The big-beaked, rock-munching fish that protect coral reefs - Mike Gil
As the sun rises over a quiet coral reef,
one animal breaks the morning silence.
Named for their vibrant scales
and beak-like teeth,
these parrotfish are devouring
a particularly crunchy breakfast: rocks.
It may not be immediately clear
why any creature would take bites
out of the seafloor.
But the diet of these flashy foragers
actually plays a key role
in defending the coral reef’s
complex ecosystem.
Massive coral reefs
begin with tiny coral larvae,
which settle on the seafloor’s
hard surfaces
and metamorphasize into coral polyps.
Over time, these polyps
generate rock-like skeletons
made of calcium carbonate.
Together, colonies of polyps produce
large three-dimensional structures,
which form the basis
of an underwater metropolis.
These coral complexes are full
of nooks and crannies
that house and protect
countless life forms.
Even though coral reefs occupy less than
one percent of the ocean floor,
these dense ecosystems are home to more
than twenty-five percent of marine life.
Many fish use corals as shelters for
sleeping and to hide from large predators
between their trips foraging for seaweed.
As the primary food source for many
of the reef’s fish and invertebrates,
seaweed is vital to this ecosystem.
But in high densities,
seaweed can become problematic,
and even lethal to corals.
Seaweed grows on the same hard open
surfaces that coral larvae rely on,
and their growth prevents new coral
from settling and expanding.
These competitors have also evolved
a variety of ways to kill existing corals,
including smothering and abrasion.
Some seaweed species
even engage in chemical warfare—
synthesizing compounds
that destroy coral on contact.
This is where parrotfish come in.
Like many reef fish,
these colorful creatures eat seaweed.
But unlike their neighbors,
parrotfish can completely remove
even the tiniest scraps of seaweed
from the ocean floor.
Their so-called beak is actually a mosaic
of tightly-packed teeth
which can scrape and grind rock,
allowing them to consume every bit
of seaweed covering a stony surface.
This helps parrotfish reach seaweed
other fish can’t consume,
while simultaneously clearing out
open space for new corals
to settle and existing colonies to expand.
Eating rocks is just one way parrotfish
help manage seaweed.
Through a dynamic system
of social networks,
parrotfish can convey information
to other coral dwelling fish.
Each fish’s presence and simple routine
behaviors produce sensory information
that nearby fish can see, hear, or smell.
They can even detect changes in water
pressure produced by their neighbors
using a special sensory organ.
All these factors can inform
the behavior of nearby fish.
For example, a fish safely entering
an open feeding ground
and not getting attacked
means it’s safe to forage.
Conversely, a fish rapidly leaving
a location can provide an early warning
that a threat is approaching.
By simply trying to stay alive,
these reef fish can incidentally help
their neighbors survive—
and more of these fish
means less seaweed.
Unfortunately, human activities
over the last several decades
have disrupted almost every part
of this complex system.
In many coral reefs, overfishing
has reduced the number of parrotfish,
as well as other seaweed eaters,
such as surgeonfish and rabbitfish.
This has led to unchecked seaweed growth,
which threatens to degrade
entire coral reefs.
The parrotfish that remain
live in much smaller communities.
Their reduced numbers
can weaken their social network,
making surviving fish more timid
and less effective at controlling seaweed.
Today, climate change and pollution
are lowering coral’s natural defenses
while contributing to runaway
seaweed growth—
leaving reef ecosystems
more fragile than ever.
Our reefs are vitally important
to both marine and human life.
Their unparalleled biodiversity offers
unique opportunities for ecotourism,
sustainable fishing,
and scientific research,
while their rocky structures guard
coastlines from waves and storm surges.
Fortunately, continued research
into reef species
like the quirky and critical parrotfish
can inform new strategies
for preserving these essential ecosystems.
