What you can do with an extra jaw - Darien Satterfield

譯者: Lilian Chiu 審譯者: Amanda Zhu

After stalking an unsuspecting cuttlefish across the seafloor,

在海底跟蹤一隻烏賊一段時間 且烏賊完全沒有起疑之後，

this moray eel is finally ready to pounce.

這隻海鰻終於準備好突襲。

As the eel snags the mollusk in its long teeth,

當海鰻用牠的長牙咬住 這隻軟體動物時，

its prey struggles to escape.

牠掙扎著想要脫身。

But before it can wriggle away,

可是在牠想擺脫之際，

a second set of teeth lunge from the eel's throat,

第二組牙齒從海鰻的喉嚨裡 往前攫住牠，

making short work of the captive cuttlefish.

快速地解決了被捕捉到的這隻烏賊。

This adaptation is called a pharyngeal jaw.

海鰻演化出的這種構造稱為「咽頜」。

And while it might seem more fitting for an alien parasite than a fish,

雖然咽頜聽起來像是 外星寄生蟲的構造而不是魚類的，

it's actually one of the most common adaptations under the sea.

但它其實是海底 最常見的演化結果之一。

At some point millions of years ago,

數百萬年前的某個時點，

early fish evolved a jointed oral jaw from one of the bony arches

早期的魚類原本用來支持 前鰓的其中一塊骨弓

演化成具有活動關節的「口頜」。

that supported their frontal gills.

This exterior, oral jaw was perfect for capturing or chewing prey,

這種外部的口頜

是用來捕捉或咀嚼獵物。

but these early fish had a problem.

但這些早期的魚類有個問題；

They had no limbs to manipulate food while eating,

在進食時，牠們沒有 可用來握住食物的前肢，

making it easy for their mobile prey to escape

牠們的獵物如果夠靈活，

就很容易在牠們 咬下第一口之前逃脫。

before they took the first bite.

Essentially, it was impossible for fish to capture and chew their prey

基本上，只有一組頜的魚類

不可能在咬住獵物的同時還能咀嚼。

at the same time with just one set of jaws.

So to hold their food and eat it too,

為了邊抓住食物邊吃，

an even deeper gill arch evolved into a secondary set of jaws.

一塊在口腔內更深處的腮弓

演化成了第二組頜。

Unlike oral jaws connected by a bone joint,

口頜是用骨頭關節來連結，

these pharyngeal jaws were suspended in muscle,

咽頜卻是直接嵌在肌肉中，

offering them a wide range of motion to turn, tug, and tear on food.

讓它們能夠做出各種動作，

來旋轉、拉扯，和撕裂食物。

Some pharyngeal jaws actively chew,

有些咽頜能咀嚼，

while others retract to pull chunks of food down the throat.

也有些咽頜會縮回，

把食物塊拉進喉嚨。

Of course, the teeth a jaw has also factor in here.

當然，咽頜上的牙齒 在這裡也很重要；

For example, an oral jaw with fangs is exceptional for catching and holding

比如，有尖牙的咽頜

能夠捕捉快速移動的獵物。

on to fast-moving prey.

But depending on what that prey is,

但要看獵物是什麼，

a fish might want sharp pharyngeal teeth to tear through tissue,

有些魚類可能會想要有 銳利的咽齒來撕裂組織

flat pharyngeal molars to grind plant matter,

或扁平的咽臼齒來磨碎植物，

or a powerful pharyngeal bite to crush shelled prey.

有些可能會有強大的咽齒咬力 來咬碎有殼的獵物。

Fortunately some fishes can change their secondary teeth

幸運的是，有些魚類 能在幾年的時間內

to match a new diet over several years.

將第二組牙齒換新， 以配合新的飲食。

This flexibility allows fish to adapt

這種彈性讓魚類能夠適應 各式各樣的水中環境，

and find food in a huge variety of aquatic environments.

並在這些環境中覓食。

Some pharyngeal jaws have formed to eat food too hard for most fishes.

有些咽頜的形成是為了能吃 對大部分魚類而言太硬的食物。

California Sheephead use pointed teeth from their oral jaws

美麗突額隆頭魚會用 牠們口頜上的的尖牙

to pry urchins off rocks before crushing their spiny exteriors

把海膽從石頭上撬下來，

然後再用和頭骨融合的強力咽頜

in strong pharyngeal jaws that have fused with the bones of their skulls.

粉碎掉牠們有刺的外殼。

Other fish prioritize eating huge quantities of food.

其他魚類演化的重點 在吃到大量的食物。

The pharyngeal jaws of grass carp have two bone components

草魚的咽頜有兩塊骨頭，

that move in unison to pull plant life into their stomachs,

這兩塊骨頭夠一起移動， 將植物拉進胃裡，

consuming over 18 kilograms of seaweed a day.

一天能吃掉超過十八公斤的海草。

Other pharyngeal jaws are adapted to suit incredibly specific circumstances.

其他的咽頜的演化 則是為了非常特定的用途。

Consider this species of pearlfish that lives inside sea cucumbers

例如這種隱魚

會住在海參裡面，

and feeds on their guts.

吃牠們的內臟維生。

Fish are the most diverse group of vertebrates

脊椎動物中最具多樣性的就是魚類，

with over 30,000 unique species,

有超過三萬種獨特的物種。

and much of that diversity is due to the hundreds of unique jaws

造就這多樣性的是

許多外型相同的魚類 卻有數百種各異的咽頜

separating otherwise identical species.

而被歸類為不同的物種。

Perhaps the best example of this dental diversity can be seen in cichlids.

最能呈現這種牙齒多樣性的例子

應該是慈鯛。

This family of fish is primarily found in select African and South American lakes.

這一科魚類主要出現在 非洲和南美洲的特殊湖泊中。

Typically, large lakes like these would be occupied

通常，這種大湖泊

會被少數種源迥異的物種給佔據，

by a handful of distantly related species,

each adapted to consume one of the lake's limited food sources.

每種都會適應演化， 變成以湖泊的單一食物來源為食。

But here, almost every corner of the ecosystem

但在這裡，幾乎生態系統的每個角落

is occupied by a different species of cichlid.

都被不同種的慈鯛佔據，

There are over 1,700 cichlid species,

種類超過一千七百種，

many almost identical save for their uniquely adapted jaws

許多種類幾乎一模一樣，

但為了吃甲殼動物、軟體動物、 蟲類、藻類、浮游生物

which have evolved to eat crustaceans, mollusks, worms, algae, plankton,

甚至其他魚類的鱗片

and even the scales of other fish.

而演化出不同的頷。

These oral adaptations are so essential for survival,

這種口腔的演化構造， 對於生存也十分重要，

that different species use their pharyngeal jaws

比如不同的物種會用牠們的咽頜

to make unique mating sounds

來發出獨一無二的求偶聲，

that allow female cichlids to identify males of their own species.

讓雌慈鯛可以找到 和自己同種的雄慈鯛。

This technique limits interspecies breeding,

這項技能會限制跨物種繁殖，

and ensures the parent fish will pass on their specialized jaws.

確保牠們有特有的頜 能夠遺傳給下一代。

Scientists are still discovering all the ways

科學家至今都還在探究

這種不凡的骨骼機制還有什麼作用，

this incredible skeletal mechanism functions.

But given what we know already, it's fair to say that for most fish,

但根據現有的知識來看，

對多數魚類而言， 兩組頜總是比一組好。

two jaws are better than one.