The fish that walk on land - Noah R. Bressman

This pond is the only home this fish has ever known.

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

Luckily, it has an option many don’t:

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

and onto bigger and better things.

However, it faces many challenges on its terrestrial journey:

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

suffering physical damage from rough terrain,

and being hunted by land predators.

We think of fish as completely aquatic animals.

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

that are actually amphibious,

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

Fish amphibiousness is a spectrum.

At one end are species like the mosquitofish

that’ll only move on land when forced.

And at the other end are species like mudskippers

that nonchalantly hop around mudflats for days at a time.

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

And how do they cope with this drastic transition?

If temperatures get too high for the mangrove rivulus

in the shallow tropical pools it inhabits,

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

During the dry period,

it can survive for two months out of the water

by staying in moist environments.

Meanwhile, the eel catfish makes its onshore voyage

to satisfy its hearty craving for beetles.

And for others, the terrestrial draw is more ritualistic.

Every year under the cover of night,

masses of California grunion flop their way onto sandy beaches,

where females deposit thousands of eggs into the sand

before re-entering the ocean.

Underwater, fish breathe with gills,

which are feathery organs packed with blood vessels

that absorb dissolved oxygen from the water.

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

so amphibious fishes need other ways to breathe.

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

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

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

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

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

Fish have thin, permeable skin that allows for essential compounds

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

But this works against them on land as their bodily moisture

diffuses into the air.

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

But the lungfish takes the cake:

the rivers it inhabits disappear during dry seasons,

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

It can survive like this for years until being resuscitated

by the next big rainstorm.

Amphibious fishes use powerful fins to move on land

and clever tools to navigate as they go.

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

scales hundred-meter-tall Hawaiian waterfalls,

inching its way up by alternately attaching the suction cups

on its mouth and pelvic fins.

To find water while on land, the mummichog,

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

Other species, like mosquitofish,

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

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

Our walking catfish, meanwhile,

uses the taste buds that coat its body for navigation.

These taste buds are concentrated in its whiskers,

which whip through the air,

sensing compounds that signal the proximity and quality

of nearby water— and prey.

The walking catfish will shimmy towards attractive volatile amino acids

while steering clear of foul waters emanating hydrogen sulfide.

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

they’ve evolved ingenious ways to overcome them.

They’re resilient in the face of droughts and floods

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

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

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

for these species, it offers an undisputed edge.