If so many sea animals evolve to “craboforms” because it is so evolutionarily advantageous, what about fish-forms?

Question

Fish are so abundant in the ocean (even given how humans overfish, and them often being prey), I kind of wonder

1. Why don’t marine animals converge to a more fish like form? (Aside from say land mammals like whales and dolphines who decided they want to go to the sea)

2. What benefits are there in having a fish-like form in the ocean? It must be evolutionarily advantageous for some reason.

3. Did the first non microscopic multicellular beings have fish-form?

In: Biology

Answer 1

Marine animals did converge to fish-like forms. That’s why there are so many species of fish.

There’s technically no proper scientific definition for exactly what constitutes a fish. What we call “fish” are just a bunch of entirely separate species which all evolved to be roughly the same shape, because that shape is good at swimming.