Do fish drink?

• I've never seen a drunk fish.

  Thomas Moore, London

• The answer depends on whether you are talking about freshwater or marine fish. Freshwater fish are hyperosmotic compared to their environment, meaning that their body contains more salts than the surrounding water. The water therefore rushes into the fish passively - through the mouth, the gills and to some extent the skin. The fish must therefore constantly eliminate water, mostly by urinating. Marine fish are HYPOosmotic, meaning that the water has more salt than they do. They lose water passively, and must therefore "drink" large quantites or water. They have adaptations to eliminate the excess salts. Thus only marine fish drink.

  Chris Blanar, Montreal, Canada

• Not if they know W.C.Fields' observation as to what they do in it.

  Pete Wigens, Stroud, Glos, UK