Several genera were able to evolve before the Atlantic and Pacific formed. Several extinct species are known. There are now 13 genera with 58 species. Seagrasses are rooted in soft mud or sand substrates and are usually found offshore of salt marshes and mangroves. They are typically protected from the full force of wave action by barrier islands or coral reefs.
All seagrasses have horizontal stems and regular roots.
Seagrasses have a thick cuticle and by themselves are not very nutritious. The organisms that do graze on seagrasses are probably getting most nutrients from the epiphytic growth. When seagrasses die, the detrital matter and decomposing bacteria do provide nutrition. Most seagrasses are euryhaline, meaning they can take a wide variety of salinities. In the division Anthophyta there are 3 families.