DO FISHES DRINK WATER AND URINATE? I love this question.
The answer is a bit complicated but I'll try to simplify it. The answer is yes and no. Fishes don't drink water the way humans and other animals do. They "drink" water through their gills.
Yes, they also urinate but we will use the word excrete here so that it will sound better. Pardon me, my hand is itching me to type it in pidgin. Dem dey piss for inside water well well.
You know what's funny? In we humans and other animals, if you drink plenty water, you'll urinate more but if you drink little water, you'll urinate less. It is the opposite for fishes o. Those who drink little water urinate plenty while those who drink plenty water urinate less. Let me explain why.
First let's remind ourselves of OSMOSIS that we learnt in secondary school. It is the movement of water from a region of lower concentration to a region of higher concentration through a selectively permeable membrane.
In freshwater fish 🐟, the surrounding water has lower salt concentration of salt while the fish's body has higher concentration. Following osmosis, water moves into their body through their skin (from lower concentration to higher concentration). Therefore, they don't need to actively drink water through the gills. Instead, they have special cells in their gills that will take in salt from their environment. Because they still need to keep cellular salt concentration balanced, they will keep excreting/urinating/peeing this extra water that gets into their body through osmosis constantly. This means fresh water fish don't drink water but they urinate a lot.
Now, let's move to saltwater fishes 🐠
In saltwater fish, the surrounding water has a higher concentration of ions than their body fluids. In this case, water tends to move out of the fish's body. So, they need to actively drink water. Remember that the water is salt water. Taking it in like that will not help. That's why they have special cells in their gills that will filter out most of the salts. Since they need the water, they keep drinking to replace the ones that leave their body through osmosis. They urinate less so a not to lose water they struggled to drink. Their kidney is so good that it filter out water and leaves them with just small water but plenty salt ti urinate. So, their urine is very small but salty (concentrated). In other words, saltwater fishes drink lots of water but urinate less.
If you didn't get this the first time. Read it again. You will surely get it on second reading. Once you get it, please repost for others to learn.
Tainkyu ☺️