Water & Salt Balance in Aquatic Animals

Account for asks you to state the reasons for something. It is close to explain, but it points at a specific fact that seems odd and asks what makes it necessary.

5 marks. State the principle once, apply it to each environment to get the direction of water movement, then derive the drinking behaviour from that, then note the cost.

Water moves across a partially permeable membrane by osmosis, from where water concentration is higher to where it is lower, which is the same as saying from the more dilute solution to the more concentrated one. A fish's gills present a very large, thin, permeable surface to the surrounding water, which is necessary for gas exchange but also makes the fish unavoidably exposed to osmosis.

A freshwater fish has body fluids more concentrated than the surrounding water, so it is hypertonic to its environment. Water therefore moves continuously into the fish by osmosis across the gills, and salts tend to be lost outward down their own concentration gradient. The fish's problem is too much water and too little salt.

A marine bony fish has body fluids less concentrated than sea water, so it is hypotonic to its environment. Water therefore moves continuously out of the fish by osmosis across the gills, and salts tend to move inward. The fish's problem is the reverse: too little water and too much salt. Despite being surrounded by water, it is in constant danger of dehydration.

The behaviours follow directly. The marine fish must drink seawater continuously because that is its only route to replace the water osmosis is removing; it then uses active transport in specialised chloride cells in the gills to pump the excess salt back out against the concentration gradient, and produces only a small volume of concentrated urine so that little further water is lost. The freshwater fish must not drink, because water is already flooding in; instead it produces a large volume of very dilute urine to remove the excess, and actively takes up salts from the water across the gills to replace what is lost.

Both solutions rely on active transport, which requires energy from respiration, so osmoregulation carries a continuous metabolic cost in both environments. This is also why most bony fish are restricted to one environment: the two problems are opposites and require opposite responses. Species such as barramundi and salmon that move between fresh and salt water must reverse their gill transport direction and their urine output, which takes time and is why such a move is a physiological transition rather than an instant one.

Explain requires the mechanism. Here the interesting part is that these animals solve the same problem as a marine fish with a completely different organ, so the comparison is worth making explicit.

5 marks. State the problem, say why the kidney alone cannot solve it, describe the salt gland and how it works, then place it as a physiological adaptation.

A marine reptile such as a sea turtle or sea snake, and a seabird such as an albatross or petrel, takes in large amounts of sodium chloride with its prey and with any sea water swallowed while feeding. Its body fluids are far less concentrated than sea water, so this salt load must be removed, and it must be removed without losing more water than the animal can spare in an environment where fresh water is unavailable.

A reptile or bird kidney cannot produce urine more concentrated than sea water, so excreting the salt load in urine would require more water than the salt arrived in. Using the kidney alone, the animal would lose water faster than it gained it and would dehydrate. The kidney can handle a moderate salt load but not a marine one.

These animals have salt glands, specialised excretory glands located near the eye in marine turtles and above the eye in the nasal region in seabirds. The gland contains many narrow secretory tubules richly supplied with blood, whose cells use active transport, driven by energy from respiration and by sodium-potassium pumps, to move sodium and chloride ions from the blood into the tubule against their concentration gradient. The result is a fluid far more concentrated in salt than sea water, which is excreted from the eye as tears in turtles and dripped or sneezed from the nostrils in seabirds.

Because the secreted fluid is more concentrated than sea water, the animal removes the salt in less water than the salt arrived in, so it comes out ahead on water balance. This allows a marine turtle or seabird to obtain water from its prey and from sea water and still maintain body fluid concentration, without ever drinking fresh water.

The salt gland is a physiological adaptation, because it is an internal functional process rather than a body shape or a behaviour, though the gland itself is a structure. It solves the same problem as the chloride cells in a marine bony fish's gills and by the same underlying means, active transport of ions against a gradient at an energy cost, but in a different organ. Two distantly related groups arriving at functionally equivalent solutions under the same selective pressure is an example of convergent evolution at the physiological level.