What are the differences in osmoregulation between a fish, amphibian, lizard and a mammal?

Question

Compare the different strategies of osmoregulation between a fish, amphibian, lizard and a mammal.

Answer 1

Osmoregulation, of course, is the process of making sure you have the right amount of osmotic pressure. There's a fine difference between osmoregulation (water pressure) and hydroregulation (water volume)... and that difference is salt. Differing solute concentrations do most of the driving of osmosis, so much of osmoregulation involves controlling salt in one way or another.

There are quite a few animals that AREN'T osmoregulators, but since none of your examples are marine invertebrates, that's not something we need to worry about. These critters conform to ambient salinity instead of maintaining a separate internal one. (It's worth mentioning, though, that since most invertebrates are osmoconformers and most animals (97%) are invertebrates, this is by far the most common mechanism biologically!)

As far as the groups you mention, it is hard to classify them concretely because there is so much variety within a group. For example, some freshwater fish and some saltwater fish are going to have opposite osmotic challenges. The freshwater fish selectively uptakes salt in the gills and urinates very dilute urine, while the saltwater fish is going to do the exact opposite. And each eventually dies if put in the other's environment because their systems will work in ways opposite of what they need!

Pretty much all these animals' primary means of osmoregulation involved the kidneys and the hypothalamus. The hypothalamus in the brain detects blood salinity, and sends signals to the kidneys to either remove more or less water. Marine reptiles actually cannot produce highly saline urine, so they have developed salt glands near their eyes that secrete out all the extra salt and allow them to drink sea water. As for the rest of us, sea water is far too salty for our systems to deal with, so it spells rapid dehydration!

Another major means of osmoregulation (as hinted at above) is diet. Many animals obtain a lot of salt from other creatures they eat, and I'm sure we've all heard of herders putting out blocks of salt for their animals to lick. A certain amount of water is also produced as an end product of metabolism as well, which is how desert animals tend to get most of their water.

Many animals avoid loss of water through thermoregulation (obviously excluding reptiles and fish). Camels, for example, allow their body temperature to increase during the day to reduce the amount of water lost through sweat and other cooling mechanisms.

Amphibians, of course, are special in spending part of their life in water and part out. So naturally this plays largely into their means of osmoregulation. But even so, while in the water they tend to resemble fishes, and while out they tend to resemble reptiles. So it's not like they have any specifically special means... just that they switch from one to another.

Answer 2

Osmoregulation In Amphibians

Answer 3

Hey Footy... how did you do that thing where you added to your question at a later point? I'm new here, help me out. And I am not being condescending! It's difficult to express yourself on these things without different fonts, or italics. I truly don't mean to be condescending. I'm only trying to have a rational discussion, so I'm sorry if it came across that way. You want me to tell you why God made us mammals and not reptiles??? Is that seriously your question? ^_^ How the heck should I know? Something tells me if he'd made us reptiles, you'd be demanding to know why we weren't mammals! But my not knowing the WHY does not negate the HOW of all this. (.... and don't freak out about the capital letters! I'm not condescending!) And if you have emotional empathy with your food, nothing is stopping you from being a vegetarian, certainly not God. I once had a cat who befriended a pet rat... so how do you explain that? Did God not create the cat because of that anomaly? I really don't get your point there.