The characteristics of fish are: living in water, the surface of fish is mostly covered with scales, which reduces the resistance of water, breathing with gills, swimming with fins, coordinating fins by swinging tail and trunk left and right, and constantly swimming forward. ① ⑤ It conforms to the characteristics of fish and belongs to fish. ② Whales are viviparous mammals, belonging to mammals. ③ Octopus, ④ Mo Dou and ⑥ Squid are mollusks, which have a soft body, a mantle and a degenerated shell.

So choose: B.