Life is everywhere. It is found in rocks deep beneath the surface of the earth, in a lake buried under the ice in Antarctica, and even in ponds of liquid tar-asphalt. So, although the physical environment-in the 5,000 m deep ocean floor is challenging, numerous species of organisms call it home.

To protect and manage our Marae Moana, we first need to understand it. This is not something unique to the deep sea, most other industries that interact with nature like farming, building or fishing use an essentially identical approach.

A desert under the sea?

Life on the abyssal plains is very different to the life we see when swimming along our reefs. Sunlight stops at about 200m depth, and most of the fish and other marine species we’re familiar with don’t dive much deeper than 1,000 m. Animals at 5,000 m have had to adapt to the extreme conditions: dark, cold, and intense pressure.

Deserts are usually defined as areas of land that receive little rainfall (or snow) and thus have small amount of life (usually called biomass) e.g. [1]. The abyssal plains are clearly a lot wetter than your typical desert but are thought to have even less life e.g. [2], [3]. One reason is that there are no plants on the seabed. On land they make up as much as ~80% of the biomass [4]. Then the central part of the Cook Islands is part of the nutrient poor South Pacific Gyre, which means there is not much food available. So it is thought to have even less life again [5].

It’s hard to make a direct numerical comparison between deserts and the abyssal plains, because much of the life in both places is composed of tiny, buried microbes [6], [7]. But the deep seabed is thought to have about 1/10th to 1/100th as much biomass as a non-polar sand desert, which in turn has less than 1/100th the biomass of a forest.

A question of biodiversity

Life may not be abundant on the deep ocean floor, but it is diverse. Basically, biodiversity is the variety of life found in a particular place. This can be a small place (e.g. within a pot plant in your backyard) or a large place (the entire Cook Islands Marae Moana, the Pacific Ocean or even the world). And we agree that it is important to ensure that human activities do not cause extinction of any type of life [8]. Also important is how particular species contribute to the environment and how we value it both culturally and for livelihood etc

But then it gets a little complicated when scientists try to measure it. Plants and animals have been named and described for thousands of years, and a formalised system was invented in the eighteenth century. Today scientists can use the physical aspects of the organism—size, colour, internal structure, etc, or ‘simply’ measure their genetic makeup (DNA) of the organisms.

One other way to classify seabed animals then is by their size. A key reason to do it that way is so that specialists can use the best tools to collect enough of the right specimens. There are four size classes of animals [9]:

Megafauna are over 2 cm in size;
Macrofauna are smaller down to about ¼ of a mm
Meiofauna are smaller again down to 0.032 of a mm
Microfauna the smallest again.

As the animals get smaller, they get easier to sample, but harder to describe accurately. A good-sized good quality box-core sample might contain millions if not billions of microfauna (mostly bacteria), hundreds of meiofauna, dozens of macrofauna and usually no megafauna (photo surveys are used for megafauna counting).

Two types of scavenger fish (species currently unknown) photographed from a baited lander in the eastern high seas pocket northeast of Aitutaki September 2024. Scale bar 5cm.

Sampling of different sizes of organisms requires different techniques and expertise. The equipment used by the Women in Science Expedition 2024 (WISE) illustrates how scientists sample different sizes of organisms. Three different instruments and teams worked on the three larger size classes. The megafauna team used baited landers to attract and study scavengers (fish, prawns and larger amphipods). The macrofauna team used multiple box-core samples at each site to get the large and replicate volumes of animal hosting water and sediment they needed. The meiofauna team used a single multicorer at each site to obtain the replicate cores they needed.

Women In Science Expedition (WISE) setting up a multi-corer for deployment for sampling. September 2024

The issue of understanding our biodiversity applies in both land and sea. Even if we take the simplest question “How many species are there on Earth?” The answer is we don’t really know. To quote [10] “Current (2019) estimates for the number of species on Earth range between 5.3 million and 1 trillion. That’s a massive degree of uncertainty. It’s like getting a bank statement that says you have between $5.30 and $1 million in your account.”.

One thing is for sure, deep-sea biologists frequently report high biodiversity in their samples and sampled areas [11]. Comparison among areas can be difficult because a well-sampled area might look more diverse than neighbouring, poorly sampled areas. One reason for this might be because the world has one global ocean [12], interconnected by continuous circulation of currents. This means that while some species differences can be seen between areas [11], many seabed species have tremendous overlapping range. For example, the first Cook Islands deepsea megafauna known to have been DNA sequenced1 is a Freyastera basketa sp. which is also known to inhabit the Clarion Clipperton Zone some 4,000 to 5,000 kilometres away.

So what is being done about it?

Quite a lot. The Government is partnering with overseas scientists to do its own research. It is also inviting other researchers from other countries to come and work here. Finally, it is condition of the seabed minerals exploration licence holders to do an Environmental Management Plan that involves a biodiversity and biomass studies.

It is also the law that all environmental data is in the public domain. SBMA manages the Cook Islands Seabed Data Repository (CSD;https://www.sbma.gov.ck/csd-1), which hosts environmental and legacy exploration data for access by all.

[1] Olson, J. S., J. A. Watts, and L. . Allison, “LBA Regional Carbon in Live Vegetation, 0.5-Degree (Olson),” Oak Ridge USA, 2003. doi: http://dx.doi.org/10.3334/ORNLDAAC/672.

[2] C.-L. Wei et al., “Global Patterns and Predictions of Seafloor Biomass Using Random Forests,” PLoS One, vol. 5, no. 12, p. e15323, Dec. 2010, doi: 10.1371/journal.pone.0015323.

[3] V. Tilot, R. Ormond, J. Moreno Navas, and T. S. Catalá, “The Benthic Megafaunal Assemblages of the CCZ (Eastern Pacific) and an Approach to their Management in the Face of Threatened Anthropogenic Impacts,” Front. Mar. Sci., vol. 5, Feb. 2018, doi: 10.3389/fmars.2018.00007.

[4] Y. M. Bar-On, R. Phillips, and R. Milo, “The biomass distribution on Earth,” Proc. Natl. Acad. Sci., vol. 115, no. 25, pp. 6506–6511, Jun. 2018, doi: 10.1073/pnas.1711842115.

[5] S. D’Hondt et al., “Subseafloor sedimentary life in the South Pacific Gyre,” Proc. Natl. Acad. Sci., vol. 106, no. 28, pp. 11651–11656, Jul. 2009, doi: 10.1073/pnas.0811793106.

[6] C. Bienhold, L. Zinger, A. Boetius, and A. Ramette, “Diversity and Biogeography of Bathyal and Abyssal Seafloor Bacteria,” PLoS One, vol. 11, no. 1, p. e0148016, Jan. 2016, doi: 10.1371/journal.pone.0148016.

[7] P. M. Leung et al., “Energetic Basis of Microbial Growth and Persistence in Desert Ecosystems,” mSystems, vol. 5, no. 2, Apr. 2020, doi: 10.1128/mSystems.00495-19.

[8] Conference of the parties to the convention on biological diversity, Kunming-Montreal Global Biodiversity Framework. United Nations: https://www.cbd.int/doc/decisions/cop-15/cop-15-dec-04-en.pdf, 2022, p. 15. [Online]. Available: https://www.cbd.int/doc/decisions/cop-15/cop-15-dec-04-en.pdf

[9] Legal and Technical Commission, “Recommendations for the guidance of contractors for the assessment of the possible environmental impacts arising from exploration for marine minerals in the Area,” Kingston, Jamaica, 2013.

[10] T. Latty and T. Lee, “How many species on Earth? Why that’s a simple question but hard to answer,” The Conversation. Accessed: Oct. 09, 2024. [Online]. Available: https://theconversation.com/how-many-species-on-earth-why-thats-a-simple-question-but-hard-to-answer-114909

[11] M. Rabone et al., “How many metazoan species live in the world’s largest mineral exploration region?,” Curr. Biol., vol. 33, no. 12, pp. 2383--+, Jun. 2023, doi: 10.1016/j.cub.2023.04.052.

[12] National Oceanographic Centre, “One Ocean,” Support Us. Accessed: Oct. 09, 2024. [Online]. Available: https://noc.ac.uk/about-us/support-us/one-ocean

Glossary:

Abyssal Plain: seabed at depths between 4,000 and 6,500 m deep (some workers use slightly different depths)

Baited lander: food/bait is attached to equipment with cameras to record what types of scavenger animals are present

Biomass: amount of life in a given area or volume, usually measured in grams

Biodiversity: diversity of life in a given area or volume, usually measured by species (or higher taxonomic unit) or morphospecies (visible animal form)

Gyre (ocean): a large system of circular ocean currents formed by global wind patterns and forces created by Earth's rotation

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All creatures great and small—life in the abyss