027小说网

绪论 虎鲸类型和种群（第4页）

Orca beaching to capture sea lion at Valdes Peninsula

A small population of orcas seasonally visits the northern point of the Valdes Peninsula on the east coast of Argentina and hunt for sea lions and elephant seals on the shore， temporary stranding themselves. Off South Africa， a distinctive “flat-tooth“ morphotype exists and preys on sharks. A pair of male orcas， Port and Starboard， have bee well known for hunting great whites and other sharks off the South African coast.

Orcas occur throughout the waters of Australia， New Zealand and Papua New Guinea. They are sighted year round in New Zealand waters， while off Australia， they are seasonally concentrated off the northwest， in the inshore waters of Ningaloo Reef， and the southwest， at the Bremer region. Genetic evidence shows that the orcas of New Zealand， and northwest and southwest Australia form three distinct populations. New Zealand orcas mainly prey on sharks and rays.

本小章还未完，请点击下一页继续阅读后面精彩内容！

Antarctic

Around 25，000 orcas are estimated around the Antarctic， and four types have been documented. Two dwarf species， named Orcinus nanus and Orcinus glacialis， were described during the 1980s by Soviet researchers， but most cetacean researchers are skeptical about their status， and linking these directly to the types described below is difficult.

Some examples of variations in orcas

Type A or Antarctic orcas look like a “typical“ orca， a large， black-and-white form with a medium-sized white eye patch， living in open water and feeding mostly on minke whales.

Type B1 or pack ice orcas are smaller than type A. It has a large white eye patch. Most of the dark parts of its body are medium grey instead of black， although it has a dark grey patch called a “dorsal cape“ stretching back from its forehead to just behind its dorsal fin. The white areas are stained slightly yellow. It feeds mostly on seals. Type B1 orca are abundant between Adelaide Island and the mainland Antarctic peninsula.

Type B2 or Gerlache orcas are morphologically similar to Type B1， but smaller. This ecotype has been recorded feeding on penguins and seals， and is often found in the Gerlache Strait.

Type C or Ross Sea orcas are the smallest ecotype and live in larger groups than the others. Its eye patch is distinctively slanted forwards， rather than parallel to the body axis. Like type B， it is primarily white and medium grey， with a dark grey dorsal cape and yellow-tinged patches. Its only observed prey is the Antarctic cod.

Type D or Sub-Antarctic orcas were first identified based on photographs of a 1955 mass stranding in New Zealand and six at-sea sightings since 2004. The first video record of this type was made in 2014 between the Kerguelen and Crozet Islands， and again in 2017 off the coast of Cape Horn， Chile. It is recognizable by its small white eye patch， narrower and shorter than usual dorsal fin， bulbous head (similar to a pilot whale)， and smaller teeth. Its geographic range appears to be circumglobal in sub-Antarctic waters between latitudes 40°S and 60°S. Although its diet is not determined， it likely includes fish， as determined by photographs around longline vessels， where Type D orcas appeared to be preying on Patagonian toothfish (Dissostichus eleginoides).

这章没有结束，请点击下一页继续阅读！

Types B and C live close to the ice， and diatoms in these waters may be responsible for the yellowish colouring of both types. Mitochondrial DNA sequences support the theory that these are recently diverged separate species. More recently， plete mitochondrial sequencing indicates the types B and C be recognized as distinct species， as should the North Pacific transients， leaving the others as subspecies pending additional data. Advanced methods that sequenced the entire mitochondrial genome revealed systematic differences in DNA between different populations. A 2019 study of Type D orcas also found them to be distinct from other populations and possibly even a unique species.

喜欢海洋霸主崛起史请大家收藏：()海洋霸主崛起史