|
Marine mammals, which include seals, whales, dolphins, porpoises, manatees, dugongs, marine otters, walruses, and polar bears form a diverse group of 129 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the aquatic environment for feeding.〔Jefferson, T. A. , Webber, M. A. & Pitman, R. L. (2009) ''Marine Mammals of the World A Comprehensive Guide to their Identification'' London ; Burlington, MA: Academic ISBN 978-0-12-383853-7. 7–16〕 The level of dependence on the aquatic environment for existence varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life, whereas seals feed in the ocean, but breed on land.〔 Marine mammals can be subdivided into four recognised groups; cetaceans (whales, dolphins, and porpoises), pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), and fissipeds, which are the group of carnivores with separate digits (the polar bear, and two species of otter). Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to ocean living.〔 While the number of marine mammals is small compared to those found on land, their total biomass is large. They play important roles in maintaining marine ecosystems, especially through regulation of prey populations. These two factors make them an integral component of the marine environment. This is of particular concern considering 23% of marine mammal species are currently threatened. ==Taxonomy== Mammals have returned to the water in at least nine separate evolutionary lineages: Cetacea, Sirenia, Desmostylia, Pinnipedia, ''Ursus maritimus'' (polar bear), ''Kolponomos'' (marine bear), ''Thalassocnus'' (aquatic sloth), ''Enhydra lutris'' (sea otter) and ''Lontra feline'' (marine otter); the eutriconodont ''Ichthyoconodon'' might have also been marine in habits. Four of these lineages are extinct (Desmostylia; ''Kolponomos''; ''Thalassocnus'', ''Ichthyoconodon'').〔 Despite the diversity in morphology seen between groups, improving foraging efficiency has been the main driver in the evolution in these lineages. Today, marine mammals belong to one of three orders: Cetartiodactyla, Sirenia, or Carnivora. Based on molecular and morphological research, the cetaceans genetically and morphologically fall firmly within the Artiodactyla (even-toed ungulates). The term Cetartiodactyla reflects the idea that whales evolved within the ungulates. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. Under this definition, the closest living land relative of the whales and dolphins is thought to be the hippopotamuses. Use of Order Cetartiodactyla, instead of Cetacea with Suborders Odontoceti and Mysticeti, is favored by most evolutionary mammalogists working with molecular data and is supported the IUCN Cetacean Specialist Group〔(Cetacean Species and Taxonomy ). iucn-csg.org〕 and by Taxonomy Committee〔("The Society for Marine Mammalogy's Taxonomy Committee List of Species and subspecies" ).〕 of the Society for Marine Mammalogy, the largest international association of marine mammal scientists in the world. Some others, including many marine mammalogists and paleontologists, favor retention of Order Cetacea with the two suborders in the interest of taxonomic stability. Within the Order Sirenia are the manatees and the dugongs. Order Carnivora contains the pinnipeds (sealions, walruses and seals), the polar bear (''Ursus maritimus''), and the two otters (''Enhydra lutris'' and ''Lontra feline'').〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「marine mammal」の詳細全文を読む スポンサード リンク
|