15 August 2017

A Small Giant Makes Up For a House


Let's say we're the curious scientists here about to embark on a deep dive, an excursion to around 400 m down Monterey Bay. 

What what we're about to witness is a giant, floating... "tadpole"

They're called giant larvaceans, pelagic basal chordates [1]. Most adult species are barely half an inch [2] while giants are typically the size of a pinkie finger, even reaching 3.5 inches. They might look like tadpoles with a distinct head and tail. Aptly so, Larvacea derives its name from its semblance to a larval tunicate [1].

Now, back to our interest, let's maneuver with our remotely operated vehicle (ROV), a video, and a laser. Reaching the deep, dark waters, we spot a head, an undulating tail - a larvacean as we expected it to be. Until...

The Big, Transparent House

Lean closer and we see a fragile, mucus "house" which encases the larvacean. Depending on the species, the creature makes it a home either by being attached to it or by being cocooned within it. For instance, Fritillaria species attach to the outside while Oikopleura species usually live within it [1]. 

Photo of the giant larvacean Bathochordaeus mcnutti from Monterey Bay Aquarium Research Institute (MBARI).

It doesn't take much to build a bubble house. An Oikopleura sp. can inflate its house in a minute, and can construct 4 to 16 houses a day depending on the temperature and food availability. In a lifetime, it can construct 46 houses [1].

What's the house for?

It is a feeding apparatus: an efficient filter-feeding and trapping system [1,2,3].

Take the giant larvacean Bathochordaeus.

The Bathochordaeus' house can reach a diameter greater than a meter. It is basically an outer structure with a mucus screen that excludes larger particles, plus an inner filter which sieves and concentrates plankton and organic material. Attached to one another, the house's tail chamber receives the particle-laden water pumped by the creature's tail. The water is then directed into the inner feeding filter. When it's done, the concentrated particulate is delivered into the animal's mouth [2].

Food particles are shown in some species to be 100 to 1000 times more dense than the surrounding water: It's a rich broth like no other [2]. 

Fastest Zooplankton Filter Feeder

Larvaceans are second to copepods as ubiquitous marine creatures. In ideal conditions, their number could be massive. In British Columbia, each cubic meter of water holds 25,260 individuals. That's a lot of whipping tails and house expansion [1].

However, attempts to grow a larvacean's house in the laboratory proved impossible; the houses are way too fragile [2,3]. A recent study led by Katija used a tool called DeepPIV which permitted them to directly measure the filtration rates of giant larvaceans on site. To their surprise, each individual can filter 11 gallons (almost 42 liters) of water per hour [2]. 

That makes them the fastest zooplankton feeder, spearing them ahead of copepods, euphausiids, salps and small larvaceans [2]. 

At peak density and maximum filtration rate, giant larvaceans have the potential to filter their 200-m depth range in Monterey Bay within 13 days [2].

Fertilizing Discards

When its house gets clogged, the larvacean simply discards it and makes another one. These large, nutrient-laden houses immediately sink to the seafloor without time for mineralization by microbes.  Soaked with the ocean's upper productivity, these mucus houses contribute about one-third to vertical carbon flux from near-surface waters to the deep sea benthos [2,3]. 

Probing further into the mid-water filter feeders and their filtration capacities could someday shed more insights on the connection between deep water biota and their long-term removal of atmospheric carbon [2,3]. 

If you have more information on larvaceans and other non-fish organisms, we'll be happy to have you as one of SeaLifeBase collaborators. Let us know by sending us an email or visiting our FaceBook page.

[1] Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). Invertebrate zoology (7th ed.) A functional evolutionary approach.

[2] Katija, K., Sherlock, R. E., Sherman, A. D., & Robison, B. H. (2017). New technology reveals the role of giant larvaceans in oceanic carbon cycling. Science Advances, 3(5), e1602374.

[3] Yin, S. (3 May 2017). In Disposable Mucus Houses, These Zooplankton Filter the Oceans. The New York Times. Retrieved from http://nyti.ms/2qBhwgD

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