Gulf of Mexico numerous bays and smaller inlets. A number of rivers empty into the gulf, most notably the Mississippi River and Rio Grande in the northern gulf, and the Grijalva and Usumacinta rivers in the southern gulf. The land that forms the gulf's coast, including many long, narrow barrier islands, is almost uniformly low-lying and is characterized by marshes and swamps as well as stretches of sandy beach. The Gulf of Mexico is an excellent example of a passive margin. The continental shelf is quite wide at most points along the coast, most notably at the Florida and Yucatán Peninsulas. The shelf
Gulf of Mexico such as bacteria and other micro – benthos, meiofauna, macrofauna, and megafauna (larger organisms such as crabs, sea pens, crinoids, and demersal fish and cetaceans including endangered ones) are living in the Gulf of Mexico. Recently, resident Bryde's whales within the gulf were classified as an endemic, unique subspecies and making them as one of the most endangered whales in the world. The Gulf of Mexico yields more finfish, shrimp, and shellfish annually than the south and mid-Atlantic, Chesapeake, and New England areas combined. The Smithsonian Institution Gulf of Mexico holdings are expected to provide an important baseline of understanding