|
The dugong (, ; ''Dugong dugon'') is a medium-sized marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow (''Hydrodamalis gigas''), was hunted to extinction in the 18th century. The dugong is the only strictly marine herbivorous mammal, as all species of manatee use fresh water to some degree. The dugong is the only sirenian in its range, which spans the waters of some 40 countries and territories throughout the Indo-West Pacific. The dugong is largely dependent on seagrass communities for subsistence and is thus restricted to the coastal habitats which support seagrass meadows, with the largest dugong concentrations typically occurring in wide, shallow, protected areas such as bays, mangrove channels, the waters of large inshore islands and inter-reefal waters. The northern waters of Australia between Shark Bay and Moreton Bay are believed to be the dugong's contemporary stronghold. Like all modern sirenians, the dugong has a fusiform body with no dorsal fin or hind limbs. The forelimbs or flippers are paddle-like . The dugong is easily distinguished from the manatees by its fluked, dolphin-like tail, but also possesses a unique skull and teeth. Its snout is sharply downturned, an adaptation for feeding in benthic seagrass communities. The molar teeth are simple and peg-like unlike the more elaborate molar dentition of manatees. The dugong has been hunted for thousands of years for its meat and oil. Traditional hunting still has great cultural significance in several countries in its modern range, particularly northern Australia and the Pacific Islands. The dugong's current distribution is fragmented, and many populations are believed to be close to extinction. The IUCN lists the dugong as a species vulnerable to extinction, while the Convention on International Trade in Endangered Species limits or bans the trade of derived products. Despite being legally protected in many countries, the main causes of population decline remain anthropogenic and include fishing-related fatalities, habitat degradation and hunting. With its long lifespan of 70 years or more, and slow rate of reproduction, the dugong is especially vulnerable to extinction. ==Etymology and taxonomy== The word "dugong" derives from the Tagalog term ''dugong'' which was in turn adopted from the Malay ''duyung'', both meaning "lady of the sea". Other common local names include "sea cow", "sea pig" and "sea camel".〔Reeves, R. R. (2002). ''National Audubon Society Guide to Marine Mammals of the World''. Knopf. ISBN 0-375-41141-0. pp. 478–481〕 ''Dugong dugon'' is the only extant species of the family Dugongidae, and one of only four extant species of the Sirenia order, the others forming the manatee family.〔Marsh, H. ''et al.'' (2002). (Dugong: status reports and action plans for countries and territories ). IUCN.〕 It was first classified by Müller in 1776 as ''Trichechus dugon'',〔(Dugong dugon ). The Paleobiology Database. Retrieved on 22 July 2007.〕 a member of the manatee genus previously defined by Linnaeus.〔(Trichechus ). The Paleobiology Database. Retrieved on 22 July 2007.〕 It was later assigned as the type species of ''Dugong'' by Lacépède〔(Dugong ). The Paleobiology Database. Retrieved on 22 July 2007.〕 and further classified within its own family by Gray〔(Dugongidae ). The Paleobiology Database. Retrieved on 22 July 2007.〕 and subfamily by Simpson.〔(Dugonginae ). The Paleobiology Database. Retrieved on 22 July 2007.〕 Dugongs and other sirenians are not closely related to other marine mammals, being more related to elephants.〔 Dugongs and elephants share a monophyletic group with hyraxes and the aardvark, one of the earliest offshoots of eutherians. The fossil record shows sirenians appearing in the Eocene, where they most likely lived in the Tethys Ocean. The two extant families of sirenians are thought to have diverged in the mid-Eocene, after which the dugongs and their closest relative, the Steller's sea cow, split off from a common ancestor in the Miocene. The Steller's sea cow became extinct in the 18th century. No fossils exist of other members of the Dugongidae.〔 Molecular studies have been made on dugong populations using mitochondrial DNA. The results have suggested that the population of Southeast Asia is distinct from the others. Australia has two distinct maternal lineages, one of which also contains the dugongs from Africa and Arabia. Limited genetic mixing has taken place between those in Southeast Asia and those in Australia, mostly around Timor.〔 One of the lineages stretches all the way from Moreton Bay to Western Australia, while the other only stretches from Moreton Bay to the Northern Territory.〔 There is not yet sufficient genetic data to make clear boundaries between distinct groups.〔 There are claims mentioning that several subspecies such as in Okinawan waters exist either historically or presently. 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「Dugong」の詳細全文を読む スポンサード リンク
|