Climate: Sponges could be key coral reef organisms in the future

Written by James Bell, Rob McAllen, and Valerio Micaroni *of

Conversation

Analytics Sponges are ancient marine animals, very common throughout the world’s oceans and seem less affected by ocean warming and acidification.

Golden sponge (Verongia aerophoba), Tenerife, Canary Islands.

Golden sponge (Verongia aerophoba), Tenerife, Canary Islands. There are over 8000 species of sponges in the oceans.
Photo: AFP/Sergio Hanquette/Biosphoto

Our latest research shows that it can also tolerate low levels of oxygen.

This is a surprising discovery because most sponges are rarely naturally exposed to hypoxia in modern seas.

We suggest that their tolerance is the result of their long evolutionary history and exposure to variable oxygen concentrations through geologic time.

As our oceans continue to warm due to climate change, they are expected to contain less oxygen.

The ability of sponges to survive in low-oxygen conditions means that they are likely to tolerate these potential future environments better than other seafloor organisms.

There are over 8000 species of sponges in the oceans. They are multicellular organisms with a physical structure built around a system of water channels, pores and channels that allow water to be pumped and circulated through them.

Their specialized pumping and feeding cells, called filamentous cells, are highly efficient. The sponge can pump up to its own body size in a matter of seconds.

In modern oceans, sponges are often the most abundant organisms in rocky reef environments. They perform important ecological functions as part of benthic (benthic) communities around the world.

Sponges play many roles in marine ecosystems, but their ability to process water and their efficiency in capturing small particles are most important because they connect the water column to the sea floor. Sponges also support diverse seafloor communities through carbon conversion.

Some sponge species have been shown to be highly tolerant to the stresses of climate change, particularly temperature change and acidity (measured as pH). This means that sponges could be a future winner in changing the oceans.

Sponges in the oceans past

We know sponges are ancient creatures, but recently described 890-million-year-old fossils have turned our understanding of evolution on its head.

Most major animal groups, including arthropods and worms, first appear in the fossil record during a period known as the Cambrian explosion, 540 million years ago. But if the newly described fossils were really sponges, they would have existed nearly 300 million years ago, significantly delaying the history of the oldest known animals on Earth.

If the ancestors of modern sponges were about 900 million years old, they would have evolved and lived during the Marian glaciation, 657-645 million years ago, when the oceans were very low in oxygen.

It is also possible that they experienced wide fluctuations in other environmental conditions such as pH, temperature and salinity through evolutionary time.

Sponges tolerate low oxygen

Our recent environmental tolerance experiments support this scenario, showing that they are surprisingly tolerant to low levels of oxygen.

We evaluated the response of sponges to moderate and severe hypoxic events in a series of laboratory experiments on four species from the Northeast Atlantic and Southwest Pacific.

Sponges were exposed to a total of five low-oxygen treatments, with increasing intensity (40 percent, 20 percent, 6 percent, 5 percent and 1.5 percent air saturation) over seven to 12 days.

We found that sponges in general are very tolerant of hypoxia. All but one of the species survived in the harsh experimental conditions, and those species only began to die at the lowest oxygen concentration. In most experiments, hypoxic conditions did not significantly affect the sponge’s respiration rates, indicating that they could absorb oxygen at very low concentrations in the surrounding environment.

In response to reduced oxygen, the sponge exhibited a number of morphological and structural changes, potentially increasing its ability to absorb oxygen at these low levels.

Ocean sponges of the future

Warmer ocean waters contain less oxygen, and removing oxygen from the oceans is one of the major consequences of climate change.

Warmer water also becomes more powerful than cold water, which reduces the mixing of surface oxidized water with deeper layers that naturally contain less oxygen. At the same time, warmer temperatures increase organisms’ demand for oxygen while increasing metabolic rates and initiating stress responses.

While ocean oxygen levels are expected to decrease, on average, by only 4 percent across all oceans, these effects are likely to be more extreme locally and regionally. In coastal waters, the removal of oxygen from the ocean due to climate can be exacerbated by a process called eutrophication, which is essentially an excess of nutrients. This fuels plankton multiplication, and when bacteria decompose dead phytoplankton, they use up all the oxygen.

Since the land is generally the source of these extra nutrients, shallow coastal areas are most at risk. These are areas where sponges usually predominate in rocky reefs, especially below the depth of light penetration (usually 20-30 m).

Our findings provide further support for the idea that sponges will be the survivors if our oceans continue to warm.

* James Bell is Professor of Marine Biology at Te Heringa Waka Victoria University in Wellington. Rob McAllen is Professor of Marine Conservation, University College Cork, and Valerio Micaroni is a PhD candidate in coastal and marine biology and ecology at Te Heringa Waka Victoria, University of Wellington.

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