The one thing stopping jellyfish from taking over - Mariela Pajuelo & Javier Antonio Quinones
Over the past two decades, jellyfish
have begun to overwhelm our oceans.
In New Zealand, divers swat off millions
of stinging jellyfish
each no larger than a grain of pepper.
In Sweden, a massive cluster
of moon jellyfish
shut down one of the world’s largest
nuclear reactors
by clogging the plant’s essential pipes.
And in the Sea of Japan, thousands
of 200 kilogram Nomura’s jellyfish
with bells two meters in diameter
swarm fishermen,
snapping nets and devouring local fish.
Around the world, these animals
voraciously consume fish eggs and larvae,
routinely undermine
marine farming efforts,
and outcompete adult fish by consuming
the resources they need to survive.
If things stay on their
current trajectory,
we could be headed for a future where
the entire ocean is thick with jellyfish.
So, is there anything that can keep these
gelatinous creatures under control?
Enter, the humble sea turtle.
There are a wide range of marine animals
that feed on jellyfish,
but sea turtles are among
their most ancient predators.
And while every known species
of sea turtle preys on jellyfish
at some point in their lives,
none consume quite as many
as the leatherback.
Leatherbacks are the largest species
of sea turtle,
and they eat jellyfish almost exclusively,
devouring well over 1,000 metric tons of
them over their roughly 50-year lifespans.
This is particularly remarkable
because jellyfish are 95% water
and very low in calories,
so to maintain a healthy weight,
the average 500 kilogram leatherback
needs to eat roughly 400 kilograms
of jellyfish every day.
That’s roughly the same weight
as a grand piano.
And while some sea turtle species
have been documented
selectively eating their preys’
protein-rich gonads,
leatherbacks eat jellyfish whole—
mowing down huge swaths
of unsuspecting jellies.
Normally, jellyfish aren't
quite this defenseless.
Most species have tentacles covered
in cells called cnidocytes,
which contain venomous harpoons
coiled and ready to launch.
These barbed structures,
known as nematocysts,
are released on contact.
Jellyfish frequently use this sting
to paralyze and kill their food,
and it can also irritate the skin
of would-be intruders.
But it's completely useless
against sea turtles.
Most of these reptiles have thick
scales covered in keratin—
the same material that makes
up nails and claws.
This leathery armor protects
their skin while they hunt,
and any captured prey that tries
to escape is impaled
on the keratinized spikes
lining the leatherback’s esophagus.
For most sea turtles, these adaptations
make individual jellyfish easy prey.
However, a jelly population’s
true defense mechanism
is in how quickly they can reproduce.
Almost all jellyfish species have evolved
to reproduce both sexually and asexually,
allowing them to increase their numbers
with or without a partner.
In tropical environments,
jellyfish reproduce constantly
throughout the year.
But in more temperate climates,
species often reproduce all at once
in a massive bloom,
where jellies multiply at alarming rates.
And humanity is making these blooms
much more frequent.
Fertilizer runoff from farms
introduces chemicals
that simultaneously kill other fish
and prompt blooms.
High water temperatures
caused by climate change
speed up jellyfish reproduction
and extend the reproductive season.
Meanwhile, both marine
construction and pollution
dramatically increase the surface area
for jellyfish polyps
to attach, grow, and mature.
All these issues require a wide range
of policy-based solutions.
But one major way to prevent jellyfish
populations from getting out of hand
is to protect their natural predators—
many of whom are currently under threat.
Small-scale fisheries that are essential
to communities in Mexico and Peru
often use gillnets,
which unintentionally capture and kill
hundreds of sea turtles each year.
In the Eastern Pacific, these practices
could drive leatherbacks to extinction
in the next 60 years.
Fortunately, some researchers have already
developed inexpensive tools
to minimize these risks.
Attaching green LED lights to gillnets
has proven useful
for allowing sea turtles, dolphins,
and even seabirds to avoid fishing gear.
Solutions like this should
allow small-scale fishers
to support their communities
while minimizing their impact
on our leathery ocean defenders.
