Correct answer: Yes, almost all species drink water Some 28,000 different species of fish live in our planet’s oceans, lakes and rivers. 42% of these fish are freshwater and 58% are saltwater. The amount of water they drink depends on what kind of water they live in: if a saltwater fish doesn’t drink enough, it’ll die, and if a freshwater fish drinks too much, it’ll explode. Why does water salinity matter? Because of osmosis, which is the phenomenon whereby water molecules naturally pass from a solution with a low salt concentration to one with a high salt concentration. The salt concentration is three times higher in sea water than in the water in a fish’s body. So for fish swimming in the ocean, water molecules pass through their gills, which are permeable membranes, into the surrounding sea water. That means saltwater fish must drink a lot (10-30 mL/100 g/day) to avoid dehydration. On the other hand, fresh water has a lower salt concentration than the water in a fish’s body. Therefore water molecules naturally pass into a freshwater fish’s body, meaning it needs to drink very little (1–5 mL/100 g/day). And most importantly, it has to avoid retaining too much water – otherwise it could swell to the point of explosion. To accommodate the process of osmosis, saltwater fish produce small quantities of highly concentrated urine and evacuate any excess salt through their gills. Freshwater fish produce a great deal of urine and pull salt in through their gills, subsequently absorbing it via their kidneys. And just as there is an exception to every rule, sharks and rays don’t drink water at all. They maintain the osmotic pressure in their bodies by increasing their inner concentration of urea. In other words, they become just as salty as the sea water they live in, evacuating as much water as they absorb. Salmon and eels, which alternate between freshwater and saltwater habitats, are capable of adapting to their surroundings. They either don’t drink at all – or drink like fish!