This Foot Was Made For Walking, That One Wasn't

This Foot Was Made For Walking, That One Wasn't

I get it. Some of you are skeptics. After shooting it back and forth with someone, here I am writing this blog. I am by no means a seasoned marine biologist, but this one had me laughing. I was asked, "What's the only organ system that aquatic animals have, that terrestrial ones don't?" 

Weird question. I heard the terms "organ system and aquatic", so I immediately thought of clams, more specifically mussels. Clams and mussels have something that many animals don't. Before I answer that, I need to address the flaws in this question. A lot of aquatic animals are also terrestrial. (Hippos)

Some animals can be both. These animals are defined as 'semiaquatic.' Mussels are the only animal that I could think of that are fully aquatic. Right?

Well, I learned something new throughout all of this, clams are in fact gastropods. Many other animals are gastropods, including but not limited to, the snail and sea slug. These animals are full-well capable of being what you would define as "terrestrial," while still being aquatic. This question had no actual basis.

When I was first asked this question, I answered, "a foot" to throw them off.

Not many people know that a foot is actually a muscle in osmoregulatory systems. Clams have what's called a foot. What I did not know was that some snails and other gastropods also have it. Don't worry, these animals don't need shoes.

Mussels and clams live most of their lives in the water and only wash ashore in wet environments. Mussels or clams are the only animal with a foot that are fully aquatic. Freshwater mussels are fully aquatic bivalves, a kind of mollusk that's encased in a shell made of two valves or hinging parts. Source: Google

Note: Not all gastropods are bivalve mollusks.

Video from Business Insider. 
The answer they were looking for was osmoregulatory systems, but that would have been WRONG. Being human, our own kidneys perform osmoregulatory functions. Marine mammals have been found to have it as well. I spent a lot of time researching this. I'm sure that lions have this function as well! 
I compared jellyfish to octopi. I even studied the anatomy of an extinct genus of tetrapod called the acanthostega. It was said to be fully aquatic and have gills, so I compared it to the anatomy of an axolotl. 
Mussels are the ONLY fully aquatic species that I know of that have a foot. Snails and sea slugs are classified as semiaquatic species. However, I think I actually found an answer. It only took me two hours.
Jellyfish have what's called an 'incomplete digestive system.' There is nothing like it on this planet. Planarians also have this system and are one of many flatworms of the traditional class Turbellaria. Planarians are fully aquatic. Source: Merriam-Webster
Seeing how both jellyfish and planarians have this system, what’s next? No mammals are known to have incomplete digestive systems. Could this be our answer? 

Incomplete digestive systems are not found in mammals. 

Thanks for reading!

I may not have all the answers, but I know what questions to ask. 


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