Spotlight on: Osmoregulation and ion transport in crab gills.

http://www.lafayette.edu/~hollidac/crabgillspot.html

(Also available are "Osmoregulation and ion transport in brine shrimp" and "Osmoregulation and ion transport in isopod gills".)

20K gif image Marine crabs and other marine invertebrates usually live in the relatively constant salinity of offshore waters. These animals rarely encounter low-salinity waters and most of them cannot control the salt concentration and osmotic pressure of their body fluids; they are called "osmoconformers" because the osmotic pressure of their body fluids is very close to that of the external medium (primer on osmotic pressure). If the salinity of the medium changes, salts move in or out of the crab by simple diffusion and water also moves in or out by osmosis and the body fluids quickly approach ionic and osmotic equilibrium with the external medium. 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, some crabs live in estuarine waters in which the salinity 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 on the right illustrates the "osmotic performance" of typical osmoconforming and osmoregulating crabs.

Crabs (and other animals) which can actively transport salts into their body fluids are able to keep the 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). Thus, they do not suffer from lowered blood salt concentration when they are in estuarine 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 crab by osmosis. The crab deals with this osmotic water load by increasing its urinary rate. However, the urine it makes is isoosmotic with the blood because, unlike freshwater crustaceans and vertebrates, it cannot reabsorb salts from its urine. This, in turn, means that the crab loses a lot of salt in its urine when it is in dilute waters at the head of an estuary. If the crab is to be able to keep its blood from becoming diluted, this lost salt must be replaced by increased inward salt pumping by the gills.

When a crab 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 are minimal in such a medium. 100K gif image Crabs 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 loose 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 crab. A few crabs are able to pump salts out of their body fluids, but this ability is rare in crustaceans. Brine shrimps are champion hypoosmoregulators and you may click here for more information about them.

Hyperosmoregulating crabs use their gills to actively pump salts into their blood as necessary to osmoregulate. Their gills have patches of special epithelial cells known as "ionocytes" and, for unknown reasons, the ionocytes are most densely located in the posterior gills; the anterior gills have few or no ionocytes. The picture at the right shows how the gills of a crab are organized and how the ionocytes are disposed in a typical posterior gill. The lower right frame of this picture is a diagram of the ultrastructure of a gill ionocyte (the cell is about 20 microns across in this diagram and in the picture in the next paragraph). The ionocyte has structural features typical of animal ion transport epithelia: large numbers of basal infoldings, many mitochondria to make the ATP which powers the sodium pump and "tight junctions" between cells at the apical surface. Other animals, both vertebrates and invertebrates, have similar ion transporting epithelia in such tissues as kidney, intestine, gall bladder and secretory tubes of various kinds of glands (salivary, tear and a variety of other fluid-producing glands). In crustacean gills, large numbers of apical infoldings are present just under the exoskeleton (cuticle). These apical infoldings increase in size when the crab is actively transporting ions into its blood against large concentration gradients.

The electron micrograph at the right depicts an ionocyte from the gill of the blue crab, Callinectes sapidus, and was taken by Dr. Eugene Copeland. 96K gif image If a crab is acclimated (that is, kept for 2-3 weeks) in a dilute external medium in which it can hyperosmoregulate, the ionocyte patches in its gills increase in size. The activity of the enzyme which powers the cellular sodium pump in crabs' gills and other ion transport epithelia, the Na, K-ATPase, also increases in the gills of hyperosmoregulating crabs. [Protocol for the Na, K-ATPase enzyme assay I use.] Both short-term increases in ion pump activity and long-term increases in the number of cells and ion pumps in the ionocyte patches appear to occur when crabs are acclimated in dilute media.

As noted above, hypoosmoregulating crabs are less common. Only a few species have been shown to keep their blood less salty than a concentrated external medium (i.e., saltier than 100% sea water). In these crabs it is unclear whether or not the gills are used to pump the salts out of the body fluids, as is the case in marine teleosts (bony fishes) and in brine shrimps.

If you would like to read full accounts of ion transport and sodium pump activity in the gills of four estuarine crabs, the following papers may be of interest: Click appropriately to learn more about the research interests of the other members of the Biology Department at Lafayette College, learn more about doing research in my lab or see a list of my former research students and their project titles with these links. Or, if you like, take a look at a list of my publications with research students or a complete list of my publications.

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