The anatomy of a fish (no particular type)

This year, one of my close friends introduced me to fishing, and ever since I've been interested in fish, and their anatomy. Below is some of what I know about fish. :)

(Royalty free image)

The advantage of an eye underwater

Most fish don't have eyelids, since they're underwater they don't need to keep their eyes moist. However, some fish do have eyelids but they are not used for keeping the eye moist but rather for protecting it from abrasive objects or blocking out UV light. (They will block UV light only when direct or excessive)

Gills and how they work

When a fish 'breathes,' it will take in water, which passes through the gills. The gills have a feathery structure which will act some what like a sieve. The water the fish takes in contains oxygen whilst the blood contains carbon dioxide. As the water passes through the gills, the feathery "sieve" will gather as much oxygen as it can and move it into the blood stream, and remove the carbon dioxide from the bloodstream and expel it into the water.

Fish armour

Fish scales (although it may not seem it) serve many a purpose. They are a good protection against anything that dwells in the water, such as fungi or bacteria, sharp rocks or unforgiving predators. Efficient swimming is a significant business, inefficient swimming means no survival. Scales are sleek and flexible, therefore contributing to efficient swimming. However, not all fish have scales. Fish such as eels, catfish and sharks don't have scales but instead have tough leathery skin with bony plates underneath.

Salt or not?

Fish that live in the salt water are actually different to the ones in freshwater, because of the way they tolerate salt. Freshwater fish are saltier than the water that they live in (there are sugars and salts inside all living things), and because water is seeking to balance salts and sugars, it wants to get to the place where there's salt - which would in this case be the fish - which would mean the fish would probably uh, explode. To prevent blowing themselves up, freshwater fish excrete extra water (which means weeing, a lot) and retain as much salt as they can from their surroundings.

Salt water fish on the other hand don't have the whole exploding problem, but they do have another problem, which is literally the opposite of freshwater fish. Since the water around them is more salty than themselves, the water wants to leave the fish which would really dehydrate them. Therefore these fish need to take in more water, which is salty, and excrete very concentrated wee, which is full of extra salts but not much water. In this way they can stay hydrated.