哺乳類 and 魚類: a whale's sea habitat and classification
Read 哺乳類 and 魚類 in whale, exhibition, aquarium and picture examples. Keep living in the sea and being a mammal as coexisting statements in the whale sentence.
哺乳類 in the whale sentence
哺乳類 (ほにゅうるい) means mammal. 「クジラは海で暮らしているが哺乳類だ。」 means “Whales live in the sea but are mammals.” The が following the sea habitat does not negate the mammal classification.
「展示室では地域に生息する哺乳類を紹介している。」 means “The exhibition introduces mammals that inhabit the region.” The exhibition's objects are mammals inhabiting the region. This introduction has its own scope, separate from the whale's sea-dwelling description.
魚類 at an aquarium and in pictures
魚類 (ぎょるい) means fish or fish species. 「水族館で淡水の魚類を見た。」 means “I saw freshwater fish at the aquarium.” Freshwater modifies the fish seen here; it is not a required habitat for all 魚類.
「魚類の体の形を図で比べた。」 means “We compared the body shapes of fish in pictures.” The action is comparing fish body shapes in pictures, not changing their classification because of their appearance.
Habitat and classification are both stated
The whale sentence says both that whales live in the sea and that they are mammals. Read the contrast linked by が without replacing 哺乳類 with 魚類 on the basis of the habitat.
The exhibition introduces, the aquarium example reports seeing, and the picture example reports comparing. Preserve the classification named for each object without adding new classification criteria or species lists.
Does クジラは海で暮らしているが哺乳類だ negate 哺乳類 because whales live in the sea?
Show the answer and why
No. Sea-dwelling and being a mammal are both stated. Habitat or outward resemblance alone does not justify replacing the classification in this sentence with 魚類.