ASSOCIATED PRESS
メキシコからアラスカにかけての沿岸で、1年以上もヒトデが溶ける謎の現象が起きている。
病気を発症すると最初に腕は固く曲がり、皮膚上に白い病斑ができる。次に、通常は水分をたくさん吸収してその形を保っているヒトデが収縮し始める。そして突然手足が剥がれ落ちる。症状が一回発症すると、数日で分解されて死に至る。
この疾患は「ヒトデ消耗性疾患」（SSWD）と呼ばれ、2013年に太平洋岸沿いで急速に発生し広まった。この海洋生物の大量死の原因については、海洋生物学者の間で「地球温暖化が原因なのか」といった推測がいくつかあるに過ぎない。
現在、大規模な新しい研究が行われ、ヒトデを溶かしている原因を絞り込んでいる。無脊椎動物の生物学者、遺伝学者、統計学者、獣医師やウィルス学者と様々な専門家によるグループの研究結果がアメリカ科学アカデミー紀要の機関誌で紹介されている。
研究者は次のように指摘している。「ヒトデを襲っているのはウィルスで、実際にヒトデに発見された最初のウィルスだ」
コーネル大学の海洋生物学者で報告書の共筆者であるイーアン・ヒューソン氏は次のように述べている。「今回の研究より以前は、ヒトデの病原体についてほとんど知られていなかった」
アメリカ政府からの資金援助で資金を確保したことにより、専門家たちが集められてヒトデの死の原因を明らかにした。ヒトデの大量死は海洋の生態系全体に影響を及ぼす可能性がある。
海底に無抵抗にただ横たわっているように見えるが、ヒトデ（star fish）――生物学者の間では「海の星」（sea star）と呼ぶのが好まれている。ヒトデは実際には魚とは遠い「いとこ」だからだ――は、海で最も貪欲な捕食者の一種である。下面にある口でウニ、貝のイガイやその他の海洋生物を食べる。ヒトデが食べる海洋生物の多くには他に天敵がいないので、ヒトデがいなくなると天敵がいなくなった海洋生物の個体群密度が増し、サンゴ礁を維持するサンゴなど、他の海底にいる生物種を追い出してしまうことになる。
健康なヒトデが餌を食べているタイムラプス画像
ヒューソン氏はこう述べている。「一般に、病気の原因を突き止めることは何年も何十年も要する」。彼は研究グループに対し、早急に原因を究明するように伝えた。環境汚染がこの病気の原因ではないかと世間が心配しているからだ。
研究グループはヒトデの組織に感染している新しい細菌を顕微鏡で発見できなかったが、「ニュートリブレット」（スムージーを作るミキサー）を使って「ヒトデスムージー」を作り、目に見えなかった「ウィルス」を探すことにした。病気を発症しているヒトデの組織を混ぜ合わせ、ミキサーにかけられたヒトデを分解して「ウィルスの大きさの微粒子」にする。健康な検体にその溶解を注入しタンクに入れておいた。研究者がそのスムージーを熱で低温殺菌して潜在しているウィルスを全部消滅させた場合、ヒトデは発病はしなかった。しかし、熱処理しなかった場合、それを注入したヒトデには萎縮が始まった。
また、研究グループは1942年に採取された博物館の標本までさかのぼって調査したところ、標本からもウィルスを発見した。その結果、近年の大流行は昔から長く存在する病気が原因であることが明らかになった。
オレゴン州立大学の海洋生物学教授で、今回の研究には関与しなかったブルース・メンゲ教授はこう述べている。「報告書に報告されている実験結果はかなり有力だ」
なぜウィルスが発症し、どのようにして組織を溶かすのかは不明のままだ。近年の個体群密度が高いことが今回の大流行をもたらしたかもしれない。一説には、西海岸の都市の下水が水中に浮かぶ小さなプランクトンの餌になっているかもしれない。ウニやその他の海洋生物がプランクトンを食べ、さらそれらの海洋生物がヒトデの餌となり、数が繁殖する。この接点を確かなものにするにはさらに研究が必要だとヒューソン氏は述べている。
SSWDが進行したヒマワリヒトデ（イーアン・ヒューソン氏提供）
「ヒトデの大量死は過去に海水が暖かくなったことと関連性がありました。しかし近年は、アメリカの西海岸沖の海温は通常よりも低くなっています」サザンプトン大学の海洋研究員アマンダ・E・ベイツ氏はこう述べている。「ですので、現在の病気の大発生が温暖化と関連があるとは考えていません」。ベイツ氏はこれまでの研究で海水温の上昇とヒトデの大量死の関連性を指摘している。「しかし、海洋生物が適した水温よりも暖かい温度、例えば非常に熱い温泉のような温度にさらされたら、病気を追い払う能力は低減します」。
メンゲ教授はこう述べている。「今回の現象が広範囲にわたっているということは、海洋の変化がきっかけになっているのかもしれません」。メンゲ教授は、地球温暖化や海洋酸性化などの人間の活動に関連した原因を調査すべきと考えている。
Amazing Sea Creatures
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This undated image provided by the Monterey Bay Aquarium Foundation shows Toola, a sea otter who died at the aquarium, Saturday March 3, 2012, in Monterey, Calif. Believed to be 15 or 16, Toola succumbed to natural causes and to the infirmities of age, an aquarium spokesman said. (AP Photo/Monterey Bay Aquarium) (credit:AP)
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A California sea lion and a walrus kiss each other during a show at the Hakkeijima Sea Paradise aquarium-amusement park complex in Yokohama, southwest of Tokyo, Sunday, Sept. 16, 2012. (AP Photo/Itsuo Inouye) (credit:AP)
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A red lionfish (Pterois volitans) swims in the aquarium of the Schonbrunn zoo in the gardens of the Schoenbrunn Palace in Vienna on October 16, 2012. The red lionfish is a venomous coral reef fish. ALEXANDER KLEIN/AFP/Getty Images (credit:Getty Images)
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A two-day-old female white whale swims with her mother at the Hakkeijima Sea Paradise aquarium-amusement park complex in Yokohama, southwest of Tokyo, Saturday, June 30, 2012.(AP Photo/Itsuo Inouye) (credit:AP)
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A seahorse swims in an aquarium in the zoo of Frankfurt, Germany, Tuesday, Oct. 16, 2012.(AP Photo/Michael Probst) (credit:AP)
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A Cownose Ray swims in a tank during a preview of the newly renovated Suzanne and Walter Scott Aquarium at Henry Doorly Zoo in Omaha, Neb., Wednesday, April 4, 2012.The aquarium opens to the general public on Thursday, April 5. (AP Photo/Nati Harnik) (credit:AP)
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King penguins stand in an enclosure at the Hakkeijima Sea Paradise aquarium-amusement park complex in Yokohama, near Tokyo, Sunday, Sept. 30, 2012.(AP Photo/Itsuo Inouye) (credit:AP)
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In this photo taken Thursday Aug. 2, 2012 and released by the Monterey Bay Aquarium, a male weedy sea dragon at the Monterey Bay Aquarium swims with some of his newly hatched babies on in a sea dragon display thats part of the aquariums special exhibition, The Secret Lives of Seahorses. in Monterey, Calif. The inch-long fish, Australian relatives of the seahorse, were carried as eggs on a brood pouch under the father sea dragons tail. (AP Photo/Monterey Bay Aquarium, Randy Wilder) (credit:AP)
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Activists from environment campaign group Greenpeace wearing cardboard tuna cutouts hold a protest in front of South Korea\'s embassy in Manila on November 29, 2012. The activists sought conservation commitments from the fishing powers in the upcoming global summit on Pacific tuna fisheries to be hosted by the Philippines next week. NOEL CELIS/AFP/Getty Images (credit:Getty Images)
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This Sept. 5, 2012 photo shows Serena, a dugong at the Toba Aquarium in Toba, Japan. Dugongs, a sea mammal related to the manatee, are rare in captivity. The aquarium gift shop sells stuffed dugongs and dugong cookies. (AP Photo/Linda Lombardi) (credit:AP)
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This May 9, 2012 photo provided by the New England Aquarium in Boston shows a rare calico lobster that could be a 1-in-30 million, according to experts. The lobster, discovered by Jasper Whites Summer Shack and caught off Winter Harbor, Maine, is being held at the New England Aquarium for the Biomes Marine Biology Center in Rhode Island. The lobster is dark with bright orange and yellow spots. (AP Photo/New England Aquarium, Tony LaCasse) (credit:AP)
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Pterophyllum scalare fish are displayed at the 2012 Taiwan International Aquarium Expo in Taipei on November 9, 2012. More than one hundred fish tanks from many countries will be on display in the four day exhibition at Nangang Exhibition Hall from November 9 to 12. Mandy Cheng/AFP/Getty Images (credit:Getty Images)
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In this Oct. 15, 2012 photo provided by the Wildlife Conservation Society, Mitik, an orphaned Pacific walrus calf rescued off the coast of Alaska, emerges from his tank at the New York Aquarium in the Brooklyn borough of New York. Mitik suffered from a number of ailments when he was rescued in July but is making progress as he receives round-the-clock care at the aquarium. (AP Photo/Wildlife Conservation Society, Julie Larsen Maher) (credit:AP)
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Phenacogrammus interruptus fish are displayed at the 2012 Taiwan International Aquarium Expo in Taipei on November 9, 2012. More than one hundred fish tanks from many countries will be on display in the four day exhibition at Nangang Exhibition Hall from November 9 to 12. Mandy Cheng/AFP/Getty Images (credit:Getty Images)
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This photo taken July 4, 2012, at the Alaska SeaLife Center in Seward, Alaska, shows a baby beluga calf being rehabilitated at the center. The whale was approximately two days old when it was found in Bristol Bay, Alaska, and separated from its mother. Staff from the Alaska SeaLife Center is receiving help with the whale\'s care from the Georgia Aquarium in Atlanta, Shedd Aquarium in ChiCago and SeaWord in San Diego.. (AP Photo/Mark Thiessen) (credit:AP)
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A Pacific white-sided dolphin calf swims along with its mother Piquet, Tuesday, June 12, 2012, at Chicago\'s Shedd Aquarium. The baby male dolphin, which does not have a name, was born on Memorial Day. (AP Photo/Kiichiro Sato) (credit:AP)
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A seahorse (Hippocampus Reidi) is displayed during a news conference before the 2012 Taiwan International Aquarium Expo in Taipei November 5, 2012. More then one hundred tanks of fish from many countries will be on display in the four day exhibition at Nangang Exhibition Hall from November 9 to 12. Mandy Cheng/AFP/Getty Images (credit:Getty Images)
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A still unnamed King penguin chick that hatched on April 9 is unveiled at The Aquarium at Moody Gardens, Monday, April 23, 2012, in Galveston, Texas. A blood test will be conducted to determine the gender of the bird who came into life weighing about 20 ounces. This chick is the 14th King penguin chick to have been successfully bred at the aquarium. Due to space limitations, this chick will be go to another facility once weaned from its parents. The chick currently is in the main penguin exhibit at the aquarium which also home to Gentoo, Macaroni, Rockhopper and Chinstrap penguins. (AP Photo/Houston Chronicle, Johnny Hanson) (credit:AP)
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A Rock Beauty angelfish looks out from its tank at the Greater Cleveland Aquarium in Cleveland Wednesday, Jan. 18, 2012. Developed by Marinescape NZ Limited, Ohio\'s only free-standing aquarium opens Thursday with two preview days for annual pass holders and opens to the public Saturday, Jan. 21. (AP Photo/Mark Duncan) (credit:AP)
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This image provided by the Cabrillo Marine Aquarium shows a female Argonaut, or paper nautilus, a species of cephalopod that was recently scooped out of the ocean off the California coast. The baseball-sized animal is making herself at home at the aquarium, bobbing up and down in her tank furling and unfurling her sucker-covered arms. This strange octopus is rare in California, because it only lives in tropical and subtropical waters. (AP Photo/Cabrillo Marine Aquarium, Gary Florin) (credit:AP)
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In this photo released on Friday April 27,2012 by the Aquarium of the Pacific in Long Beach showing Shelby, one of the Aquariums female harbor seals, who is 16 years old and gave birth to her first pup today Friday April 27,2012. The newborn female seal weighs approximately 20 pounds. Aquarium of the Pacific mammal and bird curator Dudley Wigdahl says Shelby 16 years old and gave birth to her first pup when most seals start having pups at 4 or 5.(AP Photo/Terri Haines, Aquarium of the Pacific) (credit:AP)
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Turkey fish swim in an aquarium in the zoo of Frankfurt, Germany, Tuesday, Oct. 16, 2012. (AP Photo/Michael Probst) (credit:AP)
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A Magellanic penguin swims in the June Keyes Penguin Habitat exhibit at The Aquarium of the Pacific in Long Beach, Calif., Wednesday, May 16, 2012. (AP Photo/The Orange County Register, Mark Rightmire) (credit:AP)
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A common spider tortoise is seen in its enclosure at the South Carolina Aquarium in Charleston, S.C., on Tuesday, May 1, 2012. The creature is part of a new exhibit, Madagascar Journey, opening at the aquarium on Saturday, May 5, 2012. (AP Photo/Bruce Smith) (credit:AP)
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A blue caribbean tang swims over an artificial coral reef inside the Greater Cleveland Aquarium in Cleveland Wednesday, Jan. 18, 2012. Constructed in an old powerhouse, Ohio\'s only free-standing aquarium opens Thursday with two preview days for annual pass holders and opens to the public Saturday, Jan. 21. (AP Photo/Mark Duncan) (credit:AP)
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A new baby Fairy Penguin swims at the Sydney Aquarium in Sydney, Australia, Wednesday, Jan. 18, 2012. Three juveniles were born at the aquarium and have been separated from the main group since birth to ensure a healthy growth rate and to keep them safer while they were still young. (AP Photo/Rob Griffith) (credit:AP)
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This image provided by the Cabrillo Marine Aquarium shows a female Argonaut, or paper nautilus, a species of cephalopod that was recently scooped out of the ocean off the California coast. The baseball-sized animal is making herself at home at the aquarium, bobbing up and down in her tank furling and unfurling her sucker-covered arms. This strange octopus is rare in California, because it only lives in tropical and subtropical waters. (AP Photo/Cabrillo Marine Aquarium, Gary Florin) (credit:AP)
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In this Monday, Aug. 27, 2012 photo, Mauyak, a beluga whale at Chicago\'s Shedd Aquarium, swims with her newly born calf at the aquarium\'s Abbott Oceanarium. Shedds animal care team estimates that the calf is 4½ feet long and weighs about 150 pounds. The newborn is the sixth successful birth as part of Shedds collaboration in the beluga whale breeding cooperative. (AP Photo/Shedd Aquarium, Brenna Hernandez) (credit:AP)
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Freshwater exotics Greater Cleveland Aquarium in Cleveland Wednesday, Jan. 18, 2012. Ohio\'s only free-standing aquarium opens Thursday with two preview days for annual pass holders and opens to the public Saturday, Jan. 21. (AP Photo/Mark Duncan) (credit:AP)
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Walrus, (Odobenus rosmarus), Neseyka swims in the new aquarium in the Hamburg Hagenbeck zoo, Thursday, July 5, 2012. (AP Photo/dapd/ Philipp Guelland) (credit:AP)
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In this May 24, 2012 photo provided by the Shedd Aquarium in Chicago, Cayucos, bottom left, an orphaned sea otter pup, swims with other otters as she makes her first public appearance at the aquarium\'s Abbott Oceanarium. The sea otter was found stranded on a beach in California and named after the beach where she was rescued. The staff at Shedd has nursed the 7-month-old otter back to health. She has put on 11 pounds since arriving at the aquarium in January and now weighs 26 pounds. (AP Photo/Shedd Aquarium, Brenna Hernandez) (credit:AP)
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A male walrus pokes his tongue out at his trainer during a practice at the Hakkeijima Sea Paradise aquarium-amusement park complex in Yokohama, near Tokyo, Japan, Saturday, Feb. 4, 2012. (AP Photo/Itsuo Inouye) (credit:AP)
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In this photo taken on May 7, 2012 and distributed by Japan Coast Guard\'s 3rd Regional Coast Guard Headquarters Thursday, May 17, a fugitive Humboldt penguin swims in the water in Tokyo Bay. The one-year-old penguin Number 337 which escaped from a Tokyo aquarium in March has been spotted alive in the water busy with many ships throughout the day. (AP Photo/Japan Coast Guard\'s 3rd Regional Coast Guard Headquarters) (credit:AP)
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In this March 2012 photo provided by the Shedd Aquarium in Chicago, Piquet, a Pacific white-sided dolphin, swims at the aquarium. Care experts are preparing for Piquet to give birth this spring. Veterinarians and other staff members are planning a sleepover at the aquarium when it is shutdown because of the upcoming NATO summit May 20-21, 2012. Shedd officials say they\'re worried about being able to get to the aquarium, which is in close proximity to the summit and protest route, quickly if needed. (AP Photo/Shedd Aquarium, Brenna Hernandez) (credit:AP)
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This undated photo provided by the Monterey Bay Aquarium shows a dwarf seahorse at the aquarium in Monterey, Calif. The government will study whether the inch-long seahorse the smallest of four species found in U.S. waters should have federal protection. The dwarf seahorse lives only in seagrass beds in the Gulf of Mexico. (AP Photo/Monterey Bay Aquarium, Randy Wilder) (credit:AP)
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Georgia Aquarium\'s resident pregnant beluga whale Maris swims in the aquarium\'s tank in Atlanta, Wednesday, April 11, 2012. Maris, is expected to give birth by June. She the first mammal to conceive at the downtown Atlanta attraction since it opened in 2005. (AP Photo/John Bazemore) (credit:AP)
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FILE - In this Jan. 12, 2010 file photo, Asian bighead carp swim in an exhibit at Chicago\'s Shedd Aquarium. Officials said Thursday, Feb. 23, 2012, the Obama administration will spend about $50 million this year to shield the Great Lakes from greedy Asian carp, including first-time water sampling to determine whether the destructive fish have established a foothold in Lakes Michigan and Erie. (AP Photo/M. Spencer Green, File) (credit:AP)
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Details of a whale is seen as the Virginia Aquarium Stranding Response Team performs a necropsy on the beach in Ocean View in Norfolk, Va., on Tuesday, Feb. 21, 2012. (AP Photo/The Virginian-Pilot, Preston Gannaway) (credit:AP)
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In this January 2012 photo provided by the Virginia Aquarium & Marine Science Center, a whale swims off the coast of Virginia. Whales by the dozen are wintering in the unseasonably warm waters off the resort city of Virginia Beach, Va., attracting a flotilla of recreational boats packed with sightseers and fishermen hoping to glimpse the big mammals gorging on tons of bait fish within sight of the oceanfront\'s high-rise hotels. (AP Photo/Virginia Aquarium & Marine Science Center) (credit:AP)
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In this photo taken Tuesday, Jan. 3, 2012, a baby Grouper fish swims in an aquarium to be farmed and sold to China under new trade deals benefiting the local fish farming villages in Beimen, southern Taiwan. The fish farmers on the terraced plains above Taiwan\'s west coast are riding a China boom, exporting tons of sweet, flaky milkfish to the mainland, thanks to export duties Beijing lowered to win over the island\'s voters. (AP Photo/Wally Santana) (credit:AP)
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This undated 2010 handout photo provided by AquaBounty Technologies shows two same-age salmon, a genetically modified salmon, rear, and a non-genetically modified salmon, foreground. Salmon that\'s genetically modified to grow twice as fast as normal could soon show up on your dinner plate if the company that makes the fish can stay afloat. (AP Photo/AquaBounty Technologies) (credit:AP)
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