Sulfated polysaccharides (SP) are highly anionic compounds found in vertebrates, invertebrates and algae. Recently we reported the occurrence and determined the structure of a sulfated galactan (SG) in the cell wall of marine angiosperms (seagrass), but not in terrestrial or sweet water vascular plants (Aquino et al, Glycobiology, vol 15:1:11,2005). We now report the purification and characterization of the SP from three different species of seagrasses: two phylogenetically close related (Ruppia maritima, Halodule wrightti), and a third one phylogenetically unrelated to both (Halophila decipiens). The presence of SG in seagrasses, despite conspicuous structural differences, is a convergent evolutional adaptation. This observation raises interesting questions concerning the biological function of the sulfated polysaccharides. Are they protective molecules in the cell wall of marine angiosperms necessary due to the high osmotic pressure of the marine environment? In order to obtain a direct evidence for this proposition the marine angiosperm (Ruppia maritima) was removed from the seawater and grown in an aquarium. The salinity was gradually decreased until achieves a total absence of NaCl. Surprisingly we observed that the marine angiosperm maintained for a period of two weeks in sweet water completely abolished the biosynthesis of sulfated polysaccharide. In a reversal way, if the NaCl concentration is gradually reestablished, the marine angiosperm re-starts the biosynthesis of sulfated polysaccharide. These results strongly indicate that the presence of sulfated galactans in the cell wall of the marine angiosperm is related with the salt concentration of the marine environment. Overall, our results indicate that the sulfated polysaccharides found in the cell wall of marine plants (seagrass), but not of terrestrial or sweet water plants, is an evolutionary adaptation to the marine environment.