Larvaceans’ underwater ‘snot palaces’ boast elaborate plumbing

Secreted from the heads of tiny sea creatures, mucus homes are filtration marvels

Bathochordaeus mcnutti

Sea creatures called giant larvaceans (Bathochordaeus mcnutti, shown) construct elaborate “houses” made of mucus. A new study shows that the inner part of the house turns out to hide an engineering marvel of channels and chambers that help harvest food particles.

© 2002 MBARI

Underwater laser scans have revealed new details of how sea creatures called giant larvaceans feed themselves by flapping a filmy tail inside a cloud of snot.

But what a cloud it is. A giant larvacean produces an elaborate mucus home for itself that bioengineer Kakani Katija of the Monterey Bay Aquarium Research Institute in California on occasion calls a “snot palace.” The mucus marvels rise out of the heads of four species of spineless, roughly tadpole-shaped giant larvaceans living in the twilight depths of the bay.

To study such fragile architecture, Katija and colleagues have been working on a robotic laser imaging system called DeepPIV. It detects water flows inside the mucus clouds and lets researchers figure out the palace’s inner 3-D structure. The newest reconstructions of flow suggest how inner ducts, chambers and valves, all made of mucus, help harvest bacteria and other suitable food particles from the normally weak soup of seawater, Katija and colleagues report June 3 in Nature.

DeepPIV imaging system illustration
In this illustration of a DeepPIV imaging system, a camera can dangle 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

Frail, filmy animals like these “don’t lend themselves to traditional study methods,” says ecologist Kelly Sutherland, at the University of Oregon in Eugene, who was not involved in the research. Her lab also focuses on jellylike animals that “tend to fall apart as soon as you try to collect them.” Even some matters of basic biology in such creatures remain open questions. So devising ways to study a filmy species alive in its watery home is the way to move the science forward.

In building those homes, larvaceans remind Katija a bit of spiders. Plenty of animals build homes and traps, but larvaceans and spiders are among the few that don’t collect building material or dig and sculpt soil. Instead, they secrete all their architecture.

And much like a spider weaving a web anew each day, larvaceans are thought to make and remake their mucus houses (SN: 5/4/17). A millimeter-sized gob of mucus beads up on a larvacean’s head. Then the blob can inflate into a finished house in 45 minutes.

When fully inflated, a plump, curved, inner mucus house cradles the larvacean as the animal’s swishing tail pumps seawater through the structure. Encasing all of this plumbing and the animal cuddled against it lies the big floaty envelope of the outer house. A larvacean creates the whole palace, even ribbed walls and intricate chutes, without arms or legs or even a snout that pokes the mucus into shape or nudges parts together.

Laser scan of larvacean mucus house
Underwater laser scans have revealed the complex plumbing channels that make up the inner part of a larvacean’s mucus house. That plumbing helps filter and concentrate edible particles from seawater.© 2002 MBARI

Researchers had previously proposed that the outer house, which can stretch a meter across in larger giants, filters out bigger debris chunks that might clog the inner works. Now Katija and her colleagues propose other uses.

The outer house might protect the larvacean inside it from stinging animals that waft through the open water. That engulfing mucus might also work as an invisibility cloak. Inside, the larvacean’s tail sends seawater surging through the channels in the inner house. In the twilight dimness where larvaceans live, however, water motions can tip off a hungry fish to the possibility of a nearby larvacean lunch. Yet by the time that expelled water finds its way past the outer mucus house, the motion has slowed by three orders of magnitude. A predator might not bother to investigate.

To study the paths of water through the outer and inner houses, researchers lowered the laser imaging system to scan a series of cross-sectional views of living larvaceans floating free in Monterey Bay. Analyzing the velocities of particles whooshing this way and that in spots on the cross-sectional scans let the scientists figure out the elaborate structure.

As seawater throbs through this plumbing, the stream of food particles grows more concentrated as it approaches the animal’s mouth. As Katija, a bioengineer interested in taking design inspiration from nature, points out, these animals have evolved an inflatable filtration system.

The new insights about internal structure are welcome news to Russell Hopcroft, a zooplankton ecologist at the University of Alaska Fairbanks. He used a somewhat similar imaging tool last summer in the Gulf of Alaska and can think of plenty of fragile, filmy creatures that he would love to scan. The Monterey Bay larvaceans, however, have a special resonance: “We did a lot of head-scratching when I was a postdoc … over 20 years ago trying to understand this 3-D structure.”

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