From goldfish to sharks, fish of all sizes have lived on Barth for hundreds of millions of years. But do they ever get thirsty?
The answer is yes! Some fish do, at least. But there is a big difference between fish that live in saltwater and ones that live in freshwater.
Bony fish have some salt concentration (含盐量) in their bodies. For bony fish in saltwater, their concentration is lower than the water they live in. Through osmosis (渗透作用) these fish will lose lots of water through their bodies. In order to make up for this loss, they drink lots of water, so it’s easy to say that they can be thirsty. Although drinking saltwater is never a good idea for humans, fish can drink it because they can get rid of the salt through their gills (鳃).
But sharks are an exception. Sharks can make their concentration match the water around them. They do this by building up urea (尿素) in their blood, so they probably don’t get very thirsty. But blood with a high salt concentration can also be bad. The high concentration means sharks won’t do very well in freshwater-water can enter the sharks’ bodies by osmosis and even cause them to explode!
Freshwater fish, on the other hand, are unlikely to become thirsty. Because they live in a less salty environment, they have a different problem: water goes inwards and dilutes (冲淡) their blood. Freshwater fish therefore need to get rid of this extra water. So now we know that saltwater fish can become thirsty and drink water, while freshwater fish will avoid drinking but pee (小便) a lot.
【小题1】What’s the writer’s purpose of this article?| A．To show that fish have a long history of living on Earth. |
| B．To explain why fish of all sizes can become thirsty like humans. |
| C．To prove that fish are different in terms of whether they get thirsty. |
| A．Because they have a high salt concentration |
| B．Because they need to take in salt through their grills |
| C．Because they will lose lots of water through their bodies. |
| A．They don’t get thirsty at all because they live in saltwater. |
| B．Too much water in their blood can be bad. |
| C．They pee a lot after drinking too much water. |
| A．A science book | B．A travel guide | C．A fashion magazine |
