Larvaceans’ underwater ‘snot palaces’ boast elaborate plumbing

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.

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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