Coelacanth: Limbs vs Fins

Summary: Are they limbs or are they fins? What are those things on the side of the coelacanth? Join Kiersten and a guest host to find out!

 

For my hearing impaired listeners, a complete transcript of this podcast follows the show notes on Podbean

 

Shoe Notes: 

“Coelacanth Fossil Sheds Light On Fin-to-limb Evolution.” Science Daily, https://www.sciencedaily.com

Anatomy: https://www.pbs.org/wgbh/nova/fish/anatomy.html

Music written and performed by Katherine Camp

Oxford Languages Dictionary

Merriam-Webster Dictionary

Casey teaches her students all about cladograms!

 

Transcript

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Kiersten - This is Ten Things I Like About…a ten minute, ten episode podcast about unknown or misunderstood wildlife.

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Welcome to Ten Things I Like About… I’m Kiersten, your host, and this is a podcast about misunderstood or unknown creatures in nature. Some we’ll find right out side our doors and some are continents away but all are fascinating. 

This podcast will focus ten, ten minute episodes on different animals and their amazing characteristics. Please join me on this extraordinary journey, you won’t regret it.

This episode continues the coelacanth and their crazy interesting fins is the eighth thing I like about this deep sea fish.

Today I have a guest cohost joining me, my friend Casey. Thanks for joining me Casey.

 

Casey: You’re welcome. Thanks for having me.

 

Kiersten: Casey and I have known each other for a long time. We met as zookeepers 18 years ago and became friends very quickly. Today, Casey is a biology teacher and when she found out I was doing this podcast she was excited to help me.

 

Casey: Coelacanths are extremely interesting and their appendages, or limbs, are interesting in their history.

 

Kiersten: Great! Today we’re talking about limbs versus fins! I hope by now, listeners, you’ve all googled a picture of the coelacanth and have gotten a glimpse of their interesting fins.  In episode two, anatomy, I talked about the coelacanths special fins called lobed fins. They have six lobed fins.

 

Casey: I love their little limbs!

 

Kiersten: Me too! But I thought they were fins?

 

Casey: They do look like limbs.

 

Kiersten: I think we need to dive into this topic and learn a bit more.

 

Casey: I agree!

 

Kiersten: Let’s start with some definitions. The Oxford Languages Dictionary says a limb is "a leg or arm of a person or a four-legged animal, or a bird’s wing”. That completely leaves out the coelacanth, for sure!

 

Casey:  I agree! But the Merriam-Webster Dictionary definition is “one of the projecting paired appendages of an animal body used especially for movement and grasping but sometimes modified into sensory or sexual organs.” I think that puts the coelacanth back in the limb arena!

 

Kiersten: Maybe…Let’s hear Oxford Languages Dictionary’s definition of fin: “a flattened appendage on various parts of the body of many aquatic vertebrates and some invertebrates, including fish and cetaceans, used for propelling, steering, and balancing.” That definitely sounds like the coelacanth.

 

Casey: I can’t argue with that! But Merriam-Webster’s definition of fin is “an external membranous process of an aquatic animal used in propelling or guiding the body.” I’m on the fence with this one. I wouldn’t say that coelacanths have membranous fins.

 

Kiersten: Hmmmmm….I think what we need to do is look at what these fins can do.

 

Casey: You mean limbs.

 

Kiersten: Let’s call them appendages until we get his settled. 

 

Casey: Deal!

 

Kiersten: The coelacanth has seven appendages overall. Four are paired, two behind the gills, the pectoral fins, one on each side; two mid body on the bottom of the fish the pelvic fins, one on each side. Then there is one anal fin just in front of the tail on the underside of the fish and then two dorsal fins. They have eight if you include the tail. I think it’s interesting in itself that they have eight appendages, that’s quite a lot for a fish.

 

Casey: Yeah, what are they using all those appendages for?

 

Kiersten: Well the four paired fins on each side of the body move like paddles propelling them forward through the water. These appendages are able to rotate 180 degrees so they can probably use them to back up as well as more forward. The really cool thing about these four extremities is that they work in conjunction with each other. The right pectoral fin moves together with the left pelvic fin. 

 

Casey: You mean like the way a horse walks?

 

Kiersten: Yes! It’s a lot like how a many land mammals move their ….. oh, I see where you going with this. You just want me to say they're limbs!

 

Casey: Yes! Yes, I do! 

 

Kiersten: Well, I think we might both get what we want from the next definition. Coelacanths are classified as a lobe-finned fish. This means that the flouncy part of the fins are attached to a stalk that projects out from their body. It actually looks like a paddle with a fin attached to the end.  

 

Casey: That sounds like a limb…..and a fin.

 

Kiersten: Yes, I agree. Maybe we can agree that we’re both right?

 

Casey: I can do that. Did you know that some lungfish, who are also a lobe-finned fish, actually use their fins to walk on land when they need too? If the vernal pond they are living in becomes too shallow, they can use their limbs to drag themselves across the land to another water source. Essentially they use their limbs to walk to another pond.

 

Kiersten: I guess it really is both a limb and a fin. Speaking of limbs, I just read a research paper, from way back in the early 2000’s, that stated a coelacanth fossil actually helped scientists understand the evolution from fins to limbs in tetrapods. Tell us little about that.

 

Casey: I can! What you’re talking about is called evolutionary classification which is a strategy that we tend to use now instead of taxonomy which only looks at physical attributes. Evolutionary classification is grouping organisms together based on their evolutionary history. So, we’re looking at lines of evolutionary descent not just physical characteristics. These lines are called cladograms and they look like trees. They branch off at each different change. So, the cladogram where it’s branching off to tetrapods, which is a four limbed vertebrate, is where their is a bit of a controversy involving the coelacanth.

We are tetrapods. We may walk on two legs but we have four limbs. The big debate is whether the kingfish or the coelacanth is the direct ancestor of tetrapods. Now remember we said lungfish walk on their limbs to get to the next vernal pond. That is where the debate is coming from. I’d like to say there is an answer to this debate but there are three different cladograms and only one of them has the coelacanth as the direct ancestor to the tetrapod.

 

Kiersten: Is it descendent or ancestor?

Casey: It’s ancestor. 

 

Kiersten: Okay. Because we’re all tetrapods, right?

 

Casey: Right, but we would be a descendant of a coelacanth. Now I’m not talking like you grandmother or anything.

 

Kiersten: (laughs)

 

Casey: I’m talking millions of years ago. So coelacanths or lungfish would be the ancestors of tetrapods.  One version has the coelacanth as the direct ancestor, while another version has the lungfish as the direct ancestor, and the last version has both of them at the same branching. So, no solution to this debate as of yet. They’ve narrowed it down to these three options. No mater how you look at it though the coelacanth is Number 1 or Number 2. 

 

Kiersten: So, he’s still winning. (Laughs)

 

Casey: (laughs) Yes! He’s still winning. He still in the trifecta. Either way it’s still in the positive.

  

Kiersten: That is interesting! That’s why I picked you for this podcast! I knew you’d understand that and be able to explain it better than me! Thanks for helping me talk about the coelacanth appendages today Casey.

 

Casey: You’re welcome. I had a lot of fun!

 

Kiersten: Me too! And I think we both agree that coelacanth appendages are both limbs and fins.

 

Casey: I agree!

 

Kiersten: Well that is it for this episode, listeners. I hope you enjoyed a little debate about coelacanth appendages because it’s my eighth favorite thing about them.

 

If you're enjoying this podcast please recommend me to friends and family and take a moment to give me a rating on whatever platform your listening. It will help me reach more listeners and give the animals I talk about an even better chance at change. 

 

Join me next week for another episode about the coelacanth.  

 

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This has been an episode of Ten Things I like About with Kiersten and Company. Original music written and performed by Katherine Camp, piano extraordinaire.