The general adaptations to a saltwater environment differ greatly from adaptations to a freshwater environment. Animals living in estuaries must be able to osmoregulate quickly and effectively under multiple wildly different conditions.
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Saltwater is hypertonic to blood and freshwater is hypotonic. Most aquatic animals are adapted to osmoregulate under only one of these circumstances and could not effectively do so if their environment were to flip from one to the other. Animals living in estuaries must have the ability to do this (and do this quickly!) to survive there.