Ecological terms of recurrent use are presented, as for instance biota, biotopo, biocenosis, biotic, abiotic, ecosystem, biodiversity. The biological diversity of ostracoda (Crustacea) in the marine ecosystem is analysed, as well as the role it plays in plancton, bentos and necton, and its biology and habitats are discussed. The ICMyL, UNAM (México's National University) has carried out research, and about 760 species of ostracods have been reported, 300 in the Gulf of México, 110 in the Caribbean Sea and3 50 in the Pacific. The need for more research in the Pacific and the Caribbean Sea is emphasized.