Caecilians: Senses

Summary: This amazing amphibian’s senses are unique and almost unbelievable! Join Kiersten as she sheds a little light on caecilian senses.

 

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

 

Show Notes: 

The Amphibian Class by Rebecca Stefoff

“Underground amphibians evolved unique ear.” by PlanetEarth Online. https://phys,org/news/2014-07-underground-amphibians-evolved-unique-ear.html

Caecilians: An Overview https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/caecilians

“The comparative morphology and evolution of the eyes of caecilians (Amphibia, Gymnophiona) by Marvalee H. Wake, Zoomorphology: 105, 277-295 (1985) https://link.springer.com/article/10.1007/bf00312059

Music written and performed by Katherine Camp

 

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 caecilians and the second thing I like about this amphibian is their senses!

 

Let’s dive right in, listeners, because this is some fascinating stuff! We’re going to start off talking about vision. In the first episode my guest co-host and I touched quickly on the fact that caecilians are essentially blind. Let’s explore this in a bit more detail. 

In all the species of caecilians that have been investigated it appears that their eyes are either covered by skin or bone. Interestingly all of the examined species have a functional photoreceptor. What does this mean in layman’s terms? It means that they don’t have eyes that help them see images but they may still use their eyes to detect light. 

Ancient fossils of caecilians showed that they had larger eyes that might have been used to see images but over the millennia their eyes have become smaller and weaker. We know why this has happened. Caecilians live in dark places, underground and underwater. This behavior probably evolved slowly as they adapted to the pressures of survival in a changing world. As they relied less and less on light their eyes became smaller.

Scientists, always on the hunt for more information, asked the question “What exactly happened to the eyes of caecilians as they evolved?” Did they retain all the components of a functional eye or did they loose certain structures that were no longer needed? In 1985 a biologist in the Department of Zoology at the University of California Berkley found some answers. At the time Marvalee Wake investigated all the caecilians that were known and studied the structure of their eyes. Seven differ trends were seen. 

1. The eyes were covered by thicker skin as well as bone.

2. Nonessential ocular muscles became smaller or nonexistent

3. The retina progressed from a typical layered pattern to fewer cells to a net like formation instead of layering

4. The optical nerve starts off present then becomes smaller and weaker then to only a small amount of cells

5. The lens of the eye is originally round or spherical then progresses to crystalline to cellular to absent.

6. The vitreous liquid in the eye is lost

7. The cornea becomes attached to the covering skin and the lens becomes attached to the cornea and retina.

All of these trends show how the eyes of caecilians became obsolete, but some structures still remained. 

Another project from 2014 studied the rods and cones in the the eyes of caecilians. Rods are the structures that are active in lower light levels while cones are active at higher light levels and can detect color. They found that the caecilians optical cells only contained rods. This is further evidence supporting the thought that their eyes my still be used to detect light but are no longer used to see actual images. 

So, now we know that caecilians no longer rely on their vision to help shape their knowledge of the world around them. Some of you may be asking if they have compensated for this lack of information in some other way? I like the way you think, listeners, because this is exactly what happened!

Let’s look at the sense of smell next. Most caecilians have nostrils through which they breath but there is no evidence that indicates their sense of smell is located within the nares. Many species have another anatomical structure that helps them smell, tentacles. 

Caecilian tentacles are located on their face in-between the eye and nostrils. They have one on each side. I know when I think of a tentacle I think of the arms of an octopus, tentacles that are long, thin, and capable of grasping objects. Caecilian tentacles are not like this and thank goodness! Could you imagine having two long squiggly arms that pick up random items sticking out of your face?  Whoa! Talk about problems! Caecilian tentacles are more like short nubs. 

The tentacles of caecilians are chemoreceptors. Essentially they help them smell. They use them to gather information about their environment such as what kind of soil they are in and where they can find food. Caecilians are carnivores so they are always on the look out for terrestrial insects and other invertebrates to eat. Terrestrial and aquatic caecilians use their tentacles in the same way.

The tentacles are attached to the tear ducts and the Jacobson’s organ also known as the vomeronasal organ. Being attached to the Jacobson’s organ is a good indication that these tentacles are used for smelling. Almost all  amphibians, reptiles, and mammals on earth have a Jacobson’s organ that helps  them detect smells. 

These facial organs have a secondary purpose as well. Caecilians also use these tentacles as tactile receptors which means they use them to feel their environment as well as smell it. This is extremely helpful when you have poor or no vision. Caecilians are the only amphibians that have tentacles. They most likely evolved to help compensate for their failing vision.

The last sense we’re going to talk about is hearing. Caecilians do not have external ear holes or pinnae. When you live underground or underwater you have no real need to hear sound waves that travel through the air. But if you could feel vibrations, this would be useful. In 2014, researchers used a CT scanner to create a 3D image of a caecilians head and found something interesting. Caecilians have a much larger ventral organ in the ear than other types of amphibians. This organ is used to sense vibrations!

To deal with their underground lifestyle, caecilians have evolved a large vibration sensing organ in their inner ear. According to the study, this is different from other underground animals such as molerats. The researchers hypothesize that because the caecilians are more sedentary than other underground dwellers and they leave their head on the ground more, since they have no legs, this may be the reason their ventral organ is larger.

That’s it for caecilian senses. I know you had fun listening to this episode because it’s my second favorite thing about caecilians.

 

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Join me next week for another thing I like about caecilians!

 

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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.