But lately, it's gotten crowded and food is scarce.

這個池塘是這條魚 所知道的唯一家園。

Luckily, it has an option many don't:

但，最近，池塘變擁擠了， 食物也變稀少了。

as a walking catfish, it can dance its way out of the water

幸運的是，牠還有個選擇 是許多動物沒有的：

and onto bigger and better things.

身為鬍子鯰，牠可以 搖搖晃晃地離開水中，

However, it faces many challenges on its terrestrial journey:

去追求更大、更好的地方。

it's now in danger of suffocating, drying up,

然而，在這趟陸地旅程中， 牠要面對許多挑戰：

suffering physical damage from rough terrain,

牠現在面臨的危險包括窒息、乾掉，

and being hunted by land predators.

不平的地形造成身體傷害，

We think of fish as completely aquatic animals.

還可能被陸地上的捕食性動物獵殺。

But the walking catfish is just one of hundreds of fish species

我們認為魚類是完全 生活在水中的動物。

that are actually amphibious,

但其實有數百種兩棲魚類，

meaning that they possess adaptations that enable them to survive on land.

鬍子鯰只是其中之一，

Fish amphibiousness is a spectrum.

兩棲的意思是牠們的適應力

At one end are species like the mosquitofish

讓牠們在陸地上也能生存。

that'll only move on land when forced.

魚類的兩棲性其實是個光譜。

And at the other end are species like mudskippers

一端是像食蚊魚這類物種，

that nonchalantly hop around mudflats for days at a time.

只有在不得已時才會登上陸地。

But why do fish make the exodus from water to land?

另一端則是像彈塗魚這類物種，

And how do they cope with this drastic transition?

可以在淤泥灘裡蠻不在乎地 彈跳數日都沒關係。

If temperatures get too high for the mangrove rivulus

但魚類為什麼要從水中 外移到陸地上？

in the shallow tropical pools it inhabits,

牠們又如何應付這種巨大的轉變？

it'll flip itself onto a bank and cool off in the shade.

斑紋隱小鱂棲息在熱帶淺池中，

During the dry period,

如果牠覺得溫度太高，

it can survive for two months out of the water

牠會將自己輕彈到岸邊， 在陰影下冷卻。

by staying in moist environments.

乾旱時期，

Meanwhile, the eel catfish makes its onshore voyage

在離開水的情形下，

to satisfy its hearty craving for beetles.

牠能在潮濕環境中存活兩個月。

And for others, the terrestrial draw is more ritualistic.

至於線紋鰻鯰會上岸一遊，

Every year under the cover of night,

是為了滿足牠對甲蟲大餐的渴望。

masses of California grunion flop their way onto sandy beaches,

對其他魚類而言， 爬到陸地上只是儀式性的慣例。

where females deposit thousands of eggs into the sand

每年，在夜幕低垂時，

before re-entering the ocean.

成群的加州滑銀漢魚

Underwater, fish breathe with gills,

會靠著拍打一路爬到沙灘上，

which are feathery organs packed with blood vessels

雌魚會在沙灘上產卵數千枚，

that absorb dissolved oxygen from the water.

然後才重返海洋。

But in the open air, their gills collapse and are rendered useless,

在水中，魚類是用鰓呼吸，

so amphibious fishes need other ways to breathe.

鰓是柔軟的器官，內有血管，

The armored catfish's stomach is packed with blood vessels,

能吸收水中溶解的氧氣。

so it can gulp down air and breathe through its stomach lining.

但在開放的空氣中， 牠們的鰓就會失效，不再有用，

And lungfish, being related to the ancestors of all tetrapods,

因此兩棲魚類需要 其他的呼吸方式。

or four-limbed vertebrates, are equipped with true lungs.

甲鯰魚的胃部有血管，

They'll actually drown if they're kept underwater too long.

因此牠能大口吞下空氣，

Fish have thin, permeable skin that allows for essential compounds

並透過胃黏膜來呼吸。

to diffuse into and out of their bodies while they're underwater.

肺魚和四足動物（也稱為 四肢脊椎動物）的祖先是近親，

But this works against them on land as their bodily moisture

牠們具有真的肺。

diffuses into the air.

如果牠們在水裡待太久還會溺死。

To dodge dehydration, mudskippers roll in the mud like puppies.

魚類的皮膚很薄，具有可滲透性，

But the lungfish takes the cake:

在水中時，維生必要的化合物能夠

the rivers it inhabits disappear during dry seasons,

透過擴散的方式進出牠們的身體。

so it buries itself in the earth and coats its body in a mucus cocoon.

但這項特點在陸地上對牠們不利，

It can survive like this for years until being resuscitated

因為牠們身體的水分 會擴散到空氣中。

by the next big rainstorm.

為了避免脫水，

Amphibious fishes use powerful fins to move on land

彈塗魚會像小狗一樣在泥巴中打滾。

and clever tools to navigate as they go.

但拿到頭獎的是肺魚：

The Nopoli rock-climbing goby, no bigger than a few centimeters,

牠所棲息的河流 在乾旱季節時會消失，

scales hundred-meter-tall Hawaiian waterfalls,

所以牠會把牠自己埋在土裡，

inching its way up by alternately attaching the suction cups

用黏液做成的繭把身體包起來。

on its mouth and pelvic fins.

牠可以像這樣存活數年，

To find water while on land, the mummichog,

直到下一次暴雨時恢復。

like most amphibious fishes, is on the lookout for reflective surfaces.

在陸地上，兩棲魚類 靠強大的鰭來移動，

Other species, like mosquitofish,

並靠聰明的工具來找到方向。

exercise their inner ear to determine where they're oriented on a slope,

那不勒斯攀岩蝦虎魚 尺寸不到幾公分，

relying on the probability that they'll find water by moving downhill.

能攀上數百公尺高的夏威夷瀑布，

Our walking catfish, meanwhile,

不斷交換嘴部和腹鰭上的吸盤，

uses the taste buds that coat its body for navigation.

一吋一吋向上爬。

These taste buds are concentrated in its whiskers,

為了在陸地上找到水，

which whip through the air,

加拿大底鱂和大部分兩棲魚類一樣

sensing compounds that signal the proximity and quality

會注意尋找反射性的表面。

of nearby water— and prey.

其他物種，如食蚊魚，

The walking catfish will shimmy towards attractive volatile amino acids

在斜坡上會用內耳 來判斷牠們的方位，

while steering clear of foul waters emanating hydrogen sulfide.

仰賴朝下坡方向前進 較有可能找到水的機率。

While amphibious fishes face a multitude of new challenges upon leaving the water,

而我們的鬍子鯰

they've evolved ingenious ways to overcome them.

用身體外部覆蓋的味蕾來導航。

They're resilient in the face of droughts and floods

這些味蕾集中在牠的小鬍子上，

and have access to new prey as well as a plan B

小鬍子揮過空氣便能感測到化合物，

if they need to escape competitive, polluted, or unhealthy environments.

提供資訊讓牠知道 附近水域有多近、

While being a “fish out of water” is generally regarded as a bad thing,

品質如何——及有沒有獵物。

for these species, it offers an undisputed edge.

鬍子鯰會被易揮發的胺基酸吸引， 搖搖擺擺地移動過去，