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We usually think of scallops as tasty, pan-seared delicacies,
or the classic shape of clipart seashells.
But last week, biologists revealed that they’re worthy of way more
than a special spot on a menu:
Their visual system is amazingly complex,
and includes up to 200 telescope-like eyes.
Most eyes, from the fancy compound eyes of bees to our own simple ones,
focus light with a lens onto a retina,
which contains light-sensitive photoreceptors.
Scallop eyes, on the other hand, use mirrors to do this focusing,
which is why people sometimes compare them to telescopes,
like Hubble, despite their tiny size.
This much had been known for half a century,
thanks to a British vision researcher.
But what the mirrors are made out of,
and exactly how sight worked for the scallops, remained a mystery.
That’s mostly because their eyes are only about a millimeter big.
And every time scientists tried to prepare the delicate eye tissue
for a microscope, samples dried out and fell apart.
Enter cryogenic scanning electron microscopy, or cryo-SEM.
The technique uses liquid nitrogen to quickly chill samples,
allowing researchers to keep tissue intact and hydrated.
Reporting in the journal Science,
a team used this method and discovered that scallop eye mirrors
are made out of guanine.
You might have heard of guanine because it’s one of the four bases
that makes up DNA.
But crystals made of guanine have weird optical properties
that many organisms use to survive.
Like, it’s what makes fish scales look iridescent or shiny.
In scallops, the guanine crystals are in the shape of square tiles.
These tiles are stacked 20 to 30 sheets thick,
each layer separated by a bit of cytoplasm.
And they’re arranged to form a curved mirror at the back of each eye.
This is similar to how segmented mirror telescopes—
the largest and most powerful telescopes in the world—focus light.
So, scallop eyes are more like the James Webb Space Telescope
than Hubble.
The researchers think the precise layering of the crystals
causes them to mostly reflect blue and green wavelengths of light—
which is what you’d expect a scallop to need to live under the sea.
And maybe even more impressive is the fact that
scallop eyes have two retinas.
Instead of being at the back of the eye, like ours,
the retinas are two strips of tissue in the middle of the eye,
onto which light gets focused after it’s reflected off the back mirror.
The team took a 3D scan of the eye,
and simulated some views with a computer.
From these models, they saw that the guanine mirror
is uniquely shaped and oriented to focus different light on each retina.
So they think scallops see two images at once.
The smaller, outer retina appears to be specialized for quick changes in light
that are directly in front of the eye,
like the sudden shadowy presence of a predator.
And the larger inner retina is better at picking up light in the periphery.
It’s also more sensitive to light overall, and gives the clearest picture.
So biologists think this retina is important for the scallop
to keep tabs on its surroundings, even in low-light.
Which is probably pretty helpful if you live under a rock in the ocean.
And now: More sea creature news!
Because last week, a team of biologists also announced that
they think the first animals to break off from all others
and form a separate branch on the tree of life were sponges, not comb jellies.
It’s a debate you probably didn’t know was happening.
But it has huge ramifications in thinking about animal evolution.
And for years, scientists have been split on the matter.
Sponges, also known as poriferans, are very simple animals.
So simple that many people confuse them with plants.
They don’t move or have any organs to speak of.
Instead, their bodies are full of pores that let water pass through.
And specialized cells called choanocytes capture food particles in the water.
Comb jellies, or ctenophores, are also pretty simple,
although not as much as sponges because
they actually have a basic nervous system.
And they look kind of like jellyfish, but they don’t have any stinging cells.
The confusion has been over which of these animals is the sister to all others,
or what scientists call the sister group.
In other words, if you look at the evolutionary tree,
which group is split off by itself from other animals.
This doesn’t necessarily mean one animal is older than another,
or that we humans evolved from one or the other.
What it does tell us, though, is what the common ancestor
of all animals might have been like.
The problem is that researchers have gotten different answers
when they look at the genomes of these animals,
compare them to others, and use computer models of evolution
to make a phylogenetic tree.
If you include specific amino acid changes, you usually get sponges.
But if you exclude those, or include distant organisms, like fungi,
you usually get jellies.
In the latest study, researchers used more statistics
to filter out noise in all the data, and concluded that
sponges were the real deal.
Some scientists may still nit-pick at this result,
but others seem convinced that the debate is almost over.
And it kind of makes intuitive sense that the simpler animal
is at the root of the family tree.
The opposite finding would have meant that
sponges would have had to lose their more complicated organ systems
from a shared common ancestor.
So, there you have it.
An updated family tree for all of us…
at least until we get more data to complete the puzzle.
Alright, time for an announcement!
You may have heard of this by now,
but we here at SciShow are trying a new thing.
Sometimes—all the time—it's hard to figure out what presents
to get for people.
But we come across weird, cool science-y things all the time
that we love and we think you or someone you know might love.
So we've created a store called SciShow Finds.
It's got a limited list of items that we have in very limited quantity.
Stuff that lets you learn, or experiment on yourself,
or just show how enthused you are about the wonders of the universe.
Someone you know is guaranteed to love these socks or this lapel pin!
And we'll add new items as we find them
to replace these ones as they sell out.
A lot of them already sold out.
And, when you buy from SciShow Finds,
you're also helping support SciShow, which I appreciate a lot.
So, thanks for doing that!
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