What you can do with an extra jaw - Darien Satterfield
After stalking an unsuspecting cuttlefish
across the seafloor,
this moray eel is finally ready to pounce.
As the eel snags the mollusk
in its long teeth,
its prey struggles to escape.
But before it can wriggle away,
a second set of teeth lunge
from the eel's throat,
making short work of the captive
cuttlefish.
This adaptation is called
a pharyngeal jaw.
And while it might seem more fitting
for an alien parasite than a fish,
it's actually one of the most common
adaptations under the sea.
At some point millions of years ago,
early fish evolved a jointed oral jaw
from one of the bony arches
that supported their frontal gills.
This exterior, oral jaw was perfect
for capturing or chewing prey,
but these early fish had a problem.
They had no limbs to manipulate food
while eating,
making it easy for their mobile prey
to escape
before they took the first bite.
Essentially, it was impossible for fish
to capture and chew their prey
at the same time with just one set
of jaws.
So to hold their food and eat it too,
an even deeper gill arch evolved
into a secondary set of jaws.
Unlike oral jaws connected
by a bone joint,
these pharyngeal jaws were suspended
in muscle,
offering them a wide range of motion
to turn, tug, and tear on food.
Some pharyngeal jaws actively chew,
while others retract to pull chunks
of food down the throat.
Of course, the teeth a jaw has
also factor in here.
For example, an oral jaw with fangs
is exceptional for catching and holding
on to fast-moving prey.
But depending on what that prey is,
a fish might want sharp pharyngeal teeth
to tear through tissue,
flat pharyngeal molars to grind
plant matter,
or a powerful pharyngeal bite
to crush shelled prey.
Fortunately some fishes can change
their secondary teeth
to match a new diet over several years.
This flexibility allows fish to adapt
and find food in a huge variety
of aquatic environments.
Some pharyngeal jaws have formed
to eat food too hard for most fishes.
California Sheephead use pointed teeth
from their oral jaws
to pry urchins off rocks before crushing
their spiny exteriors
in strong pharyngeal jaws that have fused
with the bones of their skulls.
Other fish prioritize eating
huge quantities of food.
The pharyngeal jaws of grass carp
have two bone components
that move in unison to pull plant life
into their stomachs,
consuming over 18 kilograms
of seaweed a day.
Other pharyngeal jaws are adapted
to suit incredibly specific circumstances.
Consider this species of pearlfish that
lives inside sea cucumbers
and feeds on their guts.
Fish are the most diverse group
of vertebrates
with over 30,000 unique species,
and much of that diversity is due
to the hundreds of unique jaws
separating otherwise identical species.
Perhaps the best example of this
dental diversity can be seen in cichlids.
This family of fish is primarily found in
select African and South American lakes.
Typically, large lakes like these
would be occupied
by a handful of distantly related species,
each adapted to consume one
of the lake's limited food sources.
But here, almost every corner
of the ecosystem
is occupied by a different species
of cichlid.
There are over 1,700 cichlid species,
many almost identical save
for their uniquely adapted jaws
which have evolved to eat crustaceans,
mollusks, worms, algae, plankton,
and even the scales of other fish.
These oral adaptations are so essential
for survival,
that different species use their
pharyngeal jaws
to make unique mating sounds
that allow female cichlids to identify
males of their own species.
This technique limits
interspecies breeding,
and ensures the parent fish will
pass on their specialized jaws.
Scientists are still discovering
all the ways
this incredible skeletal mechanism
functions.
But given what we know already,
it's fair to say that for most fish,
two jaws are better than one.
