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

Question

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

In: Biology

Answer 1

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.