C1 Advanced US 37 Folder Collection
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Can you guess what you're looking at?
Is it a fuzzy sock? An overripe banana? A moldy tube of toothpaste?
In fact, this is the humble sea cucumber, and while it might look odd,
its daily toil paves the way for entire ecosystems to thrive.
Sea cucumbers are members of the phylum Echinodermata,
along with sea urchins, starfish and other radially symmetrical,
“spiny-skinned” marine invertebrates.
Some sea cucumbers have feathery tentacles flowing from their mouths,
some are puffed like bloated balloons,
and others simply look like Headless Chicken Monsters—
the actual name given to a rare deep-sea species.
But they are generally characterized by their long, cylindrical shape.
A sea cucumber is essentially a brainless, fleshy form surrounding a digestive tract,
bookended by a mouth and an anus.
Adhesive tube feet run the length of their bodies
and allow them to scoot along the seafloor.
Specialized tube feet can be used for feeding and respiration,
though many sea cucumbers actually breathe through their anuses.
Rhythmically contracting and relaxing their muscles,
they draw water in and out over an internal lung-like structure
called a respiratory tree that extracts oxygen from seawater.
Certain species of crabs and pearlfish
take advantage of this rhythmic action and,
once the sea cucumber's anus is dilated, they shimmy in and take shelter.
The rear end of a single sea cucumber can harbor up to fifteen pearlfish at a time.
However, it seems that not all sea cucumbers
put up with this intrusive behavior.
Some species are equipped with five teeth around their anus,
suggesting that they may have taken an evolutionary stand
against unwanted guests.
But even sea cucumbers that lack anal teeth
are outfitted with tools to defend themselves.
They evade threats and launch counter-attacks
using their mutable collagenous tissue, or MCT.
This gel-like tissue contains bundles of collagen, called “fibrils.”
Proteins can interact with these fibrils to slide them together,
stiffening the tissue, or apart, softening it.
This versatile tissue has many advantages:
it aids in efficient locomotion,
enables sea cucumbers to fit into small spaces,
and allows them to reproduce asexually by splitting apart.
But MCT's most explosive application is employed when a predator attacks.
By loosening the attachments of internal tissues
then quickly softening and contracting their muscles,
many species are capable of shooting a wide range of organs
out of their anuses.
This act is called “evisceration”
and it's a surprisingly effective defense mechanism.
In addition to startling and distracting predators,
the innards of some sea cucumber species are sticky and toxic.
Evisceration may seem drastic,
but sea cucumbers are able to regenerate what they've lost to their gut reaction
in just a few weeks' time.
Aside from the few species that have evolved to swim
and those that feed without moving,
many of these cumbersome creatures pass their time grazing the seabed.
Sea cucumbers are found everywhere from shallow shores
to abyssal trenches 6,000 meters below sea level.
On the deep sea floor, they comprise the majority of animal biomass,
reaching up to 95% in some areas.
As these sausage-shaped wonders trudge along, they vacuum up sand,
digest the organic matter it contains, and excrete the byproduct.
In this process, sea cucumbers clean and oxygenate the seafloor
by breaking down detritus and recycling nutrients.
This creates the conditions for sea grass beds and shellfish to thrive.
Sea cucumber excretions can also aid in coral formation
and may play a role in buffering marine environments from ocean acidification.
As the ocean's vacuum cleaners, they are very good at their job:
about half of the sandy seafloor
is thought to have passed through the digestive tract of a sea cucumber.
So next time you're rejoicing in the feeling of sand
crunching between your toes, consider this:
those very grains of sand might have, at one point or another,
been excreted by a pickle that breathes through its butt.
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This sea creature breathes through its butt - Cella Wright

37 Folder Collection
王杰 published on May 19, 2020
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