Coelacanth: Anatomy

Summary: Join Kiersten as she takes you on a surprising journey from the head of the coelacanth to the tail. 

 

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

 

Show Notes: 

A Fish Caught in Time: The Search for the Coelacanth by Samantha Weinberg

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

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 the coelacanth and their anatomy is the second thing I like about this ancient animal.

In the first episode we established that the coelacanth is a fish, so the anatomy should be like fish anatomy, right? Maybe, but this fish has been around for 400 million years and from comparisons between living specimens and fossils, they don’t seem to have changed much at all. So the coelacanth has some anatomical surprises that other fish do not possess.

Let’s start at the head of the coelacanth and work our way back. In the center of the snout there is a large jelly-filled cavity. It’s called the rostral organ. Scientists initially thought this might be an electrosensory organ for detecting weak electrical impulses given off by prey. This hypothesis was supported by examining the organ’s intricacies including nerves and how it interacted with the brainstem. When submersible exploration became available to scientists, we were able to test this on a living coelacanth. Using electrical fields, researchers confirmed that coelacanths do indeed respond to electrical fields under water. There is no other vertebrate, that we know of, alive today that has a rostral organ. So we’re already off to an interesting start. Let’s see what else the coelacanth has that other animals may not!

Moving to the mouth we find teeth. That’s not entirely unique in fish that eat other fish and the coelacanth is a carnivore. They have three different shapes of teeth, one is a high, sharp cone-shaped tooth that could be called a fang, the second is a midsized, sharp cone-shaped tooth, and the last is a small rounded tooth.  What is unique to the coelacanth is that the small rounded teeth are embedded in a bony dental plate that lies beneath their chin. We’re not entirely sure what the advantage of having a dental plate gives the coelacanth. In general, the teeth seem more like a way to keep fish in the mouth once they are sucked in versus tearing or chomping on their prey.

The eyes are just above the mouth and they are attached to thick optic nerves. The eyes are large. I’d say they are in proportion to their body and they are a five foot long fish, so…large eyes. Each eye does have a few cones, which allows for color vision, but they many rods, which help detect light. This is perfect for the coelacanth because they live deep under water where there is very little light available. The rods help them see in almost near darkness.  

They also have something else that helps them see in the dark and this is a layer behind the retina of the eye called the tapetum lucidum. If you’ve ever seen light flash in your cat’s eyes at night, you’re seeing the tapetum lucidum. This layer acts like a mirror reflecting the light that comes into the eyes back out of the eye to increase the amount of light that passes over the retina. This enhances the coelacanth’s ability to see in low light.

Just behind the eye toward the top of the head is the intracranial joint. Until we rediscovered the coelacanth this joint had only been seen in fossils of primitive fish. This joint allows the coelacanth to open its mouth exceptionally wide to swallow fish and other prey. Two powerful muscles cross the intracranial joint providing strength to the coelacanth’s jaws. The coelacanth is the only fish alive that still has an intracranial joint. 

Just under this joint and near the eye is the brain. The brain is small. It takes up only 1.5% of the brain case in mature adults. In a 90 lbs coelacanth the brain weighs less than a tenth of an ounce. That’s a pretty tiny brain. There is no extant vertebrate with this much of a difference between the size of the brain and the size of the body. Although, they have been alive for 400 million years so it must be working. 

Coelacanths do have gills like other species of living fish and they are located behind the eye in the usual place that you find fish gills. They are relatively small in comparison to the overall body size, but they are similar to other fish species found at the same depth as the coelacanth. The relatively small surface area of the lungs is indicative of a slow-moving fish as opposed to an active fish. All the evidence we have to date does show that the coelacanth is a relatively slow-moving fish. When you’re a five foot long fish, you take your time getting places.

This large fish is covered in scales and those scales are woven tightly together like armor. I think that seems appropriate for such an ancient animal. The scales are hard and rough to the touch. Each scale has tiny, tooth-like spikes called denticles all over the surface which creates the roughness. The hard scales and denticles provide protection against predators and rocks. The scales are a beautiful steely-blue color with random white spots throughout. The pattern of the spots is unique to each individual and scientists have used that to their advantage. Researchers use the spots like name tags to identify individual coelacanth.

The fins of this fish are quite spectacular. They have six fins that are lobed and this puts them into a group of fish known as the lobed-finned fish. Lungfish and coelacanth are both included in this group.  What’s the difference between a lobed fin and a normal fin? Great question! Lobed fins are fins that are attached to stalks that project out from the body rather than fins that are attached directly to the body. They look kind of like paddles sticking out from the side on the animal. The coelacanth has six lobed fins, one on each side just behind the gills called pectoral fins (2), one on each side of the pelvis called pelvic fins (4), one small secondary dorsal fin on the top of the body (5), and one anal fin on the underside of the body before the tail (6). 

Overall the coelacanth has seven fins, not including the tail. The first dorsal fin is the only non-lobed fin. It’s larger than the other fins and attaches directly to the body. It can be raised and lowered to change its surface area.

We call the coelacanth a vertebrate because it has an internal skeleton which usually implies that it has vertebrae or bones of the spine. Once again the coelacanth surprises us. They have a notochord in place of a bony spine. A notochord is a thick-walled, fibrous, and elastic tube that is filled with oil. This is what the coelacanth has in place of a bony spine. Most creatures with a backbone replace the notochord with vertebrae in the embryonic stage. But adult coelacanth use the notochord for their longitudinal support.

Like almost all other fish species,  the coelacanth has a swim bladder. Fish use the swim bladder to maintain buoyancy in the water. Most fish use air to inflate the swim bladder and they are able to modify the amount of air depending on the depth in which they wish to swim. I bet you didn’t see this coming but…, the coelacanth’s swim bladder is not filled with air! Okay, maybe you did see that coming. The coelacanth’ s swim bladder is filled with oil and fat, but it works the same as the air filled bladder, helping maintain buoyancy. 

That brings us to the end of the fish, also known as, the tail. Hopefully, you’ve listened to the first episode of this series where I talk about the rediscovery of the coelacanth, if not definitely check it out. When Marjorie Courtenay-Latimer rediscovered the coelacanth in 1938, she described the tail as looking like a “puppy dog tail.” The tail is actually divided into three sections with a small tail fin in the middle. The tail is flat and powerful giving the coelacanth the ability to dart forward forcefully when catching prey or escaping predators. The tail can rotate and flex from side to side and is thought to help the fish with trim and balance.

That is all I have for coelacanth anatomy. Thanks for joining me on this head to tail adventure because it’s my second favorite thing about the coelacanth.  

 

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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.