deep sea

Taking a long, hard look at a classic deep-sea theory

Reviewing: Bridges, A. E. H., Barnes, D. K. A., Bell, J. B., Ross, R. E. & Howell, K. L. Depth and latitudinal gradients of diversity in seamount benthic communities. J. Biogeogr. 1–12 (2022). doi:10.1111/jbi.14355

A slice of the deep sea

Profile of Earth’s ocean. The deep sea begins at about 650 ft and includes all the water below. Different topographic habitats exist in the deep-sea such as continental slopes, abyssal hills, seamounts, and trenches. Figure courtesy of NOAA.

Welcome to the deep sea! This vast ocean habitat starts at depths where sunlight dims (about 650 ft, or about the length of two football fields) and continues to the seafloor below. There are different types of habitats in the deep sea based on the topography (the shape of the seafloor) and whether the seafloor is rock or sand. Soft-bottom habitats are dusted with fine sediment and dominate the deep sea in the form of abyssal plains and continental slopes. Hard-bottom habitats are rare comparatively but are important sites for diversity, feeding, and reproduction. They are present in the form of seamounts (unexposed islands) and oceanic island slopes, hydrothermal vents, and volcanic ridges.

Hard-bottom habitats are known to host different types of animals than soft bottom sediments. While soft-bottom sediments are mostly composed of infauna, animals like snails, bivalves, and worms that live and eat in the sediment, hard-bottom habitats predominately host filter-feeding animals that attach themselves to rocks like corals, anemones, and sponges.

Photo left: Example of a soft-bottom deep-sea habitat found along the abyssal plains of the Pacific. Although seemingly barren, this habitat is teeming with animals living in the sediment such as bivalves, snails, and worms. Photo right: Example of a hard-bottom habitat, a coral garden occupying the volcanic rock of the Sibelius seamount in the deep waters off of Hawai’i. Images courtesy of NOAA.

Hungry for more

With limited light penetrating these waters, these deep-sea habitats cannot support the primary production of food, thus animals rely strictly on the export of food from the waters above. As you go deeper into the ocean, or further away from food-rich surface waters, fewer nutrients are available for animals in the deep sea. In these situations, the ecosystem is food limited and unable to support as many different animal species. Scientists have observed this phenomenon numerous times in soft seafloor deep-sea habitats where diversity is lower the deeper you go. However, this theory has never been tested on hard-bottom habitats, which may have similar gradients in food availability.

Testing new waters

Sampling locations in the South Atlantic as part of research by Bridges et al. 2022. Ascension Island (a), Saint Helena (b) and Tristan da Cunha (c).

Researchers based out of the UK sought to see if hard-bottom deep-sea habitats also have decreases in species diversity related to food availability. To test this hypothesis, the researchers used underwater images to characterize animals on three different seamounts and oceanic islands in the South Atlantic. These sites have steep sides descending into deep water and are predominately composed of hard-bottom, volcanic substrates. The team also recorded the environmental conditions at each site, including latitude and depth, which are considered proxies for food availability.

Fractures in a rock-solid theory

 A total of 624 images were collected across the three oceanic islands with many different animals observed. The number of species did vary across the different oceanic islands where more species were found in the temperate site (c) than those of tropical latitude (a,b). However, within each site, diversity did not change with depth. Rather than fewer species being found in deeper waters as expected, there were just different species occupying deeper waters.

The major takeaway from these findings is that the theory of less diversity where food is lower may not universally apply in the deep sea! Maybe species adapt to low food availability differently across habitats or there is an unaccounted food source in these habitats? Though we may imagine the deep sea as dark, deep, and monotonous, there are different forces at play across the deep-sea such that what we may consider “rules” don’t apply everywhere. We can appreciate the complexity of diversity in the deep sea as new doors are opened from this research.

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