The adaptation that would be more useful for an animal in a marine biome than for an animal in a freshwater biome would be tolerance to high levels of salinity.
Marine biomes are characterized by a high level of salinity. The saline environments are often hypertonic to the cells of organisms. Thus, organisms will ordinarily lose water to such environments and die as a result of dehydration.
Hence, organisms that will successfully inhabit marine biomes must find a way to tolerate the hypertonic environment. One of such adaptations is the possession of salt glands in the body.
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