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.