The sharks that hunt in forests - Luka Seamus Wright
In the coastal waters of the Bahamas,
a young lemon shark is on the run
from a surprising predator:
an adult shark of her own species.
Half of her 17 siblings have already
been eaten by the older generation,
and it looks as though
she’s about to join them.
But just as the predator closes in,
she disappears into a thicket
of underwater roots,
safe in the refuge of the mangrove forest.
Forests don’t usually come to mind
as a habitat for sharks.
But various marine forests cover roughly
4.2 million square kilometers
of the planet,
providing food and shelter
for 35% of the world’s sharks.
Deadly tiger sharks
blend into seagrass meadows
before pouncing on sea cows
and sea turtles.
White sharks hunt down seals in forests
of kelp towering 65 meters tall.
And lemon sharks stalk
forests of mangroves—
the only trees on Earth
that live in the ocean.
All these ecosystems have their quirks,
but mangroves may be the most unique
of all.
Harboring life between their roots
and among their crowns,
mangroves function as an essential bridge
between land and sea.
And to survive between these worlds,
different mangrove species
have evolved various adaptations that
protect them and their resident sharks.
Taking root in the unstable ground
of muddy coastal regions is difficult,
so mangrove seedlings germinate
attached to their mother plant.
Once they've grown large enough
to survive on their own,
these partially developed plants begin
to ride the current.
Most take root nearby,
while some travel for several months
before landing in a different part
of the world.
Once they’ve settled down,
mangroves deploy tall, skinny stilt roots,
crutch-like prop roots
or wavy buttress roots,
to support themselves
in their unsteady terrain.
These newly established mangroves have
to contend with two additional problems:
seawater is high in dehydrating
and potentially toxic salt,
and the mud contains little to no oxygen.
This combination would be
lethal to most trees,
but mangroves make the most
of their marshy surroundings.
Rather than being completely buried,
mangrove roots
are largely above the ground.
This allows the microscopic pores on these
roots to take in oxygen during low tide
before closing to create
a waterproof seal during high tide.
Many mangroves also grow snorkel roots,
which can take in oxygen
through the same mechanism,
or directly produce it via photosynthesis.
To stop salt from entering their system,
some mangrove species use
incredibly fine filters in their roots.
Others concentrate salt
inside special cellular compartments,
bark or dying leaves, that then drop off.
Some species can even excrete
the excess minerals
through specially adapted salt glands.
All these processes make mangroves
more than a little salty,
but that doesn’t deter coastal life
from living in their nooks and crannies.
While birds nest among mangrove branches,
fish lay eggs amidst
their sprawling, complex root systems.
Symbiotic sponges and sea squirts
protect their host trees
from hungry woodboring crustaceans.
Crabs, snails, and shrimp
eat algae, mussels, barnacles,
and salty mangrove detritus.
These animals in turn feed fish,
which are devoured by shark pups
roaming the roots—
alongside occasional
vegetarian meals of seagrass.
But sharks aren’t just the beneficiaries
of marine forests,
they’re part of the glue
that holds them together.
Sharks limit the abundance of animals
which would otherwise overgraze
these essential plants.
Just as marine forests provide shelter
to vulnerable baby predators,
those predators grow up
to protect their forest homes.
Unfortunately, both sides of this delicate
balance are under threat.
Overfishing has decimated
shark populations worldwide,
and many marine forests are being polluted
or cut down for coastal development.
This destruction
is especially dangerous
because marine forests are one
of the single most important ecosystems
in mitigating climate change.
Mangroves and seagrasses trap
carbon between their roots,
and fast-growing kelps export vast
amounts of carbon to the deep ocean.
Together, marine forests sequester around
310 million tonnes of carbon every year,
capturing 3% of our annual
global carbon emissions.
So, like the sharks that inhabit them,
humans need to fight tooth and nail
to protect these essential ecosystems.
