As with MasterChef, fish also like just the right amount of salt
New research sheds light on how fish cope with changes in salinity and the potential advantages that estuaries offer when moving from oceans to freshwater and vice-versa. It has long been suspected that such a move would require a whole lot of energy and be potentially stressful for fish making these journeys. But is this always the case? Researchers show how a widely distributed southern hemisphere fish (Inanga, Galaxias maculatus) can cope well with a wide range of salinities and that moving back to less salty estuarine waters may actually have subtle benefits for the species.
Like most organisms, osmotic regulation is very important to fish. Having an appropriate concentration of ions in the body ensures optimal cell functioning. To maintain their desired ion concentration fish have to spend energy to pump ions into the body (fresh water species) or out of the body (salt water species). This energy demand becomes larger when the gap between body and environmental concentration widens. Furthermore, as energy expenditures increase when these diametrically opposing challenges alternate (when moving between fresh and seawater and vice versa), most fish prefer stable environments, living either exclusively in fresh or salt water .
But what happens when fish are confronted with a wide range of salinities over the course of a day or their life stages? Estuaries vary greatly in salinity over the course of a single day but despite this challenge for its inhabitants are key habitats for aquatic life. Some fish life cycles ensure exposure to different salinities. A small group of fish is amphidromous - meaning that they retain the ability to freely move between waters of different salinities. The inanga is one of these amphidromous fish, born in estuaries, they drift off into the ocean during their larval stage, to then return to fresh water conditions as adults to spawn the next generation. Yet, it also has another interesting feature: the inanga is a scaleless fish. The loss of scales could enable to fish to use its skin for osmotic regulation and waste product disposal. These characteristics attracted the interests of Dr Mauricio Urbina and his colleague Dr Chris Glover. They aimed to understand how fish like inanga cope with changing salinity by measuring its effect on metabolism and waste excretion, ion and water balance, and untangling the roles of the skin and gills during salinity acclimation.
“We hypothesized that as inanga had no scales, it could use their skin as an epithelia for one process (i.e ion exchange) and the gills for the other (i.e gas exchange), and so avoiding potentially conflicting processes being carry out by one single tissue” Dr. Urbina said.
This would not only explain why amphidromous fish are the champions in tolerating changes in salinity, but also potentially meaning considerable energy savings as they moved between waters of different salinities.
They found that inanga’s skin played a significant role in oxygen and ammonia exchange processes, however most exchanges occur at the gill – as is the case in other fish species. Dr Urbina said “it was a surprise to not find a change in the role of skin as we acclimated them to different salinities, as our previous research have suggested”. Data showed that inanga could successfully acclimate to the different salinities, proving that it is an excellent osmoregulator. Determination of metabolic rate showed no changes at salinities ranging from freshwater to saltier than normal seawater. This indicates thriving at different salinities does not necessarily incur stress, nor extra expenditures for this fish. Interestingly at intermediate salinities inanga appears to fuel most of its energy requirements from lipids and moves away from protein breakdown for energy. Dr. Urbina speculates that this bioenergetic adaptation might be common for many species living in estuaries. “This study is the first to provide evidence to suggest that estuaries are not only used by fish as a refuge from predators and as a feeding area, but may also provide advantages for the energy budget or the species that live there” Dr Urbina said.
The results of this study also shows that keeping fish like inanga in intermediate salty water could be beneficial to growth rates, a discovery that is of importance to aquaculture of inanga as well as other estuarine fish species.
Date: 7 September 2015