Surprisingly fast growth rates discovered in deep-sea corals

New research from the University of Hawai’i has shown that the deep-sea corals Leptoseris can grow at a much faster rate than previously realised. It challenges the widely held assumption that deeper corals living on the brink of darkness grow extremely slowly.

L hawaiiensis
A colony of deep sea Leptoseris spp. in a deep sea coral reef

When you think of corals you normally picture diverse and colourful structures in crystal clear tropical waters surrounded by an abundance of vibrant marine life. However corals can actually grow at depths much greater than most people realise. In fact entire reefs, much less diverse and colourful than their tropical relatives but equally important, can grow hundreds of feet below the ocean’s surface on the brink of total darkness. A new study by researchers from across Hawai’i has shown that despite the lack of light and freezing waters, these corals actually grow much quicker than anybody realised and highlights just how important they really are.

Past assumptions

Leptoseris is the dominant group of coral species that help to from deep-sea coral reefs throughout the Indo-Pacific region. Like their vibrant tropical cousins they rely heavily on their symbiotic relationship with photosynthetic algae known as zooxanthellae. However unlike shallow water corals they receive much less light. In fact at the lower end of their depth range, at around 360 feet below the surface, they only receive 0.2% of the light available to their warm water cousins. As a result of this scientists have long assumed that these deep-sea corals grow at an agonisingly slow rate of around 0.04 inches per year, based on the limited data they have collected from this challenging research environment.

Leptoseris hawaiiensis (315 feet) HURL
A colony of Leptoseris hawaiiensis located in the Au’au channel (photo by HURL)

Collecting corals

Step forward Samuel Khang, an affiliate with the University of Hawai’i at the Manoa School of Ocean and Earth Science and Technology (SOEST), who wanted to properly test the hypothesis of slow growth in deep-sea corals. Along with researchers from the Hawai’i Underwater Research Laboratory (HURL), Waikiki Aquarium, National Taiwan University, Hokkaido University and Hawai’i Pacific University, where he is also a professor. The team took samples of Leptoseris at depths between 225 and 360 feet in the Au’au Channel using HURL’s Pisces IV/V submersibles. They then used uranium-thorium radiometric dating to accurately determine the age of the coral skeletons, before measuring growth rates by looking at their radial growth axis, much like counting the rings of a tree.

Leptoseris specimen
One of the samples of L. hawaiiensis collected by the researchers

Surprising growth

What they found was not only very surprising, but also extremely interesting. “The corals were found to be relatively young with growth rates comparable to that of many non-branching shallow water corals” explained Khang in a press release. In fact at a depth of 225 feet the corals grew at around an inch per year, whereas at the deepest points around 360 feet the rate was a slower 0.3 inches per year (still almost 10x faster than previous predictions). These results were included in the recent paper released by Khang and his colleagues in the journal Coral Reefs. In it the researchers suggest that the reason behind the corals faster growth rates is due to the way they grow, which maximises the amount of light they can absorb whilst reducing energy output.

Leptoseris magnified (Khang)
A close up shot reveals spaced out polyps which might also increase their rate of growth (photo by Samuel Khang)

This is achieved by growing outwards rather than upwards. Leptoseris corals have thin skeletons with a wide surface area, which is continually spreading out as they grow. Unlike lots of other corals they also don’t get thicker when they grow, which means light can easily access the photosynthetic algae in their tissue. The team also suggest that the corals aim to create a convex shape in which they can trap light in a sort of ‘reflective-chamber’ meaning they can make the most out of the light that reaches them.

An important ecosystem

This discovery is very interesting because it highlights how important these deep-sea ecosystems could be in the future. Once considered to be ancient outliers, deep-sea coral reefs are now known to be widespread and capable of growing much quicker than previously realised. Khang also thinks that his work “suggests that these communities may be able to develop and recover from mortality events much faster than previously thought”. This is especially important given that their tropical shallow water cousins are facing so many anthropogenic problems today. However just because these reefs are deep down doesn’t mean they are safe, because new practises such as deep-sea mining can pose a very real threat to them. It is therefore important that we learn more about them in order to better protect them in the future.

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