How do marine mammals differ from fish?

Marine mammals differ from fish primarily in their need to breathe air. Unlike fish, which extract oxygen from water using gills, marine mammals possess lungs and must surface to breathe atmospheric air. This adaptation is a key distinguishing feature, as it underscores their classification as mammals rather than fish. Marine mammals include species like whales, dolphins, and seals, all of which are warm-blooded and have other mammalian characteristics such as live births and, in some cases, nursing their young with milk.

The other points highlight significant differences but do not characterize marine mammals accurately. For instance, marine mammals are warm-blooded rather than cold-blooded, and while some may give birth to live young, not all lay eggs—making the statement about egg-laying inaccurate. Additionally, size can vary greatly among marine mammals, and many species are larger than typical fish, further emphasizing that size is not a definitive characteristic distinguishing them from fish.