Why is saltwater marine life usually so much more colorful and vibrant?
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While I haven’t researched it, my best guess would be habitat. In freshwater environments the watch is much more muddy and shallow and the best places to hide are rock formations. That would explain why larger predatory fish sometimes have splashes of color while smaller ones don’t.
Ocean fish mainly inhabit coral reefs so they have evolved to be much more vibrant. Also in the ocean there are many more venomous fish that are brightly colored to ward of predators. Along with this, the fish that inhabit open water, such as bluefin tuna, are shades of blue to blend in with the open water around them. This isn’t to say that all ocean fish are bright and colorful, take the stone fish for example. They are sandy browns and greys that blend in with the sea floor because they are ambush predators. Another senerio is deep sea fish, which are often darker colored as color doesn’t matter when you can’t see.
I hope that helped and I hope I’m not spewing false information, remember I didn’t fact check any of this and am just making an educated guess.
If you’re talking about aquarium fish, it isn’t because they’re saltwater. Plenty of saltwater fish are boring. It’s because they’re tropical. Animals throughout the tropics are more colorful and if you were going through the trouble of having a saltwater tank (which is very difficult to take care of), then you might as well select the most colorful and interesting looking ones. And in the ocean, there are way more to choose from.
People think the reason tropical animals are more colorful is that given the biodiversity and density it’s more important for species to find each other for mating than to avoid predators.
Most fish aren’t really. You’re specifically thinking of coral reef fish. Most fish, saltwater fish included, are just muted browns, greys, and silvers.
Reefs are a very unique and colorful environment and some of the most densely populated habitats in the sea. The leading theory is that reef fish are so colorful to help differentiate in such a crowded environment.
There are so many different fish species going every which way on a reef that the colors and patterns help them recognize rivals and potential mates of their own species.
Many colorful reef fish actually change color at night. Their colors fade, their patterns become blotchy. This helps them hide at night while they sleep, tucked away in nooks and crannies in the reef while the nighttime predators roam.
In addition to the other answers (it’s reef fish only), it’s a similar reason to why many birds are very colourful – it’s a sign to the opposite sex that they’re so good at getting food and avoiding being food, that they don’t have to worry about being seen.
A slow fish has to hide, whether it’s to ambush others or to avoid being eaten. The faster you are, the less you have to hide from everyone else, so being colourful sends a signals to mates that you’re a good partner.
But there are other types of camouflage as well. Lots of stripes are actually a way to hide, the same way a spotted cheetah is actually less visible than a solid-coloured one.
Then there are fish like the [Four eye butterfly fish](https://en.wikipedia.org/wiki/Foureye_butterflyfish) that has eye spots like a butterfly, which leads predators to think the head is at the wrong end, and will assume it will flee the opposite way.
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