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Strange Animals Podcast

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Welcome to Strange Animals

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Podcast. I'm your host,

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Kate Shaw. It's been way

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too long since we talked about an

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invertebrate, so this week we'll look at

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one suggested by Morgan, the Antarctic Death

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Star. It has a lot

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of other names, too, including the Antarctic

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Sun starfish and the Wolf Trap or

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Bear Trap starfish. Its

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scientific name is Labidiaster

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annulatus. I'm going to

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call it the Death Star because I

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think that's hilarious. As

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you may have guessed from its

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common names, the Antarctic Death Star

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is a starfish that lives in

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cold ocean waters near the Antarctic,

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aka the South Pole. But

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its common names also hint at how it

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gets its food, and this would be a

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good time to take a moment and be

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glad you're not a copepod that also lives

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in the Antarctic Ocean. The

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Death Star is reddish brown on

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its dorsal side, white underneath. It's

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a large starfish, up to two feet

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across, or 60 centimeters. And

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it also has a lot of

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legs, more properly called rays, up

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to 50 of them. The

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rays are long, narrow, and very

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flexible, and the undersides have rows

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of little structures called tube feet.

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All echinoderms, including starfish, have

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these tube feet, and they're

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used for several purposes. One

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important purpose is helping the animal stick

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to a hard surface, which allows it

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to climb around more easily and ride

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itself if it gets flipped over. For

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over 150 years, scientists thought the

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tube feet acted like little suction

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cups, but that didn't explain how

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a starfish or other echinoderm could

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stick to porous surfaces. It

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wasn't until 2012 that a study was

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published explaining how the tube feet were

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feet actually work. The

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tube feet exude tiny amounts of

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a sticky chemical that acts like

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glue. The Death Star's

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body also has little spines and

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bumps all over it, but it

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also has some structures that give

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the animal its other names, the

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Wolf Trap or Bear Trap starfish.

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The structures are called pedicillarii, which

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are also common in echinoderms. Most

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echinoderms seem to use them to

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keep algae and other organisms from

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settling on the body, although scientists

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aren't completely sure. Pedicillarii

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have muscles and sensory receptors, and

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when something touches them, they snap

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shut like a trap. In

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the case of the Antarctic Death

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Star, its pedicillarii are extra big

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and really sharp. When a

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krill or other tiny animal brushes against

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one of these little traps, it grabs

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the animal and then the Death Star

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can eat it. But that's

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just part of what's going on when

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the Death Star goes hunting, so let's

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discuss it in more detail. Most

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starfish spend almost all their time

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on the ocean floor, walking around

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looking for food. The Death Star

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does this too, but not all

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the time. Quite often a

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Death Star will climb on top of a

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rock or other large structure, and then it

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will extend some of its rays up and

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out into the water. It waves

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its rays around, and if it touches

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a small animal, it will wrap the

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rays around it. The pedicillarii

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also snap shut. Then

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the Death Star can eat whatever it

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caught. Usually this is krill

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or amphipods, but it's not a

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picky eater. Since it will eat

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animals it finds already dead, researchers

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aren't completely sure if the Death

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Star ever catches fish. They've certainly

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found dead fish in Death Star's

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stomachs, but the water it lives

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in is so cold that not

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many fish live there anyway. Fish

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don't make up a big part

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of the Death Star's diet, whether

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or not it's catching them at

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sea. The Death Star

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also eats other starfish, including

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smaller Death Stars. Like

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other starfish, the Death Star can

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eat surprisingly large pieces of food

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because it can avert its stomach.

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This means it can actually push

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its stomach out through its mouth

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and engulf whatever large food it's

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found or caught. The digestion

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process starts right away, which allows

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the starfish to eat food that

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can't actually fit through its mouth.

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It doesn't chew its food because it doesn't

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have any kind of teeth or jaws, but

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who needs teeth and jaws if your stomach

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can just reach out and grab food. While

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I was researching the Death

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Star, I came across a

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study published in November 2023

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about echinoderms. So let's learn

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something surprising about starfish and

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their relations in general. Echinoderms

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demonstrate radial symmetry instead

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of bilateral symmetry. That's

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why you can't tell when

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a starfish or other echinoderm

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is facing forward, because it

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doesn't actually have a forward.

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But it's actually more complicated

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than it sounds, because the

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distant ancestor of echinoderms, which

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lived during the Cambrian almost

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half a billion years ago,

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did demonstrate bilateral symmetry, and

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the larvae of modern echinoderms

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do too. When a modern

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echinoderm larvae develops into an adult, the

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left side of its body is the

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only part that grows. The right

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side of its body is absorbed,

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and from then on the body

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develops radially. It actually shows

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pinter radio symmetry with five sections

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around the central part of the

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body. That's why so many

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starfish have five rays, although obviously

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not all of them. The

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Death Star starts out with five rays, but

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adds more and more as it grows. For

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a long time, scientists have wondered

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if echinoderms technically have heads, or

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if they're just bodies. They

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don't have eyes, or nostrils or most

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other body parts that we associate with

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the head, just an oral opening in

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the middle of the underside of the

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disc. Starfish do have cells

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at the end of their rays that

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act as eye spots, which are sensitive

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to light and dark but can't actually

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see anything else. Instead of

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a brain, it has a nerve ring

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around its mouth and connected nerve nets

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in its rays, and its

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digestive system extends into its

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rays. In other words,

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it sure seems like an echinoderm

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has no head and is basically

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just a weird body, but the

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new study came to a surprising

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conclusion. The study examined

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starfish genetics and discovered that

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the genes associated with head

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development were there. It

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was the genes associated with the

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development of a body and tail that

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were missing. In other words,

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the starfish and echinoderm in general

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are just really complicated heads.

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You can find Strange

7:02

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episodes. Thanks for listening.

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And this will be a good time to take a

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moment and be glad you're not

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a copepod that also lives

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in the Antarctic Ocean. I

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mean, I'm glad for this

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fact every day. Alright, that

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typo is driving me nuts. Fix

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that.