Cetacea
Cetaceans are marine mammals belonging to the infraorder Cetacea (), a secondarily aquatic clade under the order Artiodactyla that include whales, dolphins, porpoises and extinct groups such as Basilosaurus. Most cetaceans live in marine environments, particularly the pelagic zone, but some reside solely in brackish or fresh water. Having a cosmopolitan distribution, they can be found in some rivers and all of Earth's oceans. Many species migrate seasonally over vast ranges for food advantages.
Key characteristics of cetaceans are their fully aquatic life cycle, streamlined, fish-like body shape, the need to periodically surface and breathe air, and exclusively carnivorous diet. All extant cetaceans are capable of echolocation.
As nektonic animals, cetaceans propel themselves through the water with powerful up-and-down movements of their tails, which have evolved into in a horizontal paddle-like fluke. Their hindlimbs have disappeared with only some vestigial skeleton of the pelvis and femurs, and their forelimbs have evolved into flippers which they use to paddle and steer. Some fast-swimming groups, most notably the smaller dolphins and porpoises, have a dorsal fin to facilitate directional stability. Cetaceans also have large brains and have high intelligence, complex social behaviour, and song-like communication. Some cetaceans have large bodies, such as the blue whale, which reaches a maximum confirmed length of 29.9 meters (98 feet) and a weight of 173 tonnes (190 short tons), making it the largest animal known to have existed.
There are approximately 90 living cetacean species split into two parvorders: Odontoceti or toothed whales, which contains 75 species including porpoises, dolphins, the beaked whales and other predatory whales like the beluga and sperm whale, who prey upon fish, cephalopods and other marine mammals such as pinnipeds; and Mysticeti or baleen whales, which contains 15 species of large whales including the blue whale, humpback whale and bowhead whale among others, who are mostly filter-feeding planktivores (or sometimes bottom-feeding crustacivores or molluscivores, as in the case of the gray whale) using oral bristle plates known as baleen to sieve out and feed on large swarms of small invertebrates, usually crustaceans such as krill. Despite their highly modified bodies and carnivorous lifestyle, genetic and fossil evidence places cetaceans within the terrestrial even-toed ungulates, most closely related to the hippopotamids.
Since the Industrial Revolution, cetaceans have been extensively hunted by humans for their meat, blubber and oil by commercial whaling operations. Although the International Whaling Commission has agreed on putting a halt to commercial whaling, whale hunting is still ongoing, either under IWC quotas to assist the subsistence of Arctic native peoples, or for scientific research, despite a large spectrum of non-lethal methods now available to study marine mammals in the wild. Cetaceans also face severe environmental hazards from underwater noise pollution, entanglement in ropes and nets, ship strikes (especially against propellers), build-up of heavy metals and plastic pollutions, food scarcity due to overfishing down the food web, and anthropogenic climate change, but how much they are affected varies widely from species to species, from minimally in the case of the southern bottlenose whale to the functional extinction of baiji (Chinese river dolphin) due to impacts of human shipping activity.
Надо Мной
- 2019-05-02T00:00:00.000000Z
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