[ELI5] why do some fish like Snapper have scales but other fish like Tuna do not?

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[ELI5] why do some fish like Snapper have scales but other fish like Tuna do not?

In: Biology

The simple answer is all traditional fish (the kind of fish you think of when you read fish) have scales. Snappers have large scales that are easily distinguishable. Tuna have very small scales that almost appear like one continuous membrane but they do have scales. It could come down to the type (or shape) of the scale playing a factor for example Sharks have placoid scales which are very small pointed scales.

There are a few species of fish that do not have scales such as anglerfish or catfish usually have some sort of plating that functions similar to scales and is most likely some niche evolutionary trait.

Tuna are built for speed and power, just ask any game fisherman, and they have smoother skin to allow for this. Snapper, on the other hand, are a lot tougher and more likely to fight, meaning they need the scales to protect themselves.

You may notice that, typically, larger fish, sharks, tuna, etc, tend to have smoother skin, while smaller fish, brim, carp, snapper, etc, tend to have rougher skin. The same applies for different body shapes. Whiting have smoother skin and narrower bodies, for instance