Here's a more specific breakdown:
* Fish: These are vertebrates (animals with backbones) and belong to the class Pisces. They are incredibly diverse, with over 33,000 species.
* Dolphins: These are mammals, meaning they are vertebrates that give birth to live young and nurse their offspring. Dolphins belong to the order Cetacea, which also includes whales and porpoises.
* Squids: These are invertebrates, meaning they lack a backbone. They belong to the class Cephalopoda, which also includes octopuses and cuttlefish.
While all of these animals swim in the ocean, they have different evolutionary lineages and belong to different taxonomic groups.