The Energy Saving Secret of the Jellyfish
水母：節能游泳的高手
By James Gorman

The moon jelly, or common jellyfish, is often viewed as an ecological invader, a nuisance to swimmers, or worse. A moon jelly population explosion last month clogged the water intakes of a Swedish nuclear power plant, forcing it to shut down. The jellyfish, which can be 5 to 38 centimeters in diameter, is such a concern that engineers in South Korea designed seagoing, prowling robotic blenders to liquefy them.
海月水母又稱普通水母，經常被視為生態上的入侵者，對游泳者造成困擾的東西，甚或更大的威脅。上個月的一次水母數量大爆炸，塞住了瑞典一座核能發電廠的入水口，迫使電廠暫時關閉。水母的直徑介於5公分到30公分之間。由於牠們經常構成潛在的問題，南韓的工程師還特別設計了一種海上搜尋自動攪拌機，把牠們打碎成為液態。

But some researchers were interested in another aspect of these highly successful blobs: They may be the most efficient swimmers on the planet.
然而有些研究人員對這些生存得極為成功的團狀動物的另一面向卻很有興趣：牠們可能是地球上最有效率的游泳好手。

Brad J. Gemmell at the Marine Biological Laboratory at Woods Hole in Massachusetts, and several other scientists, analyzed the movement of the jellies as part of a Navy-funded project to look at what Dr. Gemmell called “nontraditional propulsion.”
美國麻州伍茲霍爾市海洋生物實驗室的吉梅爾與另外幾位科學家，在由美國海軍出資的一項研究計畫下，共同分析水母的動作，深入研究他所謂的「非傳統推進」原理。

The scientists used a new way of calculating energy called “cost of transport” that took account of what was happening throughout the two-phase swimming motion of the jellyfish. In the first phase, the jelly contracts its open bell and pushes water behind it, propelling itself forward. Then, the bell returns to its original shape and fills with water again.
他們使用一種名為「運輸成本」的能量計算新方法。這種方法考慮到水母分為兩個二階段的游泳動作過程中所發生的現象。在第一個階段，水母會收縮牠張開的傘膜，把海水往後推，利用這股推力前進。接著，傘膜恢復原狀，再度注滿海水。

Earlier studies had shown that the jelly got a second thrust during the rest and refill phase, but they had not calculated the jelly’s energy expenditure during that time. It turned out the jelly was not actually doing any work in that phase. Instead the elastic tissue in the bell acted like a rubber band, re-forming the bell. That action produced water movement under the jelly, called a vortex, that pushed it forward.
先前的一些研究顯示，水母在休息與再度注滿水的階段中，會第二次向前推進，然而研究人員並未計算水母在這個階段的能量支出。證據顯示，在這個階段中，水母其實並沒有做任何事。實情是，傘膜的彈性組織作用有如橡皮筋，可以使傘膜回復到原先的形狀。這個動作會使水母下面的水產生流動，稱為渦流，把水母往前推進。

The study found that the secondary push was responsible for about 30 percent of the distance traveled by the jellyfish. And it worked even with anesthetized jellies that were pushed through the water. The recovery phase and its kick were purely mechanical. “That’s what makes them so energy-efficient,” Dr. Gemmell said.
研究發現，第二次推進的距離大約占了水母前進總距離的30%。即使施以麻醉，在水中被推著走的水母也是如此。恢復的階段與前進是純粹機械式的。吉梅爾說：「這是讓牠能夠如此節能的關鍵。」

The finding offers some ideas about propulsion that could be useful to the Navy. This kind of low-energy, high efficiency thrust would not power any kind of fast-moving, quick-turning ocean craft, but it might be useful for monitoring devices that need to maintain a position or move at a slower pace.
這些研究結果提供了一些有關推進的概念，可能對美國海軍會有用處。這種低能量、高效率的推進不足以驅動任何行動迅速而且可以快速轉彎的海洋船艦，然而或許可用於必須保持在一定位置或行動比較緩慢的監測裝置。

It also could be one reason, Dr. Gemmell said, why the jelly has so much energy to spend on reproduction, producing those problematic population explosions.
吉梅爾說，水母能夠有那麼多精力用於繁殖，造就足以構成嚴重問題的數量大爆炸，這可能正是原因之一。

原文參照：
http://www.nytimes.com/2013/10/08/science/energy-saving-secret-of-jellyfish.html

Video：ScienceTake: The Jellyfish Engine: The moon jellyfish is slow, but there’s a reason for that. Its undulating pace allows it to swim more efficiently than just about anything in the ocean.
http://nyti.ms/17gScu0

2013-10-22聯合報/G5版/UNITEDDAILYNEWS 陳世欽譯 原文參見紐時週報十版下