第198期：海蛞蝓

最新 08-01

閱讀是我的習慣，英語是我的興趣，寫作是我的愛好。於是，我先後開了兩個微信公眾號，一為「英語王國」，輸入為主，主要是編輯、記錄和分享高質量的英文文章；二為「曹帥讀書筆記與生活隨筆」，輸出為主，主要是用英文整理學術筆記、發表所思所想和記錄生活軌跡。

讀萬卷書，行萬里路，嘗千家食，交一二友。月下，燈前，捧一本佳作，品一杯茗茶。窗外霓虹閃爍，川流不息。桌前清淡如水，萬物靜謐。任思緒馳騁，憑大浪滔天，我自埋頭於知識的象牙塔，享受一時之歡愉。

【中文導讀】

海蛞蝓是軟體動物門後鰓亞綱無楯目海兔科海洋腹足類的統稱，屬於淺海生活的貝類，是甲殼類軟體動物家族中的特殊成員，因為它們的貝殼已經退化為內殼，呈透明或半透明狀。

海蛞蝓的另一特殊之處是它是動植物嵌合體，它是科學家發現的第一種可生成植物色素葉綠素的動物。其實，它體內的葉綠素是從海藻那裡「偷來」的。但是，它是如何把海藻的葉綠素「偷來「，又是如何利用葉綠素進行光合作用，科學家對此還一無所知。對海蛞蝓的研究也許能對免疫學和基因療法做出貢獻。但最早一批研究海蛞蝓的科學家大都退休，又加之海蛞蝓目前變得越來越稀少，使得對它們的研究前景變得越來越迷茫。

具體內容請參考下文。

Elysia cholorotica, a sea slug（蛞蝓，俗稱鼻涕蟲）found off the U.S. East Coast, can steal photosynthetic（光合的；光合作用的）plastids（質體）from algae（藻類）and survive by basking in the sun（曬太陽）.PHOTOGRAPH BY PATRICK J. KRUG

Solar-Powered Slugs Hide Wild Secrets—But They』re Vanishing（消失）

The photosynthetic sea slug, which lives off the U.S. East Coast, is becoming almost too rare to research.

BY DOUGLAS MAIN

PUBLISHED JULY 22, 2018

Life has certain rules and patterns. Plants, with their incredible ability to harness（利用）the sun』s energy, don』t go roaming around. They don』tneedto. Butanimals, lacking the wondrous（奇異的；令人驚嘆的）power of photosynthesis（光合作用）, do. They trot（小跑）, slither（滑行；滑動）, flap（拍打）. They seek out plants and they eat them.

They most certainly donotphotosynthesize, the animal playbook（劇本）would seem to dictate（命令）. That』s a plant』s role.

But one small sea slug does not care for such rules, thank you very much.

These animals,Elysia chlorotica, which live off the U.S. East Coast, are not merely content to glide（滑動）about munching（用力咀嚼）algae.Instead, they steal the molecular（分子的）engines that allow plants to harvest solar energy. The slugs take up these mini-machines, called chloroplasts（葉綠體）, into their skin, which turns them emerald green.

Experiments have shown that this sea slug, which looks like a little leaf an inch or two in length, can go without eating for nine months or more, photosynthesizing with its stolen plant-parts as it basks in the sun.

「It』s unique; it』s controversial; it』s elusive（英文解釋：Something or someone that is elusive is difficult to find, describe, remember, or achieve）; it never eats,」says Patrick Krug, a biologist at California State University, Los Angeles. 「Basically your typical L.A. celebrity.」

Though other sea slugs can purloin（偷竊）chloroplasts and use them to catch some rays, none come close to doing it as well asElysia chlorotica.

For this reason, these animal-plant chimeras（動植物嵌合體）have attracted attention from scientists, who hope that researching them could have far-reaching applications, for instance in the fields of immunology（免疫學）and gene therapy（基因治療）.

But these special slugs are becoming increasingly rare, and the small number of experts who studied them have mostly retired or have moved on to other areas.

These sea slugs resemble little leaves, and are becoming very difficult to find.PHOTOGRAPH BY PATRICK J. KRUG

Research to date on these slugs has provided few answers but many tantalizing（引誘的；誘人的；使可望不可即而受折磨的）hints, suggesting further study could unearth（發掘）a wonderland of special and useful knowledge.

As yet, nobody knows how the sea slugs keep their chloroplasts running, which normally require a bevy of（一群）special proteins produced by thousands of algal genes, most of which these slugs apparently lack—though this area remains controversial.

And why don』t the chloroplasts hurt the slug? Photosynthesis produces free oxygen radicals（氧自由基）at levels most animals shouldn』t be able to tolerate.

Other mysteries abound. Why aren』t the chloroplasts destroyed in the slug gut? Why doesnt the immune system attack these foreign entitites? How do the slugs biochemically interact with them?

Only one research group—led by Sidney Pierce, a retired researcher at the University of South Florida—has collected the slugs in the last couple years, at a single salt marsh in Martha』s Vineyard, Massachusetts. Like others who』ve studied them, he doesn』t make this information public due to the animal』s scarcity.

Karen Pelletreau, a researcher at the University of Maine who used to work extensively with the animals, has only caught the invertebrates（無脊椎動物）at Martha』s Vineyard and a spot in Nova Scotia. She has searched several areas in Maine where they used to be found, to no avail（徒勞；空手而歸）.

To find them is 「difficult, very difficult,」 says Pelletreau』s former advisor and colleague Mary Rumpho-Kennedy, who studied the creatures for decades but retired several years ago. 「If you don』t know exactly where you』re looking and what you』re looking for, you won』t find it.」

Krug, who studies sea slugs, especially those that live on the West Coast, has looked for them near Woods Hole, Massachusetts, without luck.

He mainly studies a related genus（生物學的「屬」）calledAlderia, which eat the same algae and likeE. chloroticalive in salt marshes. These areas and their animal residents are vulnerable to sea level rise, climate changes brought by warming, and development.

「This habitat may be suffering or growing increasingly ephemeral（短暫的；朝生暮死的）, more than people appreciate,」 Krug notes. Nobody, he says, has conducted population studies on these animals.

The green critters（=creatures）are also quite tough to raise in the lab. Adults need to be cared for well enough to breed, typically living less than a year. The eggs hatch into a free-swimming form that eats several different kinds of algae. Then, as young adults, they begin munching a different food,Vaucheria litorea, a slow-growing algae that』s tricky to culture and raise.

「They』ll eat it literally faster than they we could ever grow it,」 says Pierce, who』s studied the animals for more than 30 years. 「It』s like having teenagers in the house.」

Though two groups—the team of Rumpho and Pelletreau on the one hand, Pierce on the other—have bred the slugs and raised successive generations in the lab, it is difficult enough that wild collection is easier. They also produce prodigious（大量的；驚人的）levels of mucus（粘液）, which complicates DNA and molecular analysis.

However the slugs manage to maintain their chloroplasts, it must involve using algal genes or gene products in a novel, unknown way.

Studies led by Pierce suggest the slug』s genome contains genes transferred from algae, an incredible biochemical feat（功績；壯舉）that could have relevance for genetic manipulation in other animals like humans. But that finding has been disputed by Rumpho and colleagues, as well as some European researchers.

Debashi Bhattacharya, a researcher at Rutgers University, led a recent study inMolecular Biology and Evolutionshowing that the animals express genes that tamp down（用多次輕擊以向下壓緊）their immune system when they take in the chloroplasts, and increase the activity of those genes associated with neutralizing reactive chemicals.

These patterns are reminiscent of（使人想起）the biochemical interactions involved in the symbiotic relationship（共生關係）between coral（珊瑚蟲）and their photosynthetic algae.

Further research into these similarities, and what implications that might have for understanding this vital symbiosis, presents just one promising area for future exploration. He, like others, hopes work will continue onElysia chlorotica—though he doesn』t have plans to do so.

「To follow up on this, somebody would have to find a way to raise a lot of these sea slugs,」 he says. 「The scarcity of the animal is the problem.」

Source:National Geographic

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