Tidal marshes occur in mid to high latitudes, along coasts that are protected within estuaries or behind barrier islands. They are most common in North America and China. Some tidal marsh species are protected from high salinity by relatively impermeable skin, and others have kidneys that can concentrate salts from large volumes of water or specialized glands that exude salt. Many are gray or black in color, which is believed to be an advantage because it matches the dark color of the soils often found in tidal marshes. Why endemic tidal marsh species seem to be largely restricted to North America--which has 24 of the worldwide total of 25--is not clear. Although it could reflect differing taxonomic practices in different countries, it may be related to the history of glaciation or of agriculture.
Endemic tidal marsh species are vulnerable to coastal development and to sea level rise, both of which are rapidly reducing the area of tidal marshes. They are also threatened by toxic wastes and invasive species. Greenberg and his coauthors argue for an expanded research program to try to understand how species will respond to these threats.