Spotlight on: Osmoregulation and ion transport in isopod gills.
http://www.lafayette.edu/~hollidac/isopodspot.html
(Also available are "Osmoregulation and ion transport in brine shrimp" and "Osmoregulation and ion transport in crab gills.)
Isopods are classified by biologists as an order of higher crustaceans (Subclass Malacostraca; more on crustacean taxonomy) with elongated, segmented bodies. Terrestrial isopods called "pillbugs" are those most familiar to many people and you may click here to learn more about their problems of salt and water balance. However, the vast majority of isopods are marine and live in intertidal and subtidal environments in oceans and estuaries. In intertidal and estuarine environments they are often exposed to waters of varying salinity. The picture on the right is of Idotea woesnesenskii,Woesnesenskii's isopod of the U.S. and Canadian Pacific Northwest, shown approximately life size.
Although marine isopods rarely encounter waters of truly low salinity, many of them can control to some degree the salt concentration and osmotic pressure of their body fluids. These animals are called "weak osmoregulators" because the osmotic pressure of their body fluids can be kept higher than that of a moderately dilute external medium (primer on osmotic pressure). If the salinity of the external medium decreases, 
salts tend to move out of an animal by simple diffusion and water also moves in by osmosis and the body fluids quickly approach ionic and osmotic equilibrium with the external medium. This is what happens to animals which cannot pump salts into their blood when they move into dilute media and they are called "osmoconformers." Most osmoconformers cannot tolerate large reductions in the osmotic pressure of their blood and, thus, they cannot penetrate very far into the dilute waters of estuaries. However, many isopods live in nearshhore waters and in the lower reaches of estuaries where the salinity of the water changes continuously as the tides ebb and flow and as rainy seasons alternate with dry ones. These animals can control to varying degrees the salt concentration of their body fluids and are called "osmoregulators." The graph to the right illustrates the "osmotic performance" of typical osmoconforming and osmoregulating animals.
Isopods (and other animals) which can actively transport salts into their body fluids are able to keep the total ion concentration and osmotic pressure of their blood higher than that of the more dilute external medium and are called "hyperosmoregulators" ("A" on the graph above; note that sea water has an osmotic pressure of about 1000mOsm). Thus, they do not suffer from severely lowered blood salt concentration when they are in dilute waters. Because the osmotic pressure of their blood is higher than that of the medium, blood water concentration is lower and water moves into the isopod by osmosis. The isopod probably deals with this osmotic water load by increasing its urinary rate, but the urinary rates of isopods have not been measured. An increased urinary rate would cause the isopod to lose a lot of salt in its urine when it is in dilute waters. If the animal is to be able to keep its blood from becoming diluted, this lost salt must be replaced by increased inward salt pumping, probably by the gills.
When an isopod is in a medium that is the same osmotic pressure as its blood ("B" on the graph above) it is said to be isosmotic with the medium and it neither gains nor loses water by osmosis. Drinking and urination should be minimal in such a medium. Isopods which actively transport ions out of their body fluids are called "hypoosmoregulators" and their body fluids have a lower ion concentration and osmotic pressure than the medium ("C" on the graph above). This, in turn, means that they lose water to the medium by osmosis and that they must drink to replace this water loss. Such drinking increases the salt concentration of the blood and the "extra" salt must be pumped out of the animal. One isopod, Haloniscus searlei, is known to be able to pump salts out of its body fluids and it lives in hypersaline lakes in Australia (reference). Terrestrial isopods can absorb water vapor directly from damp air, probably by using their gills to make a very salty solution which "pulls" water out of the air (Discussion of this notion and references). This ability may have been a "preadaptation" which allowed the terrestrial ancestors of Haloniscus to colonize hypersaline lakes.
In order to investigate the possibility that marine isopods use their gills for inward ion transport when they are living in dilute sea water, I studied a large intertidal isopod from the U.S. Pacific coast, Idotea woesnesenskii. This animal lives in the Oregon intertidal on rocky, open coasts and is found on the stipes of macroalgae, most often species of Fucus. This isopod hyperosmoregulates well in media as dilute as 25% sea water and, when it does so, the activity of the sodium pump in its gills increases dramatically, implicating the gills in inward ion transport.
This picture shows the left fourth gill, or pleopod, of Idotea, stained with silver nitrate to show areas of high permeability to ions (protocol for this silver-staining method)
. The animal has five pairs of pleopods (3-5 mm long) and only the last three pairs show silver-staining. The medial branches of the last three pairs of pleopods, the endopods, stain most intensely (the gray structure in the picture) and they also have very high activity of the Na, K-ATPase, the enzyme which powers the cellular sodium pump (protocol for the Na, K-ATPase enzyme assay I use). The other branches, exopods, do not stain well and have low Na, K-ATPase activities. When the animal is acclimated to 50% sea water, the Na, K-ATPase activity increases only in the endopods of the last three pleopods, further implicating them as the sites of inward ion transport. A final experiment proved that the endopods of the last three pleopods are the sites of inward salt transport. Surgical removal of these appendages all but eliminated the ability of the animal to hyperosmoregulate, while removal of the exopods of the same appendages or the first two pairs of pleopods had no effect on osmoregulation.
Unlike the structurally complex gills of crabs (they have a mixture of ion-transport and non-transport tissues), the gills of Idotea and other large isopods are easily removed and studied in the lab. This should make them excellent organs for the study of crustacean ion transport.
If you would like to read a full account of ion transport and sodium pump activity in the gills of an osmoregulating intertidal isopod, the following paper may be of interest:
Holliday, C.W. (1987). Branchial Na, K-ATPase and osmoregulation in the isopod, Idotea woesnesenskii. J. Exp. Biol. 136: 259-272. Abstract.
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