Because cell membranes are semi-permeable, the substances cannot equalize their concentration by diffusing across the membrane. This process is called osmosis . The cells are considered hypertonic to the outside solution. If on the other hand, the concentration of solutes is greater outside of the cell than inside of it, water will tend to flow out of the cell. These cells are called hypotonic .
Marine environments are very different chemically than freshwater environments.
As you know, ocean water contains relatively high concentrations of salt and other minerals. Cells in ocean water tend to be more similar to their external environment in terms of salt and mineral concentrations than cells in freshwater habitats. Organisms that live in marine environments are often osmoconformers . The concentration of solutes inside of them mirrors the concentration outside. Freshwater organisms and some marine organisms, however, are often osmoregulators , meaning that they adjust the concentration of solutes inside of them by transporting materials actively across the cell membrane.