲透壓初期上升而後降回接近於淡水對照組的基準，體內水含量及離子變化不大，且此際仔魚已具有成魚適應海水時調節體內滲透壓較外界環境為低之低張特性。而海水孵化的仔魚在轉至淡水時，沒有明顯死亡，鈉、鉀及鈣三種離子略為下降，體內水含量維持在體重之62.0-64.0%之間，滲透壓由初期的下降到轉移第二天後調節至海水對照組的值。顯示海水仔魚可直接適應淡水。 採自沿海(30?)之鰻線，在轉移淡水中，其水含量急速增加而鈉、鉀及鈣離子急劇下降而後皆趨於平穩，且3小時內達到河內淡水值之基準。在河口區(15?)的稚魚直接放入淡水中，含水量和離子變化小且較海水組更快趨於穩定。顯示迴遊性鰻線已具有適應淡水的能力。

By tranferring to different environments, stenohaline goldfish larvae (Carassius auratus), euryhaline tilapia larvae (Oreochromis mossambicus), and migrating glass eels (Anguilla japonicus) were examined to study the larvae’s maintenance of internal osmolality and ion compositions. To understand the relationship of water and ion balance, we investigated the changes in the body water content, sodium, potassium and calcium ion concentrations, and tissue osmolality under various environmental conditions. When goldfish larvae were transferred to 14 ppt seawater, most of them died within one day and showed a continuous increase in ion contents and tissue osmolality. It suggested that stenohaline larvae could not survive in this hyperosmotic environment, because they were not able to regulate the ion concentrations of their body fluids. In contrast, they survived as in the control group, when they were transferred into a 7.5 ppt seawater, and they showed a slight increase in the osmolality and ionic concentrations. The larvae appeared to show a hyperosmotic regulation as did in the adults. When freshwater-hatched tilapia larvae were transferred directly into 32 ppt seawater, the water content lost remarkably in 6 hours after transfer, and more than 85 % larvae died in one day after transfer. On the other hand, no significant mortality was observed as compared to the control group, when they were transferred into 25 ppt seawater. Osmolality initially increased, followed by a decline to the control level. It suggested that the larvae had the ability of hyporegulation similar to that of adult fish. However, when seawater-hatched tilapia larvae were transferred to freshwater, the body sodium, potassium and calcium ion concentrations decreased slightly. Their body water content remained rather stable around 62-64 %, and the osmolality decreased first, and then returned to the control level. It suggested that seawater-hatched tilapia larvae could acclimate directly to freshwater. When seawater glass eels were transferred directly to freshwater conditions, their body water content increased and sodium, potassium and calcium ion concentrations declined during the first 3 hours, and then reached a steady state. In contrast, the body water content and ion concentrations changed less and reached a homeostasis state faster than those of seawater group, when glass eels collected from estuarine waters were transferred directly freshwater. It is therefore concluded that migrating glass eels were shown to be capable of freshwater adaptation.