A giant larvacean in Monterey Bay. (c) 2000 MBARI

From around Monterey Bay: Lasers & Larvaceans

We at Hopkins Marine Station are lucky to be situated in Monterey Bay where so many interesting questions in marine biology are waiting to be answered. What makes the Bay even more exciting is that we are near other institutions like the Monterey Bay Aquarium right next door and the Monterey Bay Aquarium Research Institute (MBARI) about a half-hour drive along the coast that contribute to a thriving marine-minded area. Specializing in exploring the deep sea, MBARI reveals a part of the Bay that, though vast, is rarely seen, and I’m excited to share some research from around Monterey Bay.

Underwater architects

If you’re anything like me, consistent housekeeping can be a challenge. I know my roommates can attest that the mess accumulates to a rather substantial level before a massive cleaning session takes place. I would be lying if I didn’t envy the strategy used by a small, tadpole-like deep-sea creature, the larvacean, when it comes to cleaning the house.

A giant larvacean in Monterey Bay. © 2000 MBARI

Despite being no bigger than your pinky finger, larvaceans are abundant and important players in cycling carbon from the atmosphere and surface waters all the way down to the deep sea. Don’t be fooled by their simple appearance at first glance. With just a trunk (or head with an opening to ingest food) and a tail, these animals are able to construct spectacular “houses” made of mucus. Smaller species build houses about an inch big, but giant larvacean houses can span over 3 feet in diameter. The larvacean sits inside and beats its tail to pump water through this mucus house, which catches particles of organic matter along its many grooves and directs those particles to its mouth. When the house gets clogged, the larvacean abandons it to build an entirely new house somewhere else as often as once per hour. These abandoned houses, rich with nutrients and carbon, rapidly sink to quite a few miles to the bottom of the ocean. Talk about moving on and not carrying any baggage!

The first giant larvacean was discovered in 1898 after being pulled up in a sampling net from about a thousand feet deep in the ocean. Needless to say, it was pretty beaten up by the time it got to the surface, but scientists still managed to literally make heads or tails out of the giant larvacean’s body. Researchers had no clue about the giant larvacean’s mucus house until over half a century later in the 1960’s when they could descend into the deep sea in submersibles and see them with their own eyes. Even if the houses had been captured in nets, they would have surely been shredded to bits when dragged to the surface, and none of the many collected specimens ever constructed houses in lab conditions. Because the only information on these inaccessible houses came from photos and videos taken using submersibles or remotely operated vehicles (ROVs) that could go the quarter- to half-mile below the surface to look at them, a big question remained: how much water was the giant larvacean actually filtering through its house?

Lasers illuminate the larvacean’s house

Well, if researchers couldn’t bring the mucus houses up to labs above the ocean’s surface, they’d have to figure out a way to bring the lab underwater to the giant larvacean without going down there themselves. That was not a problem for Principal Engineer Dr. Kakani Katija, who led a team at MBARI to build and develop a new tool called DeepPIV (particle image velocimetry), which goes all the way to the giant larvacean’s front doorstep and uses lasers and the ocean’s abundant particles to track how much water flows through its mucus house. DeepPIV attaches to an ROV a thousand or so feet underwater, and a skilled pilot onboard a ship floating at the surface controls the movements of the ROV and DeepPIV. 

This frame grab from video taken by MBARI’s MiniROV shows the inner and outer houses of a giant larvacean (left) and the laser and camera of the DeepPIV system (lower right). © 2015 MBARI

Seeing larvaceans through the ROVs high-definition camera, the team uses DeepPIV to shine a laser sheet that lights up not only the particles around the animal but those inside as well. Because the house is translucent, DeepPIV’s laser sheet is able to illuminate cross-sections of the house in layers like an MRI, allowing us to see all the intricate inner structures that would have been impossible to document by regular video alone.

This frame grab from video taken by MBARI’s MiniROV shows the inner house of a giant larvacean, with its inner chambers outlined by the red sheet of laser light from the DeepPIV system. © 2015 MBARI

 

By calculating how fast the particles move through the larvacean’s house, Katija and her team revealed that scientists had been massively underestimating how much water giant larvaceans filter. It turns out that giant larvaceans are highly industrious, capable of filtering over 75 liters per hour! That’s nearly 40 party-sized soda bottles — could you imagine going through that much liquid in just an hour? Until now, filtration rate models only took into account how much water the larvacean’s beating tail could move around, but these new values that incorporate the role of the mucus house knock previous estimations out of the park. It really took getting footage of the real deal to finally show us what was actually going on.

Giant larvaceans connect surface and deep waters

Besides filling in a big gap in our knowledge of this seemingly obscure deep-sea animal, the team’s findings have pretty big implications for Earth’s current and future environment. Considering the high abundance of these animals and their impressive filtration rates, giant larvaceans in Monterey Bay could go through and sift particles out from the equivalent of 500 Olympic swimming pools in as little as 13 days. Plus, all that filtering leads to a lot of abandoned particle-laden houses and dropped fecal pellets (yep, giant larvaceans poop too) sinking through the depths. Because the houses and pellets are denser than tiny individual particles, they end up bringing carbon to the seafloor much, much faster — something that scientists will need to start incorporating into their models on global carbon cycling. But, it’s not just organic matter that giant larvaceans are concentrating and transporting, especially with the growing prevalence of plastic and microplastic pollution in the ocean.

Collaborating with MBARI Postdoctoral Fellow, Dr. Anela Choy, Katija’s team found that giant larvaceans also readily filter, ingest, and poop out microplastic particles in their deep-sea habitat. Though we may not think that our interactions with the ocean don’t extend much beyond beaches or the water’s surface, human impacts manage to reach much farther than where humans are located.

A giant larvacean beats its tail to pump water and microplastic beads (red dots) through its inner filter. © 2016 MBARI

Already, recognizable garbage has been documented in the deep sea and on the seafloor, and now we’ve seen that giant larvaceans can concentrate microplastics, small but potent pollutants, and accelerate their transport to previously unimpacted parts of Earth. But this information isn’t necessarily a downer. After all, knowing is much better than not. With new technologies like DeepPIV and continued use of ROVs, scientists can keep making underwater discoveries, adding to our knowledge of Earth’s biggest habitat, and motivating us to find solutions sooner rather than later. In the meantime, I’ll just have to start with cleaning the house and being more conscious of my own plastic usage because I can’t just leave tomorrow and build a new house like the larvacean.

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