A hidden line deep in the ocean divides the animals into two camps

Deep in the ocean, not all ecosystems are built the same.

And, as an international team of scientists has now discovered, the deepest depths are dominated by a particular type of organism. Below a depth of about 4,400 meters (14,436 feet), most creatures that lurk in the dark have soft, squishy bodies. It is only above that line that hard-shelled molluscs are generally found.

The reason, according to scientists, has to do with the availability of the minerals from which shells are formed. This knowledge could help us protect biodiversity from human activity in these cold, dark and supernatural environments.

“Muddy, abyssal seabeds were initially considered almost ‘marine deserts’ when they were first explored many decades ago, given the extreme conditions for life there with lack of food, high pressure and extremely low temperatures,” says deep-sea ecologist Erik Simon-Lled of the National Oceanography Center in the UK.

“But as deep exploration and technology advances, these ecosystems continue to unveil great biodiversity, comparable to that of shallow-water ecosystems, which are found only over a much wider spatial spread.”

A glass sponge and an anemone grow next to each other on the ocean floor. (Smartex/NERC project)

The abyssal ocean covers more than 60 percent of the Earth’s surface, yet so little is known about the life that inhabits it. It’s environmental anathema to humans: crushing pressures, freezing temperatures, and perpetual darkness, away from sunlight.

However, technology has improved to the point where we can remotely explore these darkest depths, revealing the strange pale and soft underbelly of the world.

Using deep-sea robots, Simon-Lled and his team collected a large database of images from an abyssal plain known as the Clarion-Clipperton Zone, which stretches 5,000 kilometers (3,107 miles) across the Pacific Ocean floor between Mexico and Kiribati at depths ranging from 3,500 to 6,000 meters.

They painstakingly cataloged any animals they could find larger than 10 millimeters from these images. They indexed more than 50,000 abyssal creatures and noticed a marked difference in the types of animals found at shallower depths than those in deeper parts of the area.

Some of the animals found live on the abyssal bottom of the Pacific Ocean. (Simon-Lled et al., Nature Ecology & Evolution2023)

“We were surprised to find a deep province so clearly dominated by soft anemones and sea cucumbers and a shallow chasm where suddenly soft corals and brittle stars were everywhere,” says Simon-Lled.

Nor did molluscs, with their hard shells, appear below 4,400 meters, although all sorts of abyssal life populated a transitional zone between the two regions. That certain depth, the researchers found, is likely related to the carbonate clearing depth.

The hard shells are formed from calcium carbonate, which diffuses through the ocean from the surface. Below a certain depth, however, insufficient calcium carbonate remains, leading to a deficiency of it on the seabed, for hard-shelled fauna to take up.

The abysmal hedgehog Globosum plesiodiadem. (Smartex/NERC project)

This suggests a delicate balance at play in deep-ocean biodiversity, which could easily be disrupted by ocean acidification, climate change and deep-sea mining, for which the Clarion-Clipperton area is currently under investigation.

‘Overall, this reflects much higher ecological heterogeneity, at multiple scales, than previously predicted for benthic assemblages in the Northeast Pacific abyssal floor,’ the researchers write in their paper.

“This overlooked heterogeneity, resulting from geochemical and climatic forcings, has crucial implications for future ecological and macroecological research in abyssal communities and for the success of regional-scale conservation strategies implemented to protect biodiversity in the Clarion-Clipperton Zone and possibly other abyssal areas targeted by deep-sea mining around the world.”

The research was published in Nature Ecology & Evolution.

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Image Source : www.sciencealert.com

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