Sponges help coral reefs thrive in ocean
deserts

By James Morgan
Science reporter, BBC News

The ultimate recyclers – sponges soak up nutrients in
seawater and turn them into food for reef organisms

The mystery of how coral reefs thrive in “ocean deserts”
has been solved, scientists say.

Reefs are among Earth’s most vibrant ecosystems, yet they flourish in waters
lacking nutrients – a phenomenon known as Darwin’s Paradox.

A team found that sponges keep the reef alive – by recycling vast amounts of
organic matter to feed snails, crabs and other creatures.

Writing in
Science, they hope their findings will aid conservation.

Sponges recycle nearly ten times as much matter as bacteria, and produce as
much nutrition as all the corals and algae in a reef combined, the scientists
calculate.

They are the “unsung heroes” of the reef community, said lead author Jasper
de Goeij, an aquatic ecologist at the University of Amsterdam.

“Up until now no-one has really paid sponges much attention. They look nice,
but everybody was more interested in corals and fish,” he told BBC News.

The scientists tested sponges in a Caribbean
reef

“But it turns out that sponges are big players – and they deserve credit for
their role.

“If you want a reef which is colourful and biodiverse, you need a ‘sponge
loop’ to maintain it.”

It was during his voyage on the Beagle that Charles Darwin famously observed
that tropical reefs are like oases in a desert.

They are surrounded by waters lacking nitrogen and phosphorus – the building
blocks of life – which ought to prohibit their growth.

And since corals release up to half their organic matter into seawater, reefs
need a system to recover these nutrients and recycle them into the
ecosystem.

Bacteria do part of the job, but are not abundant enough to service the
chemical dependencies of a whole teeming reef community.

Sugar daddies

Sponges (poriferans) are filter feeders which live in rock crevices, sucking
up plankton and organic matter released into the sea by corals.

The idea that they could be a missing link in the reef food cycle has
been proposed before.

But it was not clear how much nutrition they could supply, nor how exactly
they feed their reef neighbours – worms, crustaceans and other sea floor
foragers.

On the Caribbean island of Curacao, de Goeij and his team studied four common
species of sponges – first in laboratory aquariums, then in a natural reef where
the scientists sealed off a cavity.

Sponges are not usually the stars of conservation
campaigns – but they hold reef ecosystems together

They fed the poriferans with labelled sugars – and traced these molecules on
their journey.

First the sugars were absorbed from the water by the sponges, then quickly
shed in dead filter cells (choanocytes) – detritus which fell to the seabed.

Within two days, the same molecules were present in snails and other
creatures feeding on the sediment containing sponge waste.

These snails are in turn eaten by larger animals, and so the cycle
continues.

It was not only the speed, but the sheer volume of food turnover which took
the authors by surprise – about 10 times more than bacteria recycle.

The sponge Halisarca caerulea for example takes up two-thirds of its
body weight in dissolved carbon each day, but it barely grows in size – because
old cells are shed to the seabed.

In total, the Dutch team estimated this “sponge loop” produced nearly as many
nutrients as all the primary producers (corals and algae) in an entire tropical
reef.

And other marine deserts, like deep-sea cold-water coral reefs or temperate
Mediterranean reefs, may also rely on poriferans to recycle their nutrients.

By recognising sponges as lynchpins – the unheralded heroes of the reef –
they hope to aid conservation efforts in these fragile havens.

A stone fish lurking behind a sponge. The big
predators of tropical reefs ultimately depend on sponges