[OANNES Foro] The secret lives of jellyfish

[Mario Cabrejos]

Nature 531, 432–434

(24 March 2016)

doi:10.1038/531432a

 

Long regarded as minor players in ocean ecology, jellyfish are actually
important parts of the marine food web

The secret lives of jellyfish

 
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#auth-1> Garry Hamilton

22 March 2016

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WS-20160324
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&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=883109801&spReportId
=ODgzMTA5ODAxS0

 

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Jeff Wildermuth/Natl Geogr. Creative

Moon jellyfish (Aurelia aurita) contain more calories than some other
jellyfish.

 

Jennifer Purcell watches intently as the boom of the research ship Skookum
slowly eases a 3-metre-long plankton net out of Puget Sound near Olympia,
Washington. The marine biologist sports a rain suit, which seems odd for a
sunny day in August until the bottom of the net is manoeuvred in her
direction, its mesh straining from a load of moon jellyfish (Aurelia
aurita). Slime drips from the bulging net, and long tentacles dangle like a
scene from an alien horror film. But it does not bother Purcell, a
researcher at Western Washington University's marine centre in Anacortes.
Pushing up her sleeves, she plunges in her hands and begins to count and
measure the messy haul with an assuredness borne from nearly 40 years
studying these animals.

Most marine scientists do not share her enthusiasm for the creatures.
Purcell has spent much of her career locked in a battle to find funding and
to convince ocean researchers that jellyfish deserve attention. But she
hasn't had much luck. One problem is the challenges that come with trying to
study organisms that are more than 95% water and get ripped apart in the
nets typically used to collect other marine animals. On top of that, outside
the small community of jellyfish researchers, many biologists regard the
creatures as a dead end in the food web — sacs of salty water that provide
almost no nutrients for predators except specialized ones such as
leatherback sea turtles (Dermochelys coriacea), which are adapted to consume
jellies in large quantities.

“It's been very, very hard to convince fisheries scientists that jellies are
important,” says Purcell.

But that's starting to change. Among the crew today are two fish biologists
from the US National Oceanic and Atmospheric Administration (NOAA) whose
research had previously focused on the region's rich salmon stocks. A few
years ago, they discovered that salmon prey such as herring and smelt tend
to congregate in different areas of the sound from jellyfish
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b1> 1 and they are now trying to understand the
ecological factors at work and how they might be affecting stocks of
valuable fish species. But first, the researchers need to know how many
jellyfish are out there. For this, the team is taking a multipronged
approach. They use a seaplane to record the number and location of jellyfish
aggregations, or 'smacks', scattered about the sound. And on the research
ship, a plankton net has been fitted with an underwater camera to reveal how
deep the smacks reach.

http://www.nature.com/polopoly_fs/7.35212.1458646521!/image/017_Aerial_Kremb
s.jpg_gen/derivatives/landscape_630/017_Aerial_Krembs.jpg

Christopher Krembs

Scientists with the National Oceanic and Atmospheric Administration use a
plane to survey swarms of moon jellyfish, which show up as curvy white
streaks in an arm of Puget Sound, Washington.

 

Correigh Greene, one of the NOAA scientists on board, says that if salmon
populations are affected in some way by jellyfish, “then we need to be
tracking them”.

>From the fjords of Norway to the vast open ocean waters of the South
Pacific, researchers are taking advantage of new tools and growing
<http://www.nature.com/news/ocean-sciences-follow-the-fish-1.13886> concern
about marine health to probe more deeply into the roles that jellyfish and
other soft-bodied creatures have in the oceans. Initially this was driven by
reports of
<http://www.nature.com/news/coastal-havoc-boosts-jellies-1.16236> unusually
large jellyfish blooms wreaking havoc in Asia, Europe and elsewhere, which
triggered fears that
<http://www.nature.com/news/marine-ecology-attack-of-the-blobs-1.9929>
jellyfish were taking over the oceans. But mounting evidence is starting to
convince some marine ecologists that gelatinous organisms are not as
irrelevant as previously presumed.

Some studies show that the animals are important consumers of everything
from microscopic zooplankton to small fish, others suggest that jellies have
value as prey for a wide range of species, including penguins, lobsters and
bluefin tuna. There's also evidence that they might enhance the flow of
nutrients and energy between the species that live in the sunlit surface
waters and those in the impoverished darkness below.

“We're all busy looking up at the top of the food chain,” says Andrew Jeffs,
a marine biologist at the University of Auckland in New Zealand. “But it's
the stuff that fills the bucket and looks like jelly snot that is actually
really important in terms of the planet and the way food chains operate.”


A mass of mush


The animals in question are descendants of some of Earth's oldest
multicellular life forms. The earliest known jellyfish fossil dates to more
than 550 million years ago, but some researchers estimate that they may have
been around for 700 million years, appearing long before fish.

They're also surprisingly diverse. Some are tiny filter feeders that can
prey on the zooplankton that few other animals can exploit. Others are giant
predators with bells up to two metres in diameter and tentacles long enough
to wrap around a school bus — three times. Jellyfish belong to the phylum
Cnidaria and have stinging cells that are potent enough in some species to
kill a human. Some researchers use the term jellyfish, or 'jellies' for
short, to refer to all of the squishy forms in the ocean. But others prefer
the designation of 'gelatinous zooplankton' because it reflects the amazing
diversity among these animals that sit in many different phyla: some species
are closer on the tree of life to humans than they are to other jellies.
Either way, the common classification exists mainly for one dominant shared
feature — a body plan that is based largely on water.

This structure can make gelatinous organisms hard to see. Many are also
inaccessible, living far out at sea or deep below the light zone. They often
live in scattered aggregations that are prone to dramatic population swings,
making them difficult to census. Lacking hard parts, they're extremely
fragile.

“It's hard to find jellyfish in the guts of predators,” says Purcell.
“They're digested very fast and they turn to mush soon after they're eaten.”

http://www.nature.com/polopoly_fs/7.35164.1458574125!/image/nature_NF_Jellyf
ish-calories_26.03.2016_WEB.png_gen/derivatives/landscape_630/nature_NF_Jell
yfish-calories_26.03.2016_WEB.png

Sources: J. Spitz et al. J. Mar. Sci. 67, 909–915 (2010); T. K. Doyle et al.
J. Exp. Mar. Biol. Ecol. 343, 239–252 (2007)

 

For most marine biologists, running into a mass of jellyfish is nothing but
trouble because their collection nets get choked with slime. “It's not just
that we overlooked them,” says Jonathan Houghton at Queen's University
Belfast, UK. “We actively avoided them.”

But over the past decade and a half, jellyfish have become increasingly
difficult to ignore. Enormous blooms along the Mediterranean coast, a
frequent summer occurrence since 2003, have forced beaches to close and left
thousands of bathers nursing painful stings. In 2007, venomous jellyfish
drifted into a salmon farm in Northern Ireland, killing its entire stock of
100,000 fish. On several occasions, nuclear power plants have temporarily
shut down operations owing to jelly-clogged intake pipes.

The news spurred scientists to take a closer look at the creatures. Marine
biologist Luis Cardona at the University of Barcelona in Spain had been
studying mostly sea turtles and sea lions. But around 2006, he shifted some
of his attention to jellyfish after large summer blooms of mauve stingers
(Pelagia noctiluca) had become a recurring problem for Spain's beach-goers.
Cardona was particularly concerned by speculation that the jellyfish were on
the rampage because overfishing had reduced the number of predators. “That
idea didn't have very good scientific support,” he says. “But it was what
people and politicians were basing their decisions on, so I decided to look
into it.”

For this he turned to stable-isotope analysis, a technique that uses the
chemical fingerprint of carbon and nitrogen in the tissue of animals to tell
what they have eaten. When Cardona's team analysed 20 species of predator
and 13 potential prey, it was surprised to find that jellies had a major
role in the diets of bluefin tuna (Thunnus thynnus), little tunny (Euthynnus
alletteratus) and spearfish (Tetrapturus belone)
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b2> 2. In the case of juvenile bluefins,
jellyfish and other gelatinous animals represented up to 80% of the total
food intake. “According to our models they are probably one of the most
important prey for juvenile bluefin tuna,” says Cardona.


Jellyfish in the food web


At the bottom of a fjord in Norway, an underwater camera captures hagfish,
crabs, lobsters and other creatures consuming dead jellyfish. Courtesy:
Andrew K. Sweetman

 

Some researchers have challenged the findings, arguing that stable-isotope
results can't always distinguish between prey that have similar diets —
jellyfish and krill both eat phytoplankton, for instance. “I'm sure it's not
true,” Purcell says of the diet analysis. Fast-moving fish, she says, “have
the highest energy requirements of anything that's out there. They need fish
to eat — something high quality, high calorie.”

But Cardona stands by the results, pointing out that stomach-content
analyses on fish such as tuna have found jellyfish, but not krill. What's
more, he conducted a different diet study
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b3> 3 that used fatty acids as a signature,
which supported his earlier results on jellyfish, he says. “They're probably
playing a more relevant role in the pelagic ecosystem of the western
Mediterranean than we originally thought.”

Researchers are reaching the same conclusion elsewhere in the world. On an
expedition to Antarctica in 2010–11, molecular ecologist Simon Jarman
gathered nearly 400 scat samples to get a better picture of the diet of
Adélie penguins (Pygoscelis adeliae), a species thought to be threatened by
global warming. Jarman, who works at the Australian Antarctic Division in
Kingston, reported in 2013 that DNA analysis of the samples revealed that
jellyfish are a common part of the penguin's diet
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b4> 4. Work that has yet to be published
suggests the same is true for other Southern Ocean seabirds.

“Albatrosses, gentoo penguins, king penguins, macaroni and rockhopper
penguins — all of them eat jellyfish to some extent,” says Jarman (see
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#cuisine> 'Lean cuisine'). “Even though jellyfish
may not be the most calorifically important food source in any area, they're
everywhere in the ocean and they're contributing something to many top-level
predators.”

And some parts of jellyfish hold more calories than others. Fish have been
observed eating only the gonads of reproductive-stage jellyfish, suggesting
a knack for zeroing in on the most energy-rich tissues.

Through DNA analyses, researchers are also discovering more about how
jellyfish function as refuges in the open ocean. Scientists have long known
that small fish, crustaceans and a wide range of other animals latch on to
jellyfish to get free rides. But in the past few years, it has become clear
that the hitchhikers also dine on their transport.

In the deep waters of the South Pacific and Indian oceans, Jeffs has been
studying the elusive early life stages of the spiny lobster (Panulirus
cygnus). During a 2011 plankton-collecting expedition 350 kilometres off the
coast of Western Australia, he and his fellow researchers hauled in a large
salp (Thetys vagina), a common barrel-shaped gelatinous animal. The catch
also included dozens of lobster larvae, including six that were embedded in
the salp itself. DNA analysis of the lobsters' stomach glands revealed that
the larvae had been feeding on their hosts
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b5> 5.

Jeffs now suspects that these crustaceans, which support a global fishery
worth around US$2 billion a year, depend heavily on this relationship. “What
makes the larvae so successful in the open ocean,” he says, “is that they
can cling to what is basically a big piece of floating meat, like a
jellyfish or a big salp, and feed on it for a couple of weeks without
exerting any energy at all.”

 

http://www.nature.com/polopoly_fs/7.35213.1458646584!/image/021_Aerial_Kremb
s_Field.jpg_gen/derivatives/landscape_630/021_Aerial_Krembs_Field.jpg

Christopher Krembs

Large masses of moon jellyfish float along the surface in an inlet near
Olympia, Washington.

 


Where did they go?


Researchers are starting to recognize that jellyfish are important for other
reasons, such as transferring nutrients from one part of the ocean to
another. Biological oceanographer Andrew Sweetman at the International
Research Institute of Stavanger in Norway has seen this in his studies of
'jelly falls', a term coined to describe what happens when blooms crash and
a large number of dead jellies sink rapidly to the sea floor.

In November 2010, Sweetman began to periodically lower a camera rig 400
metres to the bottom of Lurefjorden in southwestern Norway to track the fate
of this fjord's dense population of jellyfish
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b6> 6. Previous observations from elsewhere had
suggested that dead jellies pile up and rot, lowering oxygen levels and
creating toxic conditions. But Sweetman was surprised to find almost no dead
jellies on the sea floor. “It didn't make sense.”

He worked out what was happening in 2012, when he returned to the fjord and
lowered traps baited with dead jellyfish and rigged with video cameras. The
footage from the bottom of the fjord showed scavengers rapidly consuming the
jellies. “We had just assumed that nothing was going to be eating them,” he
says.

Back on land, Sweetman calculated
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b7> 7 that jelly falls increased the amount of
nitrogen reaching the bottom by as much as 160%. That energy is going back
into the food web instead of getting lost through decay, as researchers had
thought. He's since found similar results using remotely operated vehicles
at much greater depths in remote parts of the Pacific Ocean. “It's
overturning the paradigm that jellyfish are dead ends in the food web,” says
Sweetman.

Such discoveries have elicited mixed responses. For Richard Brodeur, a NOAA
fisheries biologist based in Newport, Oregon, the latest findings do not
change the fact that fish and tiny crustaceans such as krill are the main
nutrient source for most of the species that are valued by humans. If
jellyfish are important, he argues, it is in the impact they can have as
competitors and predators when their numbers get out of control. In one of
his current studies, he's found that commercially valuable salmon species
such as coho (Oncorhynchus kisutch) and Chinook (Oncorhynchus tshawytscha)
that are caught where jellyfish are abundant have less food in their
stomachs compared with those taken from where jellies are rare, suggesting
that jellyfish may have negative impacts on key fish species. “If you want
fish resources,” he says, “having a lot of jellyfish is probably not going
to help.”

But other researchers see the latest findings as reason to temper the
growing vilification of jellyfish. In a 2013 book chapter
<http://www.nature.com/news/the-secret-lives-of-jellyfish-1.19613?WT.ec_id=N
EWS-20160324&spMailingID=50997843&spUserID=MTc2NjE2MjAwMAS2&spJobID=88310980
1&spReportId=ODgzMTA5ODAxS0#b8> 8, Houghton and his three co-authors
emphasized the positive side of jellies in response to what they saw as “the
flippant manner in which wholesale removal of jellyfish from marine systems
is discussed”. As scientists gather more data, they hope to get a better
sense of exactly what role jellyfish have in various ocean regions. If
jellies turn out to be as important as some data now suggest, the population
spikes that have made the headlines in the past decade could have much wider
repercussions than previously imagined.

Back in Puget Sound, Greene is using a camera installed on a net to gather
census data on a jellyfish smack. He watches video from the netcam as it
slowly descends through a dense mass of creamy white spheres. At a depth of
around 10 metres, the jelly curtain finally begins to thin out. Later,
Greene makes a crude estimate. “Two point five to three million,” he says,
before adding after a brief pause, “that's a lot of jellyfish.”

A more careful count will come later. Right now there's plenty of slime to
be hosed off the back deck. Once that's taken care of, the ship's engines
come to life. The next jellyfish patch awaits.


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