Uncovering the Mysteries of the Mesopelagic

By Beth Francis, Bangor University UK

Have you ever thought about the mysterious dwellers of the deep ocean? Six months ago, I thought of these species only as scary-looking creatures in horror movies and nature documentaries. Now, after flying across the world twice to study these alien-like inhabitants of the depths up-close, I was inspired to investigate the mysteries of mesopelagic and why these deep ocean dwellers are important to all of us.


Alien? This amphipod, Phronima, is rumored to be the inspiration for the alien queen in the 1986 movie “Aliens.” These small parasitoid crustaceans hollow out salps (barrel-shaped invertebrates) to lay eggs inside. To the left is a Phronima in its natural form and to the right is a female Phronima inhabiting a salp. The small specs are her hatched eggs, protected by the salp (Photo: Bangor University/Beth Francis).

I began my PhD in October 2016, based at Bangor University in North Wales, UK. My PhD is funded by the Envision Doctoral Training Program, through the UK Government Natural Environment Research Council, but my main focus is studying gradients in productivity near to islands in Hawai‘i. In the six months since beginning my project, I have been privileged to join NOAA researchers twice on the Integrated Ecosystem Assessment (IEA) expeditions to West Hawai‘i, to learn more about the mesopelagic community there.


Mesopelagic Marvels: A tiny nautilus, a type of shelled deep-sea cephalopod, similar to an octopus or squid (Photo: Bangor University, Beth Francis).

The middle or mesopelagic depths, also known as the “twilight zone” of the ocean (between 200-1000 meters or 650-3300 feet) may seem like another world, but it is much closer to home than you may think. This region plays a crucial role in our planet’s ecosystem. An estimated 90% of the world’s fish live in this zone, and while most aren’t commercially important species, they form a key part of the food web. These deeper dwellers, such as shrimp and squid, are prey for dolphins, whales, and for the fish we eat. The mesopelagic also plays an important role in removing billions of tons of carbon dioxide from the atmosphere each year, pumping carbon from the surface water deeper into the ocean (Siegel et al., 2014).


Deploying the Cobb trawl net to target mesopelagic organisms (Photo: University of Hawai‘i/Jana Phipps).

Despite the importance of this layer of the ocean, relatively little is known or understood about species distribution and interaction in Hawai‘i—and these are a few of the mysteries we are researching through the West Hawai‘i IEA project. The West Hawai‘i IEA is an on-going project, collecting information on the oceanic communities in the region. Back in September 2016, and for part of this recent IEA expedition, we collected deep-sea organisms both nearshore and offshore of a known biological “hotspot” site. The goal is to quantify the abundance and diversity of organisms at both sites in order to understand why a greater density of these deep-sea dwellers is found closer to shore.

In order to sample the mesopelagic, we use a giant mid-water trawling net called the Cobb trawl. During Cobb trawls, we collect an array of weird and wonderful creatures of the deep, ranging from tiny to huge, cute and familiar, to very strange-looking! And most importantly, we have been able to collect a lot more information on the mesopelagic populations than ever before.


A free-swimming sea snail colloquially known as a sea butterfly because of its wing-like foot (Photo: Bangor University/Beth Francis).

So, what have we learned so far? Initial results collected last September have given us some really interesting information, suggesting that there are significantly more (roughly three times as many) organisms close to shore compared to offshore. And during the most recent expedition, the same pattern seems to be holding up. These findings potentially support the theory that there is an increased primary productivity near islands, and that this extends down into the deeper layers of the ocean. Hotspots in biological productivity, such as the one we are researching in West Hawai’i, could prove to be crucial in the longevity of the human interaction with the ocean, and act as a crucial natural refuge for changes to the climate in the future. We are hoping that the data we collect will help us to uncover some of these mysteries of the mesopelagic, and better understand the ocean in West Hawai’i and beyond.

Reference: Siegel, David A., et al., “Global assessment of ocean carbon export by combining satellite observations and food‐web models.” Global Biogeochemical Cycles 28.3 (2014): 181-196.
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