Education and Communications

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.
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