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Marine vs. Fresh
by Jim McNulty

Somebody Salt Me
March 1999 newsletter - The Tropical Fish Society of R.I.

Hey Fishheads,

This month we will thoroughly explore the process known as osmoregulation. Actually the term osmoregulation is the end result of a complex process. Before this can be achieved, a large number of physiological processes must take place utilizing several key anatomical structures. The organs involved include, but are not limited to, the gill membrane, the kidney, and blood. The desired result at the end of these processes is to achieve and maintain the proper balance of salt and water in the body, avoid dehydration, and have efficient gas exchange.

The membrane of the gill of all fishes is suspended between the blood and the water. This acts as an effective barrier, selectively allowing certain substances to pass through it while denying others passage. This is known as semipermeability. Because of the differences of salt content in their respective environments, freshwater and marine fish face very different osmotic problems. To fully grasp the problem this creates, we must first understand the basic processes at work. They are diffusion and osmosis. Both play a key role in how the gill works.

Diffusion is the random distribution of substances throughout the space available to them. Molecules are in constant motion. When temperature is increased, it causes them to move even faster. These concentrated molecules seek to move from areas of higher concentrations to areas of lower concentration thus achieving equilibrium. Often the effect of a semipermeable membrane is to override the forces of diffusion. When a solution causes a differential flow through the semipermeable membrane, it is said to exert osmotic pressure. Only an equalized osmotic pressure would stop the flow of water through the membrane. The term osmosis is simply the diffusion of water. The gill membrane allows the passage of many different molecules. Sodium (Na+), chloride (Cl-), water (H2O), respiratory gases such as carbon dioxide (CO2) and others. When concentrated solutions move to lower areas in this way, no energy is expelled. This occurs via osmotic pressure and is called passive transport. Other factors in osmoregulation do require energy such as respiration, blood flow, kidney function, etc.. Even a seemingly resting fish is still expending a great deal of energy to maintain it's internal salt to water ratio. This is especially so in captive specimens due to the fluctuating salinity level of the home aquarium.

In order to simplify and properly explore the differences between fresh and saltwater fish, in regards to osmoregulation, we must address the two groups separately. Many substances are passing in and out of the gill simultaneously. As Na+ and H2O pass inward for osmoregulation, ammonium ions (NH4) and hydrogen ions (H+) pass outward. Freshwater fish face two problems: 1) getting rid of excess water and 2) maintaining proper salt content in their bodies. Their bodies need to maintain a higher level of salt than the surrounding water. As H2O passes in through their gill, Na+ is lost. To counter act this problem, freshwater fish drink constantly to maintain proper ionic levels. These ions obtained from drinking are transferred to the blood through the kidney via the "Bowman's capsule". Ions obtained through osmosis at the gill have a direct link to the blood via specialized "Chloride cells" in the gill. The efficient kidney enables the fish to excrete H2O very rapidly as a dilute urine. Na+ loss is greatly reduced by efficient reabsorption from the urine before it is excreted.

Marine fish experience just the opposite effect because their internal salt content is less than that of the surrounding water. They loose water by osmosis and gain salts by diffusion. The effect is also worsened by a more permeable membrane. Marine fish also drink large amounts of water but their kidney functions differently. They pass small amounts of very dilute urine. Between 7 to 35% of their body weight is drank each day. Most of the H2O is retained to counteract the H2O lost via osmosis at the gill. Marine fish produce a daily urine amount about 1/10 to 1/20 of freshwater fishes. Terrestrial vertebrates produce approximately 1.5% of their total body weight in urine daily. Freshwater fish produce 20% daily! As you can see, marine fish pass considerably less urine than freshwater fishes.

Yes, both groups have the same organs but they work quite differently. Evolution has helped these fish adapt to their respective environments. How does all this relate to aquarists? By understanding a little something about fish anatomy and physiology, we can better provide the proper environment the fish need to be healthy. If you have an overcrowded freshwater tank with each fish producing 20% of it's body weight daily in urine, plus feces, plus uneaten food, and plant decay, it's easy to understand the need for a 10% H2O change weekly.

How about a salt water tank? You have a tank running with fish in it. You neglect it for 3 weeks allowing 5 gallons of H2O to evaporate. Salt water has approximately 8 tablespoons of Na+ per gallon. Only freshwater evaporates leaving the Na+ behind. Now you have 40 extra tablespoons of Na+ left in your tank. This is causing great strain on the osmoregulatory process of every fish in the tank, causing them to use more energy to battle the changing osmotic pressure. This causes increased stress levels leading to infection and disease problems.

Often, aquarists look for the "quick fix" for a problem they notice. They attempt to treat the symptom as if it were the actual problem. To be successful, we must learn to read the symptoms and let them tell us what the real problem is. By maintaining proper Na+ levels and performing regular H2O changes, we go a long way in providing what our animals need to thrive for us. All books used to research and create this column are available through the library of the "Tropical Fish Society of RI".

Jim Mcnulty

References
Saltwater Aquariums by Stephen Spotta
Aquariology: The Science of Fish Health Management by John Gratzek
The Book of the Marine Aquarium by Nick Dakin