Why are cold-water animals all so fat

I mean, I know it's too keep warm, but it seems like there should be the ability to just have a naturally lower tempurature threshhold? Or would blood freeze? Can't there be an animal that can have truly cold blood and organs without them freezing and screwing up?

Thanks

Yes, there are animals that evolved to function effectively at low temperatures. Many amphibians, for example, are adapted to cold temperatures, so are Arctic species of fish and invertebrates. Their enzymes are able to work well at low temperatures. Fish in general do not try to elevate their body temperatures above their surroundings because that would be a big waste of energy. Fish therefore do not need a layer of blubber to insulate themselves against the cold. They merely let their body temperature fall to the same level as their surroundings. Many even have anti-freeze chemicals in their blood to keep them from freezing.

Mammals cannot do the same, because mammals evolved on land and were adapted to the warm temperatures of the Mesozoic. Mammals do not have the enzymes that work well at cold temperatures. Besides, the muscles of mammals do not work well when they are cold (one reason runners and swimmers, as well as basketball and football players, warm up before competing) and their digestive system ceases to function at low temperatures. Mammals therefore need a layer of fat to insulate themselves. An alternative to that is a thick layer of dense fur which traps air for insulation (that is what the sea otter does). Sea otters, however, only recently evolved to exploit marine habitats. Their method is less efficient than the blubber used by virtually all other marine mammals, which evolved a lot earlier than sea otters.

As you say - it keeps them warm. It's also stored energy for lean times. Life in the natural world is always feast or famine - you either have too much to eat - or you're starving. So it's great that animals - including humans - can store fat.

That's for fish, they're cold blooded so that works for them.

Warm blooded needs WARMTH to allow blood to flow to the extremeties, which means fat/blubber needs to be protecting them.