Only saltwater fish must actively drink – otherwise they’d die of dehydration in the middle of the ocean. At 39 grams per litre, the salt content of the seawater is far higher than the fish’s 9 grams of salt per litre of blood. This difference means the fish’s water reserves are quickly depleted. To compensate for this loss, most saltwater fish ingest seawater through their mouths, mucosal membranes and gills. The gills are equipped with special glands that filter out the salt, converting it into water that is drinkable.