These animals are also plants … wait, what? - Luka Seamus Wright

譯者: Camila Lin 審譯者: Bruce Sung

Take a good look at this slug.

好好看看這隻海蛞蝓。

No, not that— that’s a leaf.

不，不是那個，那是一片葉子。

This slug.

這隻才是海蛞蝓。

There we go.

我們開始吧。

Elysia chlorotica may not look like much—

綠葉海天牛看起來 可能真的不怎麼樣，

okay, it looks like a bright green leaf—

好吧，牠看起來 就像一片鮮豔的綠葉，

but it’s one of the most extraordinary creatures around.

但牠是這附近最驚人的生物之一。

Living in salt marshes along the east coast of North America,

牠住在北美東部沿岸的鹽沼中，

it can go about a year without eating.

可以幾乎整整一年都不進食。

During that time, it lives like a plant.

在不進食的時候，它活得就像植物。

Generally speaking, animals are what are called heterotrophs,

通常而言，動物都是 所謂的「異養生物」，

meaning they can’t produce their own food— they’re consumers of other life.

也就是無法自己製造食物， 需要以其他生物為食的生物。

Plants, meanwhile, are autotrophs, or producers:

而植物則是「自養生物」， 也可稱為「生產者」：

they can synthesize their own fuel from sunlight, CO2,

它們能以陽光、二氧化碳 及其他無機化合物為原料，

and other inorganic compounds.

合成自己所需的養分。

Plants do this by using organelles called chloroplasts,

植物藉著使用名為 葉綠體的胞器來完成這件事。

which give them their bright colors

葉綠體賦予植物鮮豔的顏色，

and convert sunlight into food through photosynthesis.

並透過光合作用， 將陽光轉化為養分。

Elysia is what’s called a mixotroph:

海天牛則是所謂的 「混合營養生物」：

it can both consume food, like animals,

牠既能像動物一樣進食，

and produce its own through photosynthesis, like plants.

也能像植物一樣， 透過光合作用製造養分。

In fact, Elysia steals its ability to photosynthesize

事實上，海天牛的光合作用能力，

from the algae it eats by piercing the algal cells

是從牠吃掉的藻類那裏偷來的。

with specialized pointy teeth, called radula.

牠會用名為「齒舌」的 特化尖牙，穿刺藻細胞。

It sucks the cell empty and digests most of its contents,

牠會將藻細胞吸乾， 消化大部分的內容物，

but the chloroplasts remain intact.

留下完好無缺的葉綠體。

They’re incorporated into the epithelial cells lining Elysia’s digestive system

它們會被海天牛 消化系統中的上皮細胞整合，

that branches throughout its flat body.

而消化系統則分支遍佈於 扁平的身體各處。

This makes the slug look even more leaflike,

這讓海天牛看起來更像一片葉子，

providing camouflage as well as food.

提供食物的同時，也為牠加上偽裝。

As incredible as this adaptation is,

儘管這種適應力讓人難以置信，

there are more than 70 species of slug that steal chloroplasts from their food.

但 70 多種蛞蝓都能 自食物中偷走葉綠體。

What makes Elysia and a few closely related species

綠葉海天牛及其他生活於地中海 和太平洋的類似物種的特別之處，

in the Mediterranean and Pacific unique

is how long they can hold onto chloroplasts—

在於牠們能將葉綠體保留多久。

most other slugs keep them for a few weeks at most.

大部分的蛞蝓， 最多只能保留幾個禮拜。

This longevity seems to be due to the survival abilities

而這種長存的能力，似乎 和色素體及蛞蝓的存活能力相關。

of both plastids and slugs.

Specifically, the chloroplasts of certain algae

說得更清楚一點，某些藻類的葉綠體

can repair their own light-harvesting systems,

可以自行修復光合系統，

while most chloroplasts are thought to rely on their host cell

但我們認為，大多數的葉綠體 都得靠它們的宿主細胞

and its genes for repairs.

以及基因來修復。

This makes the chloroplasts able to sustain themselves

這讓葉綠體

能在海天牛體內存活更久。

for longer inside the slug.

Meanwhile, the slug adjusts its gene expression

同時，綠葉海天牛也會 調整自己的基因表現，

to improve its relationship with the chloroplasts

改善自己和葉綠體的關係，

and removes damaged plastids

並移除受損的色素體，

to avoid accumulation of potentially damaging chemicals.

避免在體內堆積 具潛在傷害性的化學物質。

Though few species can steal organelles from another species’ cell,

雖然能偷取其他物種 細胞內胞器的物種不多，

these slugs are far from alone in getting an assist from plants.

這些海天牛卻不是唯一 從植物那裏獲得幫助的生物。

Organisms as diverse as corals,

許多生物，例如珊瑚、

giant clams and sponges have symbiotic algae living inside their cells,

硨磲貝及海棉， 細胞內都存有共生藻類，

supplying them with organic compounds through photosynthesis.

藉光合作用， 為牠們提供有機化合物。

In turn, they supply their little helpers with shelter and inorganic compounds.

而牠們則提供庇護和 無機化合物給藻類小幫手。

Some of these mixotrophs even transmit the algae to their offspring.

有些混合營養生物， 還會將共生藻傳給下一代。

Without the aid of these algae,

若少了這些共生藻的幫助，

filter-feeding corals, clams, and sponges would not gain enough nutrition

濾食性的珊瑚、貝類和海綿， 就無法獲得足夠營養，

in the nutrient-poor tropical ocean,

在缺乏營養鹽的熱帶海域生活，

and the dazzling coral reefs they build simply would not exist.

也根本不會出現 牠們堆疊出的壯麗珊瑚礁。

Mixotrophy even cuts both ways:

但混合營養的進食方式有好有壞。

an alga called Tripos furca can consume several microscopic animals a day,

一種叫做叉狀角藻的藻類， 能每天吞噬數個微生物，

allowing it to survive in darkness for weeks.

讓自己在黑暗中存活數星期。

Tripos is in turn eaten by other mixotrophic algae,

但叉狀角藻也會被 其他混合營養藻類吃掉，

providing frequent opportunity for exchange of organelles

提供頻繁交換胞器的機會，

such as chloroplasts.

例如葉綠體。

This seems to allow some algae to survive in parts of the dark ocean

這似乎能讓某些藻類 生存於黑暗海洋的部分地區，

such as the Mariana Trench,

例如馬里亞納海溝，

which plants otherwise wouldn't be able to inhabit.

若非如此，植物便無法在那裏存活。

The processes by which Elysia becomes photosynthetic

綠葉海天牛演化出 光合作用能力的過程，

and Tripos switches between feeding modes

以及叉狀角藻切換進食模式的過程，

are reminiscent of what scientists believe led to the origin of all plants.

讓人想起科學家提出的植物起源。

Single-celled animals preyed on cyanobacteria.

單細胞動物捕食藍綠藻。

Some of these tiny plants were not digested and lived on in the animal cells,

某些藍綠藻並沒有被消化， 繼續活在單細胞動物體內，

eventually giving rise to chloroplasts.

最終產生了葉綠體。

But these first eukaryotic plants were soon consumed by other animals,

但這些首批真核植物， 一下子就又被其他動物吃掉，

which hijacked the precious chloroplast, just like Elysia.

牠們就像海天牛一樣， 搶走了珍貴的葉綠體。

And following the example of eating and being eaten,

就這樣，不斷進行吃與被吃的循環，

we’ve seen in the case of Tripos,

我們已經看過真叉角藻的例子，

this chloroplast heist happened up to three times,

「葉綠體搶奪戰」最多發生了三次，

giving rise to plastids with four membranes

產生了有四層膜的色素體，

and the ocean’s most productive plants and forests.

以及海洋中最多產的植物與森林。