A coastal marine mammal A harbor seal (Phoca vitulina) basking on a harbor rock in Monterey Bay, California. (more) What Is a Marine Mammal? Written by Melissa Petruzzello Melissa Petruzzello (she/her) is Assistant Managing Editor and covers plants, algae, fungi, insects, spiders, renewable energy and environmental engineering. She also handles certain topics in Christianity,... Melissa Petruzzello Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Apr. 16, 2026 •History A marine mammal is a mammal that lives in the ocean and depends on marine environments for its life cycle. Like all mammals, marine mammals breathe air with lungs, are warm-blooded, have hair at some stage of life, and bear live young that they nourish with milk. What makes them “marine” is their unique suite of physiological and behavioral adaptations to life in seawater. Some species, such as manatees and seals, may also use fresh...

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