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‘Dividing line’ for undersea wildlife discovered in the deep Pacific Ocean
The pattern of diversity in the deep sea is more complex than first thought.
Researchers studying a region of the north Pacific were surprised to find that diversity appears to increase with depth, showing that the abyss isn’t as uniform as once thought.
An invisible dividing line is drawn across the abyssal ocean, separating deep sea life into distinct underwater communities.
New research, published in the journal Nature Ecology & Evolution, suggests that this boundary is driven by a chemical gradient in the ability of organisms to make calcium carbonate, a mineral used to make shells and skeletons.
Above 4,400 metres the ecosystems are dominated by molluscs, corals and bryozoans, but travelling down to 4,800 metres and the environment gives way to soft-bodied animals which aren’t dependent on calcium carbonate, like anemones and sea cucumbers.
Despite this change, however, biodiversity doesn’t decrease with depth. In fact, the research in the Clarion-Clipperton Zone (CCZ) suggests that it could even increase.
Dr Erik Simon-Lledó, the study’s lead author, says, ‘Muddy abyssal seafloors were initially considered to be almost ‘marine deserts’ when first explored many decades ago, given the extreme conditions for life there - with a lack of food, high pressure, and extremely low temperature.’
‘But as deep exploration and technology progressed, these ecosystems keep unveiling a large biodiversity, comparable to that in shallow water ecosystems, only found on a much wider spatial spread.’
By giving a closer insight into how diversity changes within the deep sea, researchers hope this will allow better protection of the extraordinary species which live there.
Dr Adrian Glover, principal scientist at the Museum and co-author of the study, says, ‘We have known for some time that the abyssal plains are relatively high in biodiversity. What has been missing, however, is knowledge of how that diversity is distributed and how it changes across broad spatial scales.’
‘These new data revolutionise our understanding of abyssal Pacific biogeography and will be vital to inform urgent policy decisions on potential deep-sea mining.’
Crinoids are found in the shallow abyss, but cannot tolerate passing through the CCD. Image © SMARTEX Project, Natural Environment Research Council, UK.
How do deep sea animals change across the boundary?
The ocean tends to be separated into five main zones, based on the amount of light each receives. Beyond the sunlight and twilight zones in the upper surfaces, the ocean is devoid of any sunlight, with the only sources of light coming from bioluminescent animals.
The general lack of light means these dark waters are home to ecosystems very different from those in the upper 200 metres. No sunlight means that there are no plants, while the pressure means animals tend to have more flexible bodies and different behaviours than their shallower relatives.
But while the new research reveals that there are general characteristics to all life living in the dark depths, they don’t form a uniform ecosystem.
Dr Lupita Bribiesca-Contreras, another Museum researcher who co-authored the paper, says, ‘The deep sea is normally thought of as one vast ecosystem that’s all connected, with nothing to stop animals from moving. However, we’ve shown that there are soft barriers in place that can prevent movement.’
The upper reaches of the abyssal zone, from around 4,000 to 4,800 metres, are described as the ‘shallow abyss’. These ecosystems are initially dominated by soft corals and molluscs, but as depth increases the effects of the boundary layer begin to appear.
Known more formally as the carbonate compensation depth (CCD), this layer marks the point where the combination of pressure and temperature increases how likely calcium carbonate is to dissolve, and so makes it harder for animals to use it to make shells and skeletons.
This means that moving through this gradual boundary, molluscs which depend on their calcium carbonate shells disappear first, and soft corals begin to decline. While these corals may not have hard exoskeletons like their shallow relatives, they tend to have a hard core or other calcium carbonate components to help maintain their structure.
Beyond 4,300 metres, soft coral ecosystems are replaced by those dominated by brittle stars, close relatives of starfish which are less dependent on calcium carbonate.
However, towards the bottom of the CCD at around 4,800 metres even these cannot create enough calcium carbonate, and anemones and sea cucumbers take over to become the dominant groups of the ‘deep abyss’.
Sea cucumbers thrive below 4,800 metres, as they don't depend on calcium carbonate to survive. Image © SMARTEX Project, Natural Environment Research Council, UK.
While it might seem like this change in species across the CCD would mean that some areas would be less diverse than others, the researchers found that species richness was broadly the same, and that if anything deeper environments tended to have more diversity. These deeper sites also tended to have species from a wider range of animal groups from across the tree of life.
Though this study focuses on the CCZ, similar patterns are likely to occur in other areas of the deep sea. This makes it an important contribution to our understanding of deep sea diversity amid concern over the future of these ecosystems in the face of potential mining.
While proponents of deep sea mining argue that it is necessary to extract minerals to power a greener future, there are worries that the mining of the deep sea could irreversibly damage ecosystems that are still not well understood.
Negotiators from around the world are currently meeting in Jamaica to try and finalise regulations for deep sea mining following the end of a two year countdown to decide on new rules.
‘This research is vital to understand the biodiversity patterns of these deep sea ecosystems if we want to come up with a good management plan for the CCZ and other areas,’ Lupita says. ‘It’s important that this information is taken into account when deciding on which sites could be opened for mining, and which must be protected.’
With the help of papers like this, it’s hoped that a transition to a green economy can be just as kind to the deep sea as it is to the atmosphere.
