Breakthrough in Studying Mysterious Carbon Capturers

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Giant larvaceans (Bathochordaeus mcnutti) construct elaborate 'houses' made of mucus. © 2002 MBARI

Scientists have made a major breakthrough in the study of a complex, fragile and little understood creature of the deep. 

Giant larvaceans – tadpole-sized zooplankton – surround themselves with large balls of mucus which filter the surrounding ocean for food. When their filters get clogged with debris which includes significant amounts of carbon and now microplastics, they discard the mucus, dispatching the waste to the bottom of the ocean.

Scientists at the Monterey Bay Aquarium Research Institute (MBARI ) in California, using radical new technology, have for the first time been able to study these remarkable filter feeders in their natural habitat.

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A DeepPIV imaging system can hover in front of larvacean mucus structures in the wild to track the complex water flow through chutes and valves too fragile to pull to the sea surface intact. KIM FULTON-BENNETT © 2017 MBARI

Kakani Katija, the lead engineer of the project and key author of a recently published paper on giant larvaceans published in Nature, said: ‘Mucus is ubiquitous in the ocean, and complex mucous structures are made by animals for feeding, health, and protection. Now that we have a way to visualize these structures deep below the surface we can finally understand how they function and what roles they play in the ocean.’

Larvaceans live in the open ocean or ‘pelagic regions’. They can be found in various depths but mostly in the twilight zone. There are only around 70 species of larvacean currently known and recognised, with most species living in the Pacific Ocean including three species of giant larvaceans known as bathochordaeus.

Similar to jellyfish yet more complex, larvaceans have intricate mucous layerings that trap more than food. Using small cells on the top of their heads called ‘oikoplast cells’, larvaceans create a small mucus bubble which gets blown up like a balloon to sizes up to a metre wide.

The mucosal homes, referred to as ‘snot palaces’ by some scientists, are actually giant infrastructures which contain corridors and complex pathways. This elaborate feeding apparatus is constructed from two layers of filters. The outer filter captures large particles too big for the larvacean to eat. The inner sanctum traps suitably-sized food particles which are transported directly to the animal’s mouth. The larvaceans create currents inside the complex structure by beating its relatively large tail. There is also a built-in escape route for the larvacean to leave its home in case of a predator invasion.

These mucus encasements, which can take an hour to grow, are discarded every 24 hours when the filters become clogged with debris. The mucus is cast away carrying significant levels of carbon-rich particles and microplastics down to the abyssal levels of the ocean. They provide a vital food source for marine life living at greater depths.

The housing is extremely fragile, making the zooplankton notoriously difficult to catch and study. Early methods of study involved using nets to bring the larvaceans to the surface, which resulted in a mass of gelatinous goo. Scientists at the MBARI decided to take their laboratory underwater. Using a DeepPIV (deep particle imaging velocimetry), the team managed to capture a larvacean underwater and use laser-based 3D printing to compose a model of its mucus structure.

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Underwater laser scans have revealed the complex plumbing channels that make up the inner part of a larvacean’s mucus house. © 2002 MBARI

How much of the discarded mucus gets absorbed by the ocean floor and therefore takes carbon and plastic out of circulation is the subject for further research. But for the first time scientists have been able to study a little-understood one of the processes by which carbon and now microplastics move through the ocean.

‘Until now, no scientist has had the chance to examine such complicated structures in deep-sea creatures’, said Dr Katija.

The research conducted by the MBARI will hopefully shed light on new non-invasive methods of studying other animals from the deep.

Co-author and marine biologist at the MBARI Dr Bruce Robinson said: ‘The majority of lower-depth organisms are yet to be discovered. This is going to open things up in a really good way.’

 

 

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